Acoustic Guitar tube PreAmp & FX

[sluckey]

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.

[tubenit]

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

[tubenit]

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

[Jack_Hester]

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

[sluckey]

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.

[tubenit]

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.   

With respect, Tubenit

[sluckey]

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

[tubenit]

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!!   

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

[tubenit]

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 -

[sluckey]

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.

[tubenit]

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

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

[tubenit]

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

With respect, Tubenit

[sluckey]

Here's the adapter cable I had in mind...

[tubenit]

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

[DummyLoad]

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

[terminalgs]

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?

[tubenit]

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

[sluckey]

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.

[tubenit]

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

[jjasilli]

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.

[Jack_Hester]

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

[tubenit]

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

[Jack_Hester]

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

[jjasilli]

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!

[tubenit]

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

[tubenit]

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

[sluckey]

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.

[DummyLoad]

Bogen filter with multisim - sorry haven't figured out how to sweep a function (pot wiper) during a simulation with multi-sim yet.


--pete

[DummyLoad]

TN Twin-T

[DummyLoad]

schematic of what was simulated.


--pete

[DummyLoad]

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



 

[DummyLoad]

schematic referenced in reply above... (#80)

[tubenit]

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

[DummyLoad]

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

[tubenit]

Pete,


Thank you for the clarity and the help!  Please accept my apology for not initially understanding the info.


Best regards and respect, Jeff

[2deaf]

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.

[DummyLoad]

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

[2deaf]

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.

[2deaf]

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.

[DummyLoad]

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

[tubenit]

Updated layout and schematic showing Pete's revision to the "frequency control".   Pete, thanks again for your invaluable help!

With respect, Tubenit

[DummyLoad]

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

[2deaf]

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.

[tubenit]

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

[HotBluePlates]

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 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, 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 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.

[jjasilli]

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.

[tubenit]

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! 

It is a 13 hr drive one way to St. Louis ............ so that experiment is probably never going to happen. 
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

[jjasilli]

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.

[DummyLoad]

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

[HotBluePlates]

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).