Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tubenit on December 01, 2014, 06:42:37 pm
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Some of you may remember my experiment to build a D'Mars Overdrive Special PreAmp that would plug into a solid state PA system.
http://el34world.com/Forum/index.php?topic=15921.0 (http://el34world.com/Forum/index.php?topic=15921.0)
It worked, but I was not happy with the overdrive tone with it. It did have a very warm transparent tone to it though. That part, I was very happy with. Clean tone was great, overdrive was only so-so. It eventually became an amp with a line out and used a 100w 8 ohm resistor and a Hugh & Kettner Red Box cabinet simulator. Adding the LTPI and power tubes gave the overdrive tone that was needed.
Now, I have been approached by my great son-in-law about the idea of a tube preamp for his acoustic guitar. He is a great guitarist and plays in a church band and his signal is sent into a solid state PA system. We're looking at the idea of a good preamp that gives a warm tone.
There will be a mid boost option using a relay. And a foot switchable "MUTE" function also. I choose the tone stack values based on my own D'Mars ODS and playing my Larrivee acoustic thru that amp. I think the Larrivee sounds fantastic thru the amp with some slight delay.
The FX level will be the master volume. And the FX will be for a delay pedal or reverb.
I am thinking this will be a great PreAmp idea for an acoustic. IF this has the same warm transparent tone of my last tube PreAmp, it should sound fantastic.
Any glaring errors on this idea? The original D'Mars PreAmp had more stuff in it for the overdrive. I won't need OD in this design. IF it's used with an electric, then an overdrive pedal can be used.
IF my son-in-law goes with this and wants a non-tweed chassis, then I might try this in one of Doug's new blank chassis.
with respect, Tubenit
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I think that's a good concept. Some thoughts:
* What PU is used on the acoustic guitar. The point is impedance matching of the guitar output to the impedance input of the amp.
* Feedback is a big issue for amplified acoustic guitars. (BTW: A mid boost is probably counterproductive for acoustic guitar use.) Usually the problem feedback frequency is 100 - 150 HZ, but this can vary. Hence, the better guitar preamps have selectable, narrow frequency bands for attenuation. If you're interested I can dig out a vintage Bogen schematic with such a circuit: rotary SW & pot.
* you might also consider a phase reverse SW at the input.
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Yes, I would definitely like to see what the Bogen was using. THANKS!
The pickup is this one: http://jjb-electronics.com/prestige-330.html (http://jjb-electronics.com/prestige-330.html)
Reportedly, the suggested impedence is 1Meg.
I added a switch prior to the volume pot to help shift frequencies. I'm not sure what values to use, so these are a guess. Again, the shown values are what is on my D'Mars ODS and I like the tone that I'm getting with my acoustic. As an FYI, Larry Carlton plays his acoustic thru his Dumble amp. I've seen him in concert do that playing live.
Regarding the mid-boost, I envision that it would probably only be used when my son-in-law plays his thin line Tele with an F-hole thru it. In series, the capacitance value is 330p. It might be useful if playing single note lead solo on acoustic, but I've not tried that on my D'Mars? I'll try that out and see how that works sometime this week.
How would one do a phase reverse switch at the input? Schematic please?
With respect, Tubenit
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A phase SW is DPDT arrangement to reverse polarity: I googled Images: "phase reverse switch schematic dpdt" and got this: https://www.google.com/search?q=phase+reverse+switch+schematic&rlz=1C1RNBN_enUS433US433&espv=2&biw=1280&bih=601&tbm=isch&tbo=u&source=univ&sa=X&ei=Hed9VPj2B-PfsATckIKQDg&ved=0CCAQsAQ#tbm=isch&q=phase+reverse+switch+schematic+dpdt (https://www.google.com/search?q=phase+reverse+switch+schematic&rlz=1C1RNBN_enUS433US433&espv=2&biw=1280&bih=601&tbm=isch&tbo=u&source=univ&sa=X&ei=Hed9VPj2B-PfsATckIKQDg&ved=0CCAQsAQ#tbm=isch&q=phase+reverse+switch+schematic+dpdt)
Low Freq. Filter: Attached is the schematic for the Bogen MX-30a, in 2 pages. The Low Freq. Filter is cut in half, top middle both pages.
The specific guitar PU will work, but piezzo's would want to see around 10M input impedance; other PU's may be low impedance. If you want this flexibility it can be done. Merlin's preamp book has a schematic for hi-meg tube input; for lo impedance an input transformer can be used.
Another factor: multiband eq, but personally I have not found this to be necessary. In my experience an acoustic guitar through a tube amp sounds great - w/o all the bells and whistles in SS preamps such as delay, chorus, reverb, mic & guitar simulation circuits, etc., to make-up for the dry SS tone.
Still, another option is a diy Tube Buffer, receiving signal from a simple, quality SS acoustic guitar preamp with vol control, tone or eq controls, low frequency notch filter, phase reversal, and maybe a tuner built-in.
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Jjasilli,
Thanks for the schematic and the information! I appreciate the help. I found another Bogen schematic that's perhaps easier to read.
As I am understanding this, there is a pseudo James tone stack going into a "frequency" control which is either a 47p bypassing a 560k ..... OR a Frequency Control ?
I am thinking that capacitors in series after V1a would do a similar thing regarding frequency control? It might take two spdt switches OR a using a 3-way impedence selector switch to do a variety of capacitors in series? Again, I might have to experiment with this? I am thinking the way I am thinking about approaching this may be easier for me then borrowing the Bogen idea? Thoughts?
IF I am understanding your reply correctly, ........... regarding the pickup impedence, what I have in mind is OK?
The treble, mid, bass control on my D'Mars seems reasonably useful to me with my Larrivee acoustic AND if an electric is plugged into this tube preamp, then the tone controls would help with that also. While primarily thinking acoustic, I'd like it to be useful for an electric that uses pedals for overdrive.
Regarding the reverse polarity, I understand doing pickups out of phase. I am not sure how that translates to the guitar PreAmp. I'm not saying it doesn't apply, simply stating that it is beyond my ability to figure out how it applies?
:dontknow:
With respect, Tubenit
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I found another Bogen schematic that's perhaps easier to read.
For lot's of Bogen Schematics, see: http://www.makearadio.com/schematics/index.php (http://www.makearadio.com/schematics/index.php)
there is a pseudo James tone stack going into a "frequency" control. . .
The tonestack feeds into a tone recovery stage. The Low Filter is immediately after the tone recovery stage.
IF I am understanding your reply correctly, ........... regarding the pickup impedence, what I have in mind is OK?
Yes. (But it is not best for other types of acoustic guitar pickups.)
I understand doing pickups out of phase. I am not sure how that translates to the guitar PreAmp.
Here, the problem is Mixing -- the acoustic guitar signal is going to get mixed in the PA amp with signal from other devices, such as mic's, other preamps, etc. These signals may be out-of-phase with one another. This can cause a weak, nasal tone -- just like el. guitar PU's out-of-phase -- but here, it's undesirable. So, if the acoustic guitar sounds weak & nasally out of the PA, phase reversal is the solution. Another solution for PA users is to is to always carry an inline phase reverser -- they're made for guitar chords or mic cables.
Today's guitarist usually has a standalone guitar amp, so is personally absolved from the issues arising from mixing his signal with other's. But if your going into a mix, puff -- like it or not, you're instantly a sound engineer! All the quality commercial acoustic guitar preamps have a phase reversal SW.
BTW: on the Bogen Schematics page (above) is the Bogen MXM-A Mixer - Preamp. A great unit. Here's a project upgrading that device, full of useful ideas: http://www.clarkhuckaby.com/BogenMod/MxmMain.html (http://www.clarkhuckaby.com/BogenMod/MxmMain.html)
[BTW2: I forget to mention compression. SS preamps tend to need it to dress-up lifeless tone. But compression can also be useful to an acoustic guitarist who wold rather not fiddle with a vol control - so strumming, finger-picking & flat-picking all comes out at the same vol. ]
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Jjasilli,
Thanks for your continued information and help!
I am now thinking that a DPDT that switches the polarity on the XLR jack would resolve any concerns about phase cancellation?
I'll look a little more at the Low Filter and see what route I want to go. I should be able to draw that up fairly easily on a tagstrip. Still leaning towards a .01 cap into a multiple impedence switch using caps in series though.
Thanks, Tubenit
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I am now thinking that a DPDT that switches the polarity on the XLR jack would resolve any concerns about phase cancellation?
Agreed. (A phase reversal SW could be put anywhere.)
A better view of the Bogen Low Freq Filter is in the Mixer Schematic: http://www.makearadio.com/schematics/images/bogen-mxm-a-8.jpg (http://www.makearadio.com/schematics/images/bogen-mxm-a-8.jpg)
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:thumbsup: Also, I think if the DPDT phase SW is the Center Off kind, it would double as a mute SW. But maybe this would confuse the end user.
BTW: If your Mute SW pops, a SPST SW under the cathode resistor of the last tube stage might be quieter.
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V1b and V2a have power supply connection 'C' and 'D' tied together. ??
Jack
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V1b and V2a have power supply connection 'C' and 'D' tied together. ??
Not tied together. Separate. One of the problems of using "other schematics" to redraw something is errors like that.
With respect, Tubenit
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The pa will allow all kinds of eq options, right? I needed something like this and I really like the paralleled 6sl7 pre that I made. Single tube, no eq, premium parts...I perceive it as being very linear. Used with a fishman neo magnetic pickup in my case; seems you could load the front end for whatever imp your pickup wants to see.
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The wiper on the reverb return pot got connected to the top.
The 4.7M on the tone stack got connected to ground instead of the top of the relay.
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The wiper on the reverb return pot got connected to the top.The 4.7M on the tone stack got connected to ground instead of the top of the relay.
Those are done correctly and as I intended. I do appreciate your desire to help though.
with respect, Tubenit
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You will present the output of an effects device with a dead short to ground when you turn the effects return pot. all the way down.
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this is a preamp that RicharD and i designed a few years ago. very much a sweet sounding pre with decent headroom. for more headroom use a 12AU7 and re-bias V1. it was designed for bass guitar, but with some minor TS tweaks, it may make a fine acoustic guitar preamp. it's the "B" channel of an ampeg SVT.
the PT is sourced from tridoelectronics: it's the dynaco PAS2 preamp PT.
filaments are DC and the filament PS is used for the relays as well. the 12DC also powers the 555C timer IC. the 555C delay triggers after about 30 secs for warm-up: it mutes the output so that there's no preamp noise when powering up it also helps suppress power-off thumps.
i had to reverse engineer the remaining PCB and that is what has taken me so long to jump in: the original schematic was drawn in an application that is long defunct and we both lost the images including the artwork when RicharD experienced HDD crash and i got hit by a java exploit. i managed to save a jpeg of the artwork, but that's about it.
you can sub the edcor 10k:600 OT, however, the altec-lansing peerless and the cinemag iron sound much better than the edcor. if you want to audition the altec iron (15095) i can send you one. insert the FX loop between the HighZ output CF and amp.
just curious, but why do you need an effects loop with acoustic guitar?
--pete
EDIT: PT is part no. PA-211 ignore part no. in schema...
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I would breadboard it before building it. To my mind there is the potential for undue insertion loss in the Low Filter in your circuit for the following reasons. In the MX-30a schematic the Low Filter is driven by the tone recovery stage, which cures any insertion loss issues. It also buffers the input impedance of the Low Filter from the output impedance of the tonestack. In the MXM-a schematic the Low Filter is undriven, but its treble pot is 1M, (and there appears to be additional signal additional signal from the balance circuit). The treble pot is in parallel with the 1M Low Freq. pot. Your treble pot, using a different tonestack, is only 250K -- instead of 1M. You may have more signal loss to ground, depending on the output of your tonestack. In addition, you're using a lower mu preamp tube to begin with. All that being said your schematic might work fine.
I don't know what you mean by "caps in series". Remember, the Low Freq. control is not a tone-shaping device. Rather, it is a problem solver for the exact problems that are known to exist with acoustic guitar amplification in live venues. It's sole purpose is to eliminate boominess & feedback. Presumably the Bogen circuit accomplishes that. To do that it uses Notch filters. A typical notch filter is a cap in parallel with a resistor, which pair are in series with signal -- such as at the top of the Bogen filter. The component values are chosen to attenuate a certain specific frequency, or a narrow frequency band.
BTW: my solution to this problem is a Dano eq pedal. Everything is SS: ancient Bill Lawrence mag PU > ancient Dean Markely SS preamp > > SS pedal baord > SS Behringer mixing board > Bogen tube PA amplifier. But if I were to build an acoustic guitar preamp, I would build-in a low filter, first trying the Bogen schematic.
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At the bottom of the Bogen Low Filter is a Twin-T notch filter. See: http://sim.okawa-denshi.jp/en/TwinTCRkeisan.htm (http://sim.okawa-denshi.jp/en/TwinTCRkeisan.htm)
If the Bogen filter failed to accomplish our intended purpose, I would determine the specific frequency at which feedback occurs by sounding specific bass notes on the guitar through the PA system, then build a Twin-T notch filter to reject (attenuate) it. For flexibility, more than one such filter can be built, each targeting a narrow frequency band for rejection, to be selected by a SW or even a multi-position rotary SW if desired. But again, I would first assume the Bogen Low Filter will be good enough.
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Dummyload and Jjasilli & others,
Guys, thanks for the continued information and help! I will study Pete's schematic and make some mental notes there. I appreciate your sharing it (especially given that you had to redraw it. Very nice of you) !
Pete, I have two Edcor trannies left over from the D'Mars PreAmp project, so I think I'll try those out first. Thank you for the offer of the trying the Altec.
Jjasilli, that "notch" program is pretty cool.
Perhaps the low freq control would be useful and beneficial in this? I'll try to read some more on it.
Regarding the FX ..... I have become a real fan of very slight delay in my amps whether playing acoustic or electric.
I have a delay on 90% or the time and 10% use a reverb that has a delay effect in it. I have yet to find a delay pedal that I like prior to the input jack on an amp. They just don't sound the same to me and all I've tried are too noisy for me including Carbon Copy, Wampler Faux Echo and Boss digital delay. Unless I am playing acoustic unplugged, I always have delay or reverb on.
So given the delay being very important to me, the FX is also important. Secondly, the way I am using it makes for an excellent master volume, IMO. Because of that, I am trying to design a PreAmp around this approach using the FX loop.
With respect, Tubenit
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Pete, I hope you will be OK with this ......... I redrew the schematic you posted to more easily compare it with the idea I have using a CF then tone stack and recovery stage of an FX.
So, comparing the two ..................
I think I am headed the "right" direction in what I am wanting to do given 3 specific goals: 1) will sound good with an acoustic guitar, 2) enough switches, etc to be useful to an electric guitar with an overdrive pedal and 3) has an FX that is useful to delay further down into the signal chain and also useful as a master volume.
I think some of the cap & resistor values will need to be tweaked after building it.
With respect, Tubenit
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Riffing on this thread I've drawn a Variable Twin-T Notch Filter that I think would work in an acoustic guitar preamp to kill feedback at a dial-in frequency in the range from 80 - 150 Hz.
EDIT: Note that much smaller caps can be used if equally larger resistors are used, which might reduce insertion loss. E.g.: decrease cap value by a factor of 10, and increase R values by a factor of ten. The online calculator gives the same frequency rejection results.
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Jjasilli,
OK, I think you've talked me into trying a "low cut frequency filter". Looking at the PreAmp pedal Acoustimx and Tonebone Pz-Pre, I found the following ranges:
http://www.bbesound.com/products/instrument-preamps/acoustimax.aspx (http://www.bbesound.com/products/instrument-preamps/acoustimax.aspx) 70Hz-250Hz
http://www.tonebone.com/pzpre-specs.php (http://www.tonebone.com/pzpre-specs.php) 75Hz-220Hz
With respect, Tubenit
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The process: The site I posted above has 2 calculators on the twin t-notch page: 1. state the R-C component values, and the calculator gives the rejection frequency; or 2. state the frequency to be rejected, and the calculator gives the R-C component values.
Research informed me that the most likely offensive feedback frequency is 100 Hz (approx. the open A string). Or else, about 150 Hz or somewhere in between.
First I plugged 100 Hz into the online calculator, and got the component values posted on my schematic. Then I entered other rejection frequencies. It soon became apparent that the most critical component values were the series resistors R1 & R2, which always equal one another. Changing them while leaving other values the same yields functioning circuits with different useful rejection frequencies. Changing R1 & R2, from about 15K each to about 30K each, picked-out various specific rejection frequencies in about the range of the commercial preamps you mention. I realized that a dual gang pot, wired as 2 variable resistors, could replace R1 & R2. This makes the Filter variable so that the rejection frequency is selectable, just like the commercial preamps. And, if a fixed resistor is used with ea. pot, then we can maintain the useful range even if the pot is turned to -0-.
I then noticed that my schematic uses large caps, 0.1uF, while the Bogen schematic uses small caps, .0022uF. I then remembered that R-C values are scalable. Multiply the R value by any number, and divide the cap value by the same number and you get the same frequency response. This also results in a larger value for the shunt resistor, R3, which should yield less insertion loss.
Bottom line: use Doug's 250K dual-ganged pot instead of my 20K pot. Then scale the other component values. 250k/20k = a factor of 12.5; For the fixed resistors R1 & R2 (to be used in conjunction with the 250K dual-ganged pot): 15K X 12.5 = 187K each as their new value (or something commercially close or on-hand). For R3: 5.6K X 12.5 = 70K. For the 3 caps: .01 / 12.5 = 0.008uF. To double check, plug your values into the online calculator.
EDIT: For the 3 caps: 0.1 / 12.5 = 0.008uF.
OR, use your 1M pot, and do the same math exercise to get the other R & C values.
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Jjasilli,
Incredibly helpful and useful information! THANKS! I'll apply that and redraw and repost a schematic with a layout later today or over the wkend.
I didn't understand how the calculator worked. I appreciate the clarity.
Thank you! With respect, Tubenit
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What if you just paralleled a 25K resistor across each 100K pot? Would that not give you a linear 20K potentiometer for each? Then, you could keep your existing component values.
Jack
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Jack, yes I could do that. However, if I can use smaller caps and a 250kl dual gang pot, I would prefer that.
Going with Jjasilli's latest revision ....................... I'm revising the low freq filter values.
I'll draw the entire schematic and layout later.
I have tried repeatedly to use the calculator, but I don't know how to read the results that are posted.
Would someone PLEASE double check to see if these actual values will work for the 100Hz "problem area". Thanks!
With respect, Tubenit
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Just doing a quick overview so if I missed this....
If this is to be a dual purpose preamp, why not voice two channels with two inputs? That way he can leave everything plugged in at a set volume and tone/drive settings with both guitars on the stand and ready to go. I realize I may have just doubled the work but "jack of all trades, master of none" comes to mind. Not necessarily saying in tone (because we can obviously effectively switch things in and out), but in playability for the musician. Plus there is always a backup channel in case something goes wrong. I'm just thinking out loud about the things I incorporated into my live setups over the years.
Jim
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Both Dummyload & Ritchie make good, though divergent, points. Bottom line, Tubenit's plan will probably work.
Issues: For el guitar, the amp is part of the tone production process. For acoustic guitar the amp or preamp needs to; a) RE-produce acoustic tone, usually not mess with the core tone of the instrument; and b) eliminate the inherent sound engineering problems. These design goals are divergent, even incompatible. This leads to three logical alternatives:
i) per Dummyload, KISS - do only an acoustic preamp. Downside: not great for el guitar.
ii) per Tubenit, build a workable hybrid. Downsides: more complex; probably not best for either type of guitar. But very good for one type and good enough for the other. Since the user will play mostly acoustic this seems a good choice.
iii) per Ritchie: go even more complex and build a 2-channel preamp. Best for both types of guitar, but more complex, larger size, heavier, etc.
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Given that you designed your tone stack to go into a 250K load, you might want to use 220K instead of 68K for your notch filter and adjust the other values to that.
Another thing to consider is that the notch filter turns into a low-pass filter when the relay is in the up position.
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OK, this is the layout and schematic that I am going to go with & will meet the 3 objectives I listed in previous post reply #19.
I plan to build this and report back that it works. Then tweak it. Then record both an acoustic and then electric thru it.
THANKS to everyone that helped! I am genuinely appreciative. Special thanks to Jjasilli for his patience & perseverance in walking me thru the frequency control idea.
It'll probably be built before the end of the month, hopefully. I am anticipating this being a very cool and successful project.
BTW, this project will be built into one of Doug's new blank chassis!
With respect, Tubenit
EDIT: changed the switching on the low frequency control to a DPDT to remove it completely from the circuit when playing electric
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Chassis seems to be missing phase reverse & ground lift SW's.
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Chassis seems to be missing phase reverse & ground lift SW's.
Yeah, I didn't totally revise the original drawing, but they will be included.
Thanks. With respect, Tubenit
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Tubenit -
I anxiously await your finished build, as I would like to do the same for our Church. Our music leader builds and uses his own solid state pedals for his acoustic guitar. He connects it to the Church PA. I recently gave him one each of Dougs B9 and Pitchfork pedals, for his birthday/Christmas, as both dates are close together.
The Sunday before Thanksgiving, I carried one of my Guild ThunderStar heads to Church, along with these pedals. I had asked him previously to bring his Telecaster, but I brought along my '66 Harmony H-19, just in case he didn't bring his electric. After service, and while the womenfolk were preparing our Thanksgiving meal, I rolled the speaker cab (later Ampeg) and Guild head into the sanctuary and he got out his Tele. I had already given him the pedals and some patch cables, so after playing a bit through just the amp alone, he got out the pedals. The Guild has a receptacle on the back, so with Pitchfork connected on the FX loop, he gave it a try. He only briefly tried the B9, though later at home, he and his son gave them both a thorough tryout.
The Pitchfork was his favorite, and has since given me a report on it with his acoustic. He uses it primarily with a piezo pickup that he installed, but also has a clip-on magnetic pickup that he will use from time to time. He likes either with the Pitchfork. I believe that he has something planned for this upcoming Sunday, with the acoustic and Pitchfork.
Which brings me back to your design. I'm anxious to have a tube pre-amp for him to use with his acoustic, in Church. And, I would rather it be perfected by someone like yourself, before I build. Our Church is very small, in numbers. But, the sanctuary is rather high, and the sound carries well with a bit of echo. A couple just recently started attending, and the lady is an absolutely amazing pianist. We have an electronic keyboard that rarely gets used, as our primary pianist uses the baby grand piano. So now, we have a piano and organ, along with Don's acoustic.
Anyway, I'm following this thread closely. Thanks for the fine work.
Jack
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I'm starting on it this wkend and will have it finished by end of Christmas week. Thanks for your interest in this!
I am anticipating success with it and anticipating it needing some tweaking after the initial build is finished.
With respect, Tubenit
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Got the chassis punch out! Really took some time to think thru the layout as there is a lot going into that small chassis. I think this will be a very quiet preamp with an easy to follow layout.
I really like Doug's "Stout" chassis. Very nice quality to it & it was very easy to drill and punch out for preamp tubes and the XLR jack.
On all my chassis, I sand the aluminum with an orbital sander using 120-220 grit paper & then use spray cans of automotive lacquer base coat and then clear coat with about 5-7 coats. I've been doing this for yrs and yrs. They hold up really well! And you can have any color you want using this method. Never had any paint peel off at all or chip off. I just spray the front and back panel.
with respect, Tubenit
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Are you going to use the Low Filter?
How do you spray? Spat can or Spray Gun?
What do use for lettering to label the controls?
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Yes, I am going to use the Low Filter. I am calling it a frequency control. So yes that is included. (see schematic above in previous post)
Just inexpensive spray cans from a local auto parts store. They work just fine, IMO.
For lettering, I use decals or labeling on the lower wood edge below the chassis front panel. ON the back, I put labels on the chassis back panel.
With respect, Tubenit
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Once again the problem is Mixing. This time in the context of connecting your device (preamp) to the house PA, which may be even further connected to yet other devices like mic's, other preamps, etc. Hum may arise from pesky ground loops from numerous signal-ground/chassis-ground connections, which are hard to trace. The best solution is for each device to have its own ground lift switch which can eliminate offending ground loops by trial & error with the simple flicking of a SW.
Once again, guitarists with their standalone amps & speakers are absolved from these issues.
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Jjasilli,
I did install the ground lift switch as well as the phase reversal switch. Just wasn't sure the purpose of the ground switch.
You've been remarkably gracious of your time and info helping me with this build! THANK you! I am greatly appreciative.
I am probably within 3-4 hrs of being ready to fire this up and try it out
With respect, Tubenit
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My pleasure! Hope it works out well. For a long time I've had these preamp issues simmering on the backburner. But I haven't had an actual need. This thread provided me the opportunity to sort some stuff out in more detail.
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Now about an 1.5 hr from firing it up!
I may win an award for crammage into Doug's 12" chassis with this one. Just hoping it doesn't cause hum? However, it is amazing what I have gotten away with to end up with very quiet amps!
:dontknow: :l2: :icon_biggrin:
I will admit the layout is not the neatest, but with all the different colored wiring I am using, it's actually quite easy to trace out. And I think the layout does have some reasonable logic behind it.
I am NOT sure what fuse to use with this thing. It's only got a preamp and only has two 12AY7's. So, I am thinking maybe 500ma fuse?
Does that sound right? Think I need something larger?
with respect, Tubenit
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Heaters @ 6V draw 300ma ea x 2 = 600mA. Plate Current @ 3mA ea section x 4 = 12mA. Total = 612mA.
Maybe closest is .75A = 750mA.
EDIT: Fuse @ 2 X 612mA = 1.212A So use 1A (probably OK at more than 1.5X); or 1.5A . Slo Blo
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Heaters @ 6V draw 300ma ea x 2 = 600mA. Plate Current @ 3mA ea section x 4 = 12mA. Total = 612mA. Maybe closest is .75A = 750mA.
1.5x to 2x the VA rating of the PT.
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Agreed. I corrected my post above.
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Tubenit, I'm using a half amp fuse in my warbler which has 4 small tubes. I think half amp will be fine for your project also.
JJasilli, You don't simply add up the current for all the secondary windings and double that for your fuse size.
For example, I have an 18 watter amp that requires 6.3V @ 4A, 5V @2A, and 350V @ 100mA. Following your suggestion, I would have a total of 6.1A. Now double that and use a 12A fuse. That's a big fuse for this small amp!
The correct method is to sum all the secondary VA requirements. Primary VA = secondary VA in a perfect, zero loss transformer. Then divide the VA sum by 120V to get the primary current needed to fullfill the needs of all the secondaries. Now double this primary current for the fuse size.
So, 6.3V @ 4A is 25.2VA. 5V @ 2A is 10VA. And 350V @ 100mA is 35VA.
Total VA is 70.2VA
70.2/120 = .585A for primary current
2 x .585 = 1.17A fuse requirement. I'd use a more common 1A or 1.5A.
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Great info thanks! (My approach would apply only to fusing secondary windings.)
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Tubenit, I'm using a half amp fuse in my warbler which has 4 small tubes. I think half amp will be fine for your project also.
Steve,
Man, thanks for the info! I am always grateful for your excellent help!
Best regards, Tubenit
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OK, the PreAmp works. I can get sound out of it and overall the tone is decent EXCEPT I have a terrible hum with the PreAmp.
It seems to be between the FX Level pot and the XLR jack.
EDITED this post as further in the thread shows the solution.
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Disconnect the green wire from the IEC power connector. Does that help?
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Disconnect the green wire from the IEC power connector. Does that help?
I will try that tomorrow or Friday as soon as I get a chance. That green wire is the ground wire to the IEC of course.
Do you think the Edcor trannie and signal wires (to the XLR socket) running close to the HV are problematic? Or the Edcor trannie itself being close to the HV is the issue?
Thanks for the suggestion and I will definitely give it a try and report back (hopefully tomorrow)?
The only switch that stops hum is the center off position of the phase reversal. The only pot that stops hum is the FX level. No pots increase or decrease hum even the FX level doesn't increase or decrease (until the FX Level is completely to zero).
With respect, Jeff
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I will try that tomorrow or Friday as soon as I get a chance. That green wire is the ground wire to the IEC of course.
If you have one of those 'ground buster' AC adapters that will do the same thing and only takes a couple seconds to try.
Do you think the Edcor trannie and signal wires (to the XLR socket) running close to the HV are problematic? Or the Edcor trannie itself being close to the HV is the issue?
I do, but I didn't want to say so just yet. That's a low level signal and you put it in the noisiest area of the chassis.
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If
you have one of those 'ground buster' AC adapters that will do the same thing and only takes a couple seconds to try.
Didn't think about that! Duh! Ok, that helps.
Thanks, Jeff
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Man, this is kind of weird! Not ever had a hum like this before.
It hums with that ground lifted from the IEC .
It hums with ALL (both ) of the tubes removed!
With the tubes all removed, the FX level pot turned all the way off/down or the center off of the DPDT phase reverse swith still are the only things that will stop the hum, but it stops the guitar also.
I need to check voltages and will try to do that tomorrow and post if there are any real issues that I see from that.
I am not sure where to go from here?? I think I will reroute the HV wires away from the wires coming from the Edcor trannie to the XLR and then try it again. It's likely I won't have a chance to do that til Friday.
I'm stumped on this one. Never had one hum with no tubes in it. Not sure how that is possible?
With respect, Tubenit
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Maybe this isn't the solution. But, I just resolved a similar problem today, on an old Gibson Thor Bass amp. The new PS canned capacitor that I installed had one leg (20uf) that just wasn't filtering. And, I was getting a hum, even with the tube removed. I just added another leg from this junction, with 1K resistor and 22uf cap, as a new feed to that plate resistor. My hum disappeared.
Check your capacitor at the junction that you have labeled 'C'. Maybe replace it. In my case, it was difficult to replace one of four 20uf caps in a can. I believe yours are individual filters, so maybe a bit easier to swap out.
At least it's a place to start. Hope it's a fix.
Jack
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Your small signal output transformer is mounted directly under your PT with only a thin sheet of aluminum between them. Correction... Aluminum
does not shield is not a good shield for magnetic fields. You will likely have to move it to the other end of the chassis.
XLR jack is in a bad location too.
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Your small signal output transformer is mounted directly under your PT with only a thin sheet of aluminum between them. Aluminum does not shield magnetic fields. You will likely have to move it to the other end of the chassis. XLR jack is in a bad location too
I did not know aluminum didn't shield. And .............. There is NO room inside the chassis on the other end.
Is there high voltage on that Edcor trannie? Maybe I can mount it outside the chassis on the other end? And if needed, put a plastic cover over it for safety?
I think I can move the XLR to the other end, but it will be close to the preamp tube. Will that be OK?
THANK you for the help! Wished I had been smarter and known all this up front. :dontknow:
With respect, Tubenit
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I think I can move the XLR to the other end, but it will be close to the preamp tube. Will that be OK?
I didn't mean literally move to the other end of the chassis. That would put the line level output right next to the instrument level input signal. I would move it to the outside located near V2 but sitting over the board. There are no dangerous voltages on the OT so no need for concern about that.
Leave the XLR where it is for now but be prepared to move it away from the power switch, or move the power switch away from the XLR. It would be worth the effort to remove the power switch from the chassis, tape it up, and let it dangle far away from the XLR to see if that reduces hum.
Based on the fact that it hums with no tubes in, I think the majority of the hum is being magnetically coupled from the PT to the OT. Put some distance between them.
I would study the back panel layout and see if there is an easy way to slip all controls/switches/XLR toward the input end of the chassis such that you have a couple inches space between the power switch and that little red toggle switch.
EDIT... added pic
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I didn't mean literally move to the other end of the chassis. That would put the line level output right next to the instrument level input signal
YIKES! Too late!! :icon_biggrin:
It's on the outside of the chassis & nowhere near the preamp tubes. Only took about an hour to reposition everything.
I'll post a photo later today of the new positioning. Worse case scenario, I'll just use shielded wires up to the XLR jack. I think this will work out just fine. And I can't imagine it would be any problem using shielded wiring.
with respect, Tubenit
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I've made progress and have MUCH less hum then before and the guitar sounds clearer and better.
I don't have a PA system to check this out on .......... so I am using my home computer. Guitar into PreAmp into XLR into a E-MU TRACKER into the computer into MixCraft recording program. I have the MixCraft recording program set so I can hear the guitar thru the cheapo computer speakers.
IF the PreAmp is plugged into the E-MU Tracker there is some hum ................ even when the PreAmp is turned OFF and is unplugged from the wall.
Also with a Shure 57 plugged into the E-MU Tracker there is some slight hum. AND if the computer volume using the mic is at the same computer volume as the guitar .............. the hum is about the same for both.
(Mic volume = PreAmp/guitar volume = same volume of hum)
This is leading me to think that the hum is somehow a compositive of the PreAmp, the E-MU Tracker, the computer recording system etc..
Again, I don't have a PA system handy to try the PreAmp thru.
Any thoughts on this??? Anything else that you'd recommend trying?
It's a challenge for me to think that how I have the Edcor trannie and the wiring to the XLR is still a problem?
With respect, Tubenit -
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Do you have another guitar amp with a power amp input jack? If so, connect the output of your new preamp to that. You may need an XLR to 1/4" phone adapter.
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Do you have another guitar amp with a power amp input jack? If so, connect the output of your new preamp to that. You may need an XLR to 1/4" phone adapter.
Both the D'Mars and the Tweed BluezMeister have active FX, so I could get an XLR to 1/4" adapter and plug into that. However I am plugging into the return triode of the FX which goes into the LTPI ......... and so on.
Does that sound like a reasonable way to try this out?
I already have a low impedance XLR to high impedence adapter like this: http://www.zzounds.com/item--AUTCP8201 (http://www.zzounds.com/item--AUTCP8201)
250-50k ohms
Question 1:
However, I am not sure that's the kind of adapter that you are talking about??? I am guessing your wanting me to try an adapter that is simply "passive" and does not change impedence? Correct?
Question 2:
I also have a second Edcor trannie that I can try IF you think it's worth the experiment. The one on there currently is 10k/600.
The one I have NOT tried is 10k/150. Think it's worth the effort to try the 10k/150?
THANKS so much for the help, I feel like I am making some progress with the project in improving the sound and also narrowing the probable areas of concern.
With respect, Tubenit
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I just noticed something. On DummyLoad's (Pete's) PreAmp , he has the jack pin 3 as being the ground pin.
In contrast, I have jack pin 1 on the XLR being the ground pin. Have I got the wrong pin grounded????
EDIT & never mind .............. I found this and it looks like pin 1 is correct for what I am doing.
http://www.raf-net.com/gallery/tools/Wiring%20diagrams.htm (http://www.raf-net.com/gallery/tools/Wiring%20diagrams.htm)
With respect, Tubenit
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Here's the adapter cable I had in mind...
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Here's the adapter cable I had in mind...
OK, understood will try to do this tomorrow and report back.
I'm thinking IF I still have hum doing this, that I might try the other trannie that I have?
With respect, Tubenit
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I just noticed something. On DummyLoad's (Pete's) PreAmp , he has the jack pin 3 as being the ground pin.
In contrast, I have jack pin 1 on the XLR being the ground pin. Have I got the wrong pin grounded????
With respect, Tubenit
no you don't! I DO! thanks for catching that. i transposed those numbers off of the header. the pin direction was reversed.
fixed and attached the updated schematic.
--pete
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is your circuit ground the same as chassis ground, or is the circuit ground lifted off the chassis?
Is your ground lift switch solely on pin 1 of the XLR? or is it some other kind of switch?
where is the center tap on the heater filaments grounded?
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is your circuit ground the same as chassis ground, or is the circuit ground lifted off the chassis?
This is on my list to explore. I typically use Hoffman's grounding scheme because it is so successful for me to have a quiet amp. However, this isn't a normal amp with the Edcor trannie and a XLR jack. Currently the Edcor trannie has it's own ground point on a bolt holding the trannie down. And currently, the XLR jack has it's own ground on a bolt holding the XLR jack down also. I may clip the grounds to the preamp buss bar and see what happens?
Is your ground lift switch solely on pin 1 of the XLR? or is it some other kind of switch?
Yes, pin 1 to a spdt to ground.
where is the center tap on the heater filaments grounded?
The PT came with a heater center tap ground wire. I currently am using that instead of the 100ohm resistors to ground. The center tap ground wire is grounded on a bolt of the PT. I may experiment with 100 ohm resistors?
Thanks for the comments and ideas. First thing I want to do is rig up Sluckey's suggestion and run the PreAmp into the PA of the D'Mars and see if there is still hum? Then I have a list of ideas written out of other things to try after that.
With respect, Tubenit
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Jeff, did the hum problem improve when you moved the OT and XLR jack?
How does the hum level compare to the hum level produced by plugging your D'Mars preamp (FX send?) into your E-MU TRACKER?
Also with a Shure 57 plugged into the E-MU Tracker there is some slight hum. AND if the computer volume using the mic is at the same computer volume as the guitar .............. the hum is about the same for both.
This statement kinda makes me think the hum issue inside your acoustic preamp is resolved. Just for reference, connect your AC voltmeter across the XLR jack and measure the AC hum voltage with no signal applied to the preamp input.
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Yes, moving the Edcor trannie and the XLR did lessen the hum. It was a good suggestion and I am grateful that you nudged me that direction. Good choice.
This statement kinda makes me think the hum issue inside your acoustic preamp is resolved
Yep, and HOORAY !!!! I took your suggestion and ran the XLR plug into an adaptor into the return of the active FX loop of the Tweed BluezMeister.
SUPER quiet !!!!!!!!!!! In fact, the acoustic PreAmp is slightly quieter then the Tweed BluezMeister clean channel.
So in light of this experiment, I am dubbing the issue resolved and believe that it will be very quiet for my son-in-law with his acoustic guitar running into his PA system. Think that is a reasonable assumption ?? (He is in St.Louis, so I won't be there when he tries it)
The ONLY noise now that I hear is the typical very slight white noise that is controlled by gain and volume knobs. The "other hum" that was annoying is simply NOT there at all!
THANKS & THANKS for the help on this! Steve, you always steer me right, my friend. I am truly grateful for the help.
I am very happy with the tone of this PreAmp. It's got a sweet musical tone to it.
The only thing I am not sure about is the Frequency Control?? I can't really tell for sure by listening IF it's really doing anything or not?
Should I be able to hear a shift in tone? I'm thinking that would not be the case really.
With respect, Tubenit
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The only thing I am not sure about is the Frequency Control?? I can't really tell for sure by listening IF it's really doing anything or not?
acoustic guitar with pickup > preamp > power amp > speaker
Turn up vol & face guitar so you get feedback on the Open A string (or G note E-string, 3rd fret). Adjust the filter & see if the feedback goes away. That is the purpose of the filter.
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Do you have some pictures of the finished board/chassis configuration?
Also, is this configuration the same as the last schematic and layout?
Great job.
Jack
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Turn up vol & face guitar so you get feedback on the Open A string (or G note E-string, 3rd fret). Adjust the filter & see if the feedback goes away. That is the purpose of the filter.
Yes, I would say it is working but subtle. Dialed clockwise, it seem to feedback more easily. Dialed counter clockwise, less feedback.
Bypass the filter (which I'm calling a Frequency control), and it definitely feeds back ALOT more easily! So the filter/freq control does work.
You can hear the difference VERY easily with the filter on vs. bypass. Big difference in tone.
I would say the Larrivee sounds very natural and transparent with the filter engaged. And the treble, bass, & mid all help the shaping of the tone. I would rate the acoustic tone in the A- range.
Switching to the electric, I'd give the tone closer to a B. All that is needed to play electric thru it is to adjust the FX Level, bypass the filter and bump the mid-boost and you've got a great electric tone.
Yes, it is the same drawn schematic and layout as previously posted in reply #37.
The most recent posted pictures (reply #59) are it's current state showing where I moved things from the original build. So, no updates to schematic, layout and photo. It's all current in this thread.
I am VERY happy with how this turned out. It excels as an acoustic PreAmp and is above average as an electric guitar tone.
With respect, Tubenit
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Yes, it is the same drawn schematic and layout as previously posted in reply #37.
The most recent posted pictures (reply #59) are it's current state showing where I moved things from the original build. So, no updates to schematic, layout and photo. It's all current in this thread.
Thank you very much for the hard work. I have them saved for future reference. Have a good one.
Jack
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Yes, I would say it is working but subtle. Dialed clockwise, it seem to feedback more easily. Dialed counter clockwise, less feedback.Bypass the filter (which I'm calling a Frequency control), and it definitely feeds back ALOT more easily! So the filter/freq control does work.
Great to know that the low pass filter actually works as intended. Thanks for building and testing the circuit.
Congratulations on your preamp build! Hope your son-in-law enjoys it.
Happy New Year!
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Great to know that the low pass filter actually works as intended. Thanks for building and testing the circuit.
Hey man, I appreciate the encouragement and direction that you offered! Thank you!
With respect, Tubenit
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The Low Pass filter (that I am calling a frequency control) doesn't make much of a audible difference. Very very subtle. I am wondering if that is because it is not changing or sweeping much in the way of frequency shifting?? I think it's working but I don't know how well it's working?
I am thinking about trying the Bogen low pass filter and comparing the two. My main question is whether the Bogen's 1MA pot will allow a more audible and maybe useful frequency shift control? I am wondering if the Bogen's would be more efficient?
Thoughts???
With respect, Tubenit
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I don't know which circuit performs better. But I wanted to say that having a signal generator would be an invaluable tool for evaluating these filters.
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Bogen filter with multisim - sorry haven't figured out how to sweep a function (pot wiper) during a simulation with multi-sim yet.
--pete
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TN Twin-T
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schematic of what was simulated.
--pete
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tubenit with 3 minor mods to your notch filter, the upper range is expanded to something useful. simulations below at min, mid and max wiper sweep. with a minor mod the bogen doesn't shave off all the low freq. see attached schematics.
tubenit the mods to your filter are with R5, R6, & R8
--pete
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schematic referenced in reply above... (#80)
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Pete,
I am truly grateful for the help! Thank you! Unfortunately, I have no background in electronics & I don't know what I am looking at with the charts?? And while you're describing the different filters ............. I am not understanding if you're saying that one is better then another? I am truly not trying to be thick, I simply have no background in this.
I can translate a schematic into an amp.
Is what you are suggesting is that I use the current "Tubenit" low pass filter and simply change R5 from 68k to 47k. And then change R6 from 180k to 100k and R8 from 180k to 100k?
(Or are you saying to use the D.L. version or the Bogen version? ) PLEASE be patient with me, I'm lost in this stuff. I am trying to understand what you are saying.
Which one do you think would be best? And would the modified Tubenit version be better then the Bogen version?
THANKS and with respect, Tubenit
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yes. mod what you have built. the DL filter is probably what i would build. mod yours... 68k to 47k and oth 180k become 100k.
--pete
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Pete,
Thank you for the clarity and the help! Please accept my apology for not initially understanding the info.
Best regards and respect, Jeff
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With the wiper clear down on the Bogen version it is a low-cut filter and with the wiper clear up it is a notch filter. With the pots at zero ohms on your filter it is a notch filter and with the pots at 250K ohms it is still a notch filter, but the notch is more severe and the notch frequency is lower than at zero ohms. With D.L.'s pots at zero ohms it is a notch filter and with his pots at 1M ohms it is a low-cut filter for all practical purposes.
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With D.L.'s pots at zero ohms it is a notch filter and with his pots at 1M ohms it is a low-cut filter for all practical purposes.
take bode initial down to 1Hz. still looks like a texbook notch to me. since the notch is is at @57Hz, most of the LP function is subsonic, if that's what you're eluding to.
--pete
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take bode initial down to 1Hz. still looks like a texbook notch to me. since the notch is is at @57Hz, most of the LP function is subsonic, if that's what you're eluding to.
I am fully aware that your filter is a notch filter. What I said was that for practical purposes it is a low-cut filter as opposed to me saying that the Bogen is a low-cut filter (as drawn in their schematic with the 1M pot tied to ground). I meant that your filter is a low-cut filter for the portion of the frequency response that we are going to use it for because the notch frequency is lower than the lowest frequency produced by a guitar with the low E string tuned to 82.4 Hz.
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Tubenit--
I have attached some graphs for your filter. As it turns out, with the pots at 250K ohms the notch frequency is below the guitar range so that I would call it a low-cut filter. Looks to me as though it should work fine.
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Tubenit--
I have attached some graphs for your filter. As it turns out, with the pots at 250K ohms the notch frequency is below the guitar range so that I would call it a low-cut filter. Looks to me as though it should work fine.
so what was wrong with the bode plots i posted?
--pete
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Updated layout and schematic showing Pete's revision to the "frequency control". Pete, thanks again for your invaluable help!
With respect, Tubenit
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Updated layout and schematic showing Pete's revision to the "frequency control". Pete, thanks again for your invaluable help!
With respect, Tubenit
you're welcome. have you modified and tested yet? just curious.
--pete
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so what was wrong with the bode plots i posted?
Maybe nothing for other people, but I couldn't ascertain with complete confidence which schematic goes with which screenshot in the Bogen-TN-DL series. Maybe the upper left schem corresponds to the top screenshots and then, going clockwise, the next schem corresponds to the middle screenshots and the last one corresponds to the lowest screenshots. But maybe, since the screenshots are stacked, you go from upper left to the schem below it and then to the right one. The only thing I was sure of was that the upper left schem corresponds to the top screenshot because of the frequency response. The frequencies are too close for me to differentiate between the remaining two by the seat of my pants, so I ran the numbers.
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have you modified and tested yet?
Yes, I have modified it. Thanks! Haven't tested it yet ........... off on other projects. Will give it a test probably New Year's Day when my schedule is more freed up.
With respect, Tubenit
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The Low Pass filter (that I am calling a frequency control) doesn't make much of a audible difference. Very very subtle. I am wondering if that is because it is not changing or sweeping much in the way of frequency shifting?? ... I am thinking about trying the Bogen low pass filter and comparing the two. My main question is whether the Bogen's 1MA pot will allow a more audible and maybe useful frequency shift control? ...
I'm late to the party.
Some Background:
The Twin-T Notch Filter (http://www.radio-electronics.com/info/circuits/rc_notch_filter/twin_t_notch_filter.php) theoretically can give infinite attenuation at a single frequency. Actual attenuation in practice will be less, and the steepness of the sides of the notch depend heavily on tight tolerances among all resistors and caps used. If a high-Q (very narrow notch) is needed, you'd really have to add opamps and feedback (http://www.docircuits.com/learn/tag/twin-t-notch-filter/), though you'd also end up with a variable-Q afterwards. Bottom-line, for significant notching, the ratios of all parts need to be according to the drawing in the first link.
The Bogen probably intentionally uses non-correct cap values to lessen the notch and widen the cut (i.e., lower-Q), and then adds the 1MΩ pot on top of it to help you remove more of the notching until you have just-enough. Let's assume the upper-T in your post (http://el34world.com/Forum/index.php?topic=17920.msg182705#msg182705) represents the "correct values." Then according to the 1st Link's drawing, R=440kΩ and C=0.0022uF. Plugging those values into the equation at the bottom of the 1st Link gives a cut-off frequency of ~164Hz.
I don't know if that's the frequency you are targeting. If it is, I'd copy the Bogen circuit exactly, possibly with a change of the cap to ground to 2 paralleled 0.0022uF caps (for 0.0044uF, matching the theoretical Twin-T filter more exactly). As I see it, Bogen adding the 1MΩ pot just reduces the effectiveness of the filter 2-ways when dialed below full-up: it drives the lower-T less, and "unbalances" the resistances in the circuit, which will reduce the notching.
I don't see a way to adjust the frequency unless you can simultaneously change all 3 resistances in the same proportion, at the same time.
As an aside, I don't kow where this filter sits in your circuit; the resistor to ground is definitely a known-load on the previous stage. So 68kΩ to ground seems low. If the previous stage is a tube plate, something closer to 220k-500kΩ seems like it would cut stage gain less.
I took a swag at a 100Hz notch twin-T using an online calculator. I used C=0.0022uF (so the cap to ground is 2 of these paralleled at 0.0044uF total), and the program spit out 390kΩ to ground, with the 2 series resistors being 750kΩ each.
Since this is not easily variable (and a decent soundboard has much better EQ, with the ability to notch out a specific & variable frequency), maybe a solution is to figure out what frequencies his guitar tends to feedback at in the place he will play, and zero-in on that frequency. The bad news is if the venue changes, the feedback frequency will probably change a bit, and if you play loudly enough, any/every frequency will eventually feed back.
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Hotblue, you make good points. I too advocated the Bogen circuit as-is. But tubenit already built a circuit.
The purpose of the circuit is to kill feedback. (It may have a side benefit of rolling off the lows of an acoustic guitar). In theory we already know the worst offending frequency: about 100Hz. However this may vary per instrument and venue.
The online calculator I posted above shows you can get useful variation in the frequency to be attenuated, by changing only the values of the series resistors R1 & R2 in the twin-t notch filter. So, R1 & R2 can be replaced by a dual gang pot. Changing only their value is not perfect, but seems to work well-enough, "proven" by plugging various likely values into the online calculator.
Meanwhile DummyLoad has recently suggested other component values based on his sim of the circuit. 2deaf also has suggestions.
BUT, still unclear to me: is the circuit as tubenit built it adequately killing feedback??? I don't see a clear answer.
I think this sums up the thread.
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a solution is to figure out what frequencies his guitar tends to feedback at in the place he will play, and zero-in on that frequency
I sure wished it were that easy! :icon_biggrin:
It is a 13 hr drive one way to St. Louis ............ so that experiment is probably never going to happen. :dontknow: :icon_biggrin:
And then even if I got there, how in the world would I know what frequency it's feeding back at?
I'm pretty much left to simply wiring up a best guess (based on somewhat conflicting advice), shipping the PreAmp and ask how it's worked out? Unfortunately, that's kind of where things stand.
My understanding is that Pete's suggested mods to the one I did would target the 100Hz reportedly "problem area"? Right?
IF you guys feel strongly that the Bogen is a better choice, I still have the amp here and can rewire it? It would not be that hard to rewire.
With respect, Tubenit
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My suggestion: Use any acoustic guitar. If feedback is killed assume the preamp is good to go.
If your son-in-law has a feedback issue with his set-up, we'll work with you & him to identify his personal problem frequency (if any). Then you & he can ship the preamp back and forth to tailor the filter if need be.
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My understanding is that Pete's suggested mods to the one I did would target the 100Hz reportedly "problem area"? Right?
your filter will sweep between 80Hz and 180Hz, so yes 100Hz problem are is in that range.
if you use a 500K pot then you could sweep from 55Hz to 180Hz. don't know if that's a really needed, but a simple part change and you could filter some mains (60Hz) hum.
--pete
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The online calculator I posted above shows you can get useful variation in the frequency to be attenuated, by changing only the values of the series resistors R1 & R2 in the twin-t notch filter. So, R1 & R2 can be replaced by a dual gang pot. Changing only their value is not perfect, but seems to work well-enough, "proven" by plugging various likely values into the online calculator.
The thing to realize is there are 2 filter circuits; one is high-pass the other is low-pass. When all circuit values line up in the ratios shown in the 1st link I posted, the frequencies of each T-section align to produce a sharp null. As you jigger any of the parts values, the center-frequency of the cut for each section becomes not-same. Along with changing the frequency, the not-same-ness reduces the amount of notching.
This is why I think Tubenit is not hearing much change when using the control. I also suspect the big boost when it is bypassed has something to do with the 68kΩ load to ground.
DL: If you have it handy, send Tubenit the Contour control (I'm on a new computer and haven't transferred it yet). That is a bridged-T filter (rather than a twin-T), which has a midrange scoop, variable over a pretty wide frequency range. We might be able to tighten the Q and adjust the sweep range to usable frequencies. And unlike the critical Twin-T, the Bridged-T is quite easy to shift with a dual-pot (which is how I did it, plus a 2nd pot to control the amount of notching).
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DL: If you have it handy, send Tubenit the Contour control ... (I'm on a new computer and haven't transferred it yet). That is a bridged-T filter (rather than a twin-T), which has a midrange scoop, variable over a pretty wide frequency range. We might be able to tighten the Q and adjust the sweep range to usable frequencies. And unlike the critical Twin-T, the Bridged-T is quite easy to shift with a dual-pot (which is how I did it, plus a 2nd pot to control the amount of notching).
i do & will do. running a sim on it now.
--pete
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on a footnote: i reviewed the entire schematic just a few moments past, and i have a concern - the two filter subsystems are in series. my thoughts are that each should be separated by an active element for isolation: either a CF/PF or another GCA.
thoughts?
--pete
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Would you guys prefer the low pass filter be between V1-A and V1-B? I can easily wire it that way.
It should be noted that in the Bogen schematic, it looks to me like a James tone stack goes directly into the low pass filter?
http://el34world.com/charts/Schematics/files/bogen/bogen_mxm-a.pdf (http://el34world.com/charts/Schematics/files/bogen/bogen_mxm-a.pdf)
HBP, are you suggesting that I copy the schematic that D.L. has given me (using the same values) and build that into the amp between V1-a and V1-b?
Adding another pot on the front is going to be a challenge. This is a tiny chassis and just about all the space is spoken for. Not saying it can't be done, but I'm going to have to carefully think thru this.
OK, using the tagboard .......... I figured out how to do the layout. The trick is going to be to find a place on the front for another pot? I might be able to use a mini-pot for the "mid-range scoop" that you referred to.
With respect, Tubenit
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It should be noted that in the Bogen schematic, it looks to me like a James tone stack goes directly into the low pass filter?
it does. so in actuality if you copied the bogen plan it should work.
--pete
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After the additional information & thoughts from HotBluePlates, .................... would you build:
1) HBP's contour control (same values or changed?) & ( between V1a & V1b .......... or after TBM tone stack?)
2) stay with the modified Tubenit low fiter that is currently wired up
3) go with Bogen low frequency control
I think I can make any of them work in the chassis? However, after the recent discussion ......... I am not clear what the current thinking is on which to use?
Is there a 1st, 2nd or 3rd choice in order of preference?
With respect, Tubenit
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It should be noted that in the Bogen schematic, it looks to me like a James tone stack goes directly into the low pass filter?
it does. so in actuality if you copied the bogen plan it should work.
--pete
I am not clear what the current thinking is on which to use?
With respect, Tubenit
I continue have a simple question that calls for a Yes or No answer. Does the current build kill feedback of an acoustic guitar? Please choose YES or NO.
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I continue have a simple question that calls for a Yes or No answer. Does the current build kill feedback of an acoustic guitar? Please choose YES or NO.
Fair question. I plan to find out, but my schedule is so busy that I may not have a chance to really experiment with that until New Years day, unfortunately.
And the other piece of it is when HBP was talking about trying it out with my son-in-law's guitar, in his setting/venue and his solid state PA system ................. that was throwing me off that my experiment with my guitar into the power amp of one of my tube amps in my home may not be a good indicator or whether it works.
Having said that, it's a reasonable starting place and I will do that and give a response back. I can say with some confidence, that the original (prior to the D.L. mods to the design) did seem to lessen feedback counter clockwise.
With respect, Tubenit
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And the other piece of it is when HBP was talking about trying it out with my son-in-law's guitar, in his setting/venue and his solid state PA system ................. that was throwing me off that my experiment with my guitar into the power amp of one of my tube amps in my home may not be a good indicator or whether it works.
Since you can't easily verify in your son-in-law's setting, it's probably close-enough for now to simply have your acoustic in front of the speaker of a powerful amp.
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I am going to try it out and see what happens here at home. IF I can not tell that the Frequency Control is obviously limiting feedback, then I am going to build the Bogen version with Bogen values and call it a day.
THANKS to everyone for the help!
Best regards, Tubenit
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OK, I hope to try out the mods to the low filter pass later today or tomorrow & will report back.
Reading up on this more to try and increase my understanding, .............. it seems like I would want two features:
low pass filter to remove bass
notch filter to shift & focus more specific areas where the bass is removed
As I am understanding it, the Bogen design with the 1MA pot shifts and focuses where the bass is removed??
So, using this calculator that Jjasilli directed me to ............ IF I am understanding this correctly, using a 100k pot with a 10k resistor, I would be able to shift and focus a specific area where the bass is removed? (or use a center off spdt for 3 resistance values)
http://sim.okawa-denshi.jp/en/TwinTCRkeisan.htm (http://sim.okawa-denshi.jp/en/TwinTCRkeisan.htm)
Am I understanding this properly and do you think it would be useful to add this 100ka pot or spdt and paralleled resistors?
With respect, Tubenit
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Yes, the low pass filter works!
After the suggested mods by Pete (DummyLoad), I can now hear an audible change in tone as well as hear that it is indeed lessening feedback.
IF the low pass filter is disengaged, ............. the acoustic guitar will feedback and howl more easily at same volume.
IF the low pass filter is turned counterclockwise, it lessens feedback and/or totally eliminates it.
So, I view this as a success!
I now am going to add a "notch" filter center-off SPDT for values of 82k, 45k and 22k. It currently has 47k to ground which is working.
Will retest that and then repost.
Switching the low pass filter on requires adjusting the FX level and bass and treble pots, but that's really easy to figure out. More FX level, more bass and less treble.
With respect, Tubenit
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This is how I am thinking about the low filter pass VS. the notch filter:
Let’s say a piece of sandwich bread represents the spectrum of frequency and tone. Somewhere on that piece of bread there is a spot about the size of a quarter that is the “problem area” of feedback.
Well, the notch filter is like having an adult take a big bite out of the bread or having a small child take a bite out of the bread. It controls how big of a bass frequency (or bite) you are removing. A larger bite obviously has a higher chance of removing the problem area.
The low pass filter controls more of where on that piece of bread either the adult or the small child is taking that bite out. Remember there is a spot about the size of a quarter that is the problem and the rest of the bread/frequency is just fine for the guitar.
The goal is to remove that quarter size area and leave as much of the good bread/frequency as you can intact.
Is that a reasonable beginning useful way to think about those two functions?
With respect, Tubenit
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The notch filter switch works! Amp is done! Success!!
THANKS guys for all the great help!
With respect, Tubenit
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:thumbsup:
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Looks great!!!!!!
Brad :icon_biggrin:
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Put the Acoustic PreAmp into a cab head. Sapele, flame maple trim and black walnut from panel.
with respect, Tubenit
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Very nice!
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Wow....that is just incredible! :worthy1: :worthy1: :worthy1: :worthy1: :worthy1:
Sound clips? Give it a workout!
Jim
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:thumbsup: :thumbsup:
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A little inside "family humor" here ............... my champ of a son, Adam, is an electrical engineer for Texas Instruments that does research and design. I cc'd him a picture of the PreAmp that I sent to the PreAmp owner (son-in-law).
His response:
Looks Super Fantastic!! Well Done The Tupperware bowl is a lil out of place. Extremely functional but different.
My response back:
What tupperware bowl?
That’s a translucent subsonic multi-thermal polysynthetic electrical isolator strategic construction containment entity.
At least that’s what the engineer I bought it from said it was. :icon_biggrin: :thumbsup:
With respect, Tubenit
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:l2:
It looks great T!!!!!!
Both the preamp chassis and it's cab, top notch!
(Did you put a couple of holes in the top of the translucent subsonic multi-thermal polysynthetic electrical isolator strategic construction containment entity for a little air flow?)
Brad :icon_biggrin:
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Thanks Brad!
There is already a "hole" in the sense of the grommet from the inside of the chassis to the outside of the chassis. And the
translucent subsonic multi-thermal polysynthetic electrical isolator strategic construction containment entity is not totally flush
with the chassis surface. I think you could slide a dime under the space. So, I think there is some breathing room?
With respect, Tubenit
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Yeah, sounds good, I should've known you were on top of it. :wink:
Brad :icon_biggrin:
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Sound clips? Give it a workout!
OK, so you guys can know ALL your great help really did pay off for me!! THANKS to everyone who contributed with special thanks to Dummyload (Pete), Jjasilli (who encouraged me to try the frequency contour and notch filter) ........... and my friend, Sluckey who still holds the trouble shooting record in setting me straight on all the builds I messed up on.
Listen to the tone of the PreAmp and not the playing, please. Fingerpicking version.
http://www.soundclick.com/player/single_player.cfm?songid=13020598&q=hi&newref=1 (http://www.soundclick.com/player/single_player.cfm?songid=13020598&q=hi&newref=1)
THANKS guys! I truly appreciate the help and encouragement. I feel like this has been a successful experiment and build.
With respect, Jeff
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A little inside "family humor" here ............... my champ of a son, Adam, is an electrical engineer for Texas Instruments that does research and design. I cc'd him a picture of the PreAmp that I sent to the PreAmp owner (son-in-law).
His response:
Looks Super Fantastic!! Well Done The Tupperware bowl is a lil out of place. Extremely functional but different.
My response back:
What tupperware bowl?
That’s a translucent subsonic multi-thermal polysynthetic electrical isolator strategic construction containment entity.
At least that’s what the engineer I bought it from said it was. :icon_biggrin: :thumbsup:
With respect, Tubenit
Should keep the tone Fresh! :angel
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NICE. :icon_biggrin:
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Before this heads off to the land of completed threads, I'd just like to add my 2 cents....
GREAT thread!
GREAT collaboration!
GREAT build!
The clips sound excellent T, and the head looks gorgeous!
Nice work Kids!!
:wav:
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It really was a tremendous collaborative effort!
One of the things I value most about the forum. Lot's of contributions from people waaayyyyy more knowledgeable then I am helped make this a success. It was a group build with me just doing the soldering.
With respect, Tubenit
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Impressive work gents! I remember playing in a country and rock band in the 70's and my main instrument was a Martin D18 with a Barcus Berry passive pickup in it. Probably every single night playing out I would be frustrated trying to get more volume out of my acoustic as the crowds got bigger and the booze made them louder. drummers and guitar players don't care on set 3 and 4 if you can be heard or not so I have vivid memories of the ghost of acoustic guitar feedback. It can be a very elusive and annoying problem. But you fellas really did the hard work on this endeavor. I would be interested to hear a report from the son-in-law about the preamp's ability to 'capture' the feedback gobblin?
Cool work amigos. :icon_biggrin:
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Update!
OK, the Edcor trannie did NOT work with my son-in-law's PA system! There was some hum.
I got him to bypass the Edcor trannie and run a 1/4" shielded cable from the FX send to a direct box to the PA as a trouble shooting idea. This worked great. (note that it bypassed the FX return triode gain stage as well as the Edcor trannie).
He said it sounded absolutely fantastic and was totally DEAD quiet !!! He said it was the best acoustic tone that he has ever gotten, that he got numerous compliments about it. He stated it was the best acoustic PreAmp that he has tried or even heard. My son in law is very very picky about acoustic guitar tone and typically has a particular sound in mind. So the fact that he liked this so well and commented it was quiet, left me feeling very pleased about the build.
So the current plan now is simply to remove the Edcor trannie and install a speaker jack which will go to a direct box to the PA system. The direct box will be permanently velcroed to the inside top of the PreAmp cab.
http://www.musiciansfriend.com/pro-audio/live-wire-pdi-double-shielded-heavy-duty-passive-direct-box (http://www.musiciansfriend.com/pro-audio/live-wire-pdi-double-shielded-heavy-duty-passive-direct-box)
(I did something kind of like this using a Huges & Kettner Redbox on an amp head several yrs ago for a D'Mars head to be used in a "zero volume" stage: http://s28.photobucket.com/user/tubenit/media/DMars6K6backpanel_zps2ec00611.jpg.html?sort=3&o=21 (http://s28.photobucket.com/user/tubenit/media/DMars6K6backpanel_zps2ec00611.jpg.html?sort=3&o=21))
I am not sure why the Edcor trannie did not work out with that PA system? However, if you remember, initially it had some hum for me that seemed to have been resolved.
That Edcor trannie was used on a project about 2 yrs ago and so it was a "used" trannie for this project, I am wondering if pulling wires thru the rubber grommets .......... OR heating the wires on to the trannie tabs or something else harmed the trannie in some fashion ........... thereby causing the hum?
My understanding is that the Edcor trannie with the phase reversal switch and ground lift etc...... was essentially an "onboard direct box"? So the fact that an actually direct box works and this didn't leaves me wondering if the used trannie was "bad"?
On the other hand, maybe the Altec-Lansing trannie that Dummyload mentioned early in the thread would resolve this or one of the Jensen type trannies?
Anyhow, I wanted everyone to know the "rest of the story". This still allows for all the potentiometers, the FX loop and the switches to work. So not much of a loss changing directions on this.
With respect, Tubenit
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Well then, it's still a success! In this scenario hum is always a possible issue, even with the "best" equipment made by established manufacturers with R & D teams, etc. A "workaround" may be needed at any time, so it's good one was found readily here.