Acoustic Guitar tube PreAmp & FX

[DummyLoad]

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

[DummyLoad]

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

[tubenit]

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


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

[DummyLoad]

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

[tubenit]

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

[jjasilli]

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.

[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.

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

[HotBluePlates]

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.

[tubenit]

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

[tubenit]

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

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

[tubenit]

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

[tubenit]

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

[tubenit]

The notch filter switch works!   Amp is done!  Success!!

THANKS guys for all the great help! 

With respect, Tubenit

[jjasilli]

 

[Willabe]

Looks great!!!!!!



              Brad   

[tubenit]

Put the Acoustic PreAmp into a cab head.   Sapele, flame maple trim and black walnut from panel.

with respect, Tubenit

[sluckey]

Very nice!

[Ritchie200]

Wow....that is just incredible!


Sound clips?  Give it a workout!


Jim

[jjasilli]

 

[tubenit]

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.   

With respect, Tubenit

[Willabe]

 

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   

[tubenit]

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

[Willabe]

Yeah, sounds good, I should've known you were on top of it.   


              Brad   

[tubenit]

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

THANKS guys!  I truly appreciate the help and encouragement.  I feel like this has been a successful experiment and build.

With respect,  Jeff 

[jjasilli]

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.   

With respect, Tubenit

Should keep the tone Fresh!   

[TIMBO]

NICE. 

[SILVERGUN]

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!!
 

[tubenit]

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

[smackoj]

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.   

[tubenit]

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

(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)

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

[jjasilli]

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.