Acoustic bass amp design... how'd i do?

a fine line between stupid and clever

Acoustic bass amp design... how'd i do?

Postby ioginy » Sun Apr 04, 2010 4:00 pm

Hey all. I have been working on designing an acoustic bass amp that will allow use with an active piezo as well as a passive. This is what I have cone up with. Sorry for the split picture, my scanner is only so big :(

The output tubes are 6L6's, the rectifiers are 5U4's, the pre's are 12ax7's and the phase inverter is a 12at7.

If anyone has any thoughts on something i have done wrong i would greatly appreciate your input.

Thanks a heap.
Cory

ImageImage
User avatar
ioginy
KT88
 
Posts: 272
Joined: Sun Oct 18, 2009 5:38 pm
Location: Edmonton, ab

Postby dhuebert » Mon Apr 12, 2010 7:22 am

I'm sorry no one felt the need to reply.

Looking at it quickly I can make a few comments. It looks vey complex, I'm a simple guy so... hmm not so good for me. With a bass amp you probably want to reproduce the signal from the pickups fairly faithfully so the Fender style tone stack might not be your best choice, lots of phase distortion there.

The whole pressure piezo section I don't understand so I have no comment to make on that.

I like ultralinear for bass amps.

For a bass amp I like the power supply to be as stiff as possible. There can be some very high energy fast attack signals from a bass and a saggy power supply will turn them to mush. Therefore no tube rectifiers. Solid state amps sound really hard to me but sand rectifiers and tube outputs are just right to my ear.

A 10 meg resistor on the grid of the first stage does the whole job: piezo, electric, what have you and it's simple which I like.

These are my comments FWIW, I don't really understand what you are trying to achieve with this design so I might be miles off base here.

Good luck with it and keep us posted.

Don
User avatar
dhuebert
KT88
 
Posts: 820
Joined: Thu May 01, 2003 9:26 am
Location: Winnipeg Manitoba Canada

Postby ioginy » Mon Apr 12, 2010 7:32 pm

Sorry that says "passive piezo". my scrawls are a little less than readable sometimes.

I did some research into ultra-linear and i am liking it. I think i'll change my power stage to that.

Is there a better preamp you could suggest. I am trying to make a dual channel amp that will provide a different style sound. The fellow i am building this for uses an acoustic bass with an active piezo and a standup bass with a passive piezo. I want him to be able to use both channels for either guitar, but to find which he likes with either one. So yeah, totally different tone stacks, but switchable at the input.

As for the 10med resistor, do i replace the 68k?
User avatar
ioginy
KT88
 
Posts: 272
Joined: Sun Oct 18, 2009 5:38 pm
Location: Edmonton, ab

Postby dhuebert » Wed Apr 14, 2010 12:19 pm

This is how I do the input for my bass amps:

Image

The 10 meg resistor gives the piezo pickup the impedance it needs to work properly. Simple and effective.

Here's the balanced output section:

Image

That I stole unashamedly from these guys:

http://www.jensen-transformers.com/as/as021.pdf

It all works beautifully together, in fact this is one of my all-time favourite devices I've ever built.

Don
User avatar
dhuebert
KT88
 
Posts: 820
Joined: Thu May 01, 2003 9:26 am
Location: Winnipeg Manitoba Canada

Postby ioginy » Wed Apr 14, 2010 6:56 pm

Awesome, thanks. I have a few questions though. As far as the preamp goes, I assume the 250v out would go to the power stage of the amp.

Also, why would the balanced line out be so complicated? As far as I can tell from everywhere i have looked, you just need to step down the signal off the output signal and add a balancing transformer. Then again, this is the first i have ever dealt with something like this.

O and one more question. Which pot does which in your preamp? Sorry for the silly question but I have never dealt with this style before and i'm baffled.

Thanks in advance for your help.
User avatar
ioginy
KT88
 
Posts: 272
Joined: Sun Oct 18, 2009 5:38 pm
Location: Edmonton, ab

Postby ioginy » Wed Apr 14, 2010 7:30 pm

I think i have it sussed as far as the tone stack goes. First pot is bass, second is treble and third is volume. it looks like an orange tone stack near as I can tell.

Also another quick question. The input is a dot, and the 10M res. is to the ground. Should i assume the input is sleeve is direct to ground, or is there something I am missing?
User avatar
ioginy
KT88
 
Posts: 272
Joined: Sun Oct 18, 2009 5:38 pm
Location: Edmonton, ab

Postby dhuebert » Thu Apr 15, 2010 8:18 am

The tone stack is called Baxandall style. It is a classic design from the RCA design handbook. The advantages are: low phase distortion and low interaction between controls; it is used in most Hi Fi applications I have seen. The tone stack calculator is here:
http://www.duncanamps.com/tsc/index.html

The sleeve is always direct to ground AFAIK.

The balanced out is as complicated as it is because the high gain of the pentode was necessary to go with the rest of the microphone schematics this output is a part of. This high gain is good because it allows the preamp to work well with a large variety of equipment including multi-track tape machines. In the final design I have a pot before the output stage.

In order for the balanced line to do its job properly, the secondary impedance of the transformer must be more or less matched to downstream inputs. This impedance is often 600 ohms so the output impedance of the transformer should be as close to this as possible. Mismatched impedances will affect frequency response adversely.

I took this preamp to a recording studio recently and plugged it into everything with a balanced input. The Bass-o-matic played nicely with everything we tried which is not always the case with even professional gear.

Don
User avatar
dhuebert
KT88
 
Posts: 820
Joined: Thu May 01, 2003 9:26 am
Location: Winnipeg Manitoba Canada

Postby ioginy » Thu Apr 15, 2010 8:39 am

Wonderful, thanks. sadly i use a mac and the duncan calculator has not been made for the operating system yet. I have my fingers crossed.

Do you have any photo's of your completed amp? I would be very curious to see what your board layout is.
User avatar
ioginy
KT88
 
Posts: 272
Joined: Sun Oct 18, 2009 5:38 pm
Location: Edmonton, ab

Postby dhuebert » Thu Apr 15, 2010 9:46 am

sadly i use a mac


Sadly?!? I think you mean gladly. I too use a Mac and unfortunately Duncan only provides the software for the POS PC.

I'll take some snaps tonite.

Don
User avatar
dhuebert
KT88
 
Posts: 820
Joined: Thu May 01, 2003 9:26 am
Location: Winnipeg Manitoba Canada

Postby ChrisAlbertson » Thu Jul 01, 2010 12:44 pm

ioginy wrote:Wonderful, thanks. sadly i use a mac and the duncan calculator has not been made for the operating system yet. I have my fingers crossed..


The calculator will run directly on the Mac if you use either "Wine" or "Crossover". Wine is the free version of Crossover

These are not virtual machines like VMWare's Fusion or Palllels which both need a copy on MS Windows. Wine is free

Wine will not run all Windows programs but it will run many of them including the ones on Ducan's web site.
ChrisAlbertson
 
Posts: 120
Joined: Tue Jun 23, 2009 10:33 pm
Location: Redondo Beach, California

Postby dhuebert » Fri Jul 02, 2010 8:10 am

Funny thing about my Mac, the only trouble I've ever had with it was with a third party program called Final Vinyl, for recording vinyl onto iTunes. As much as possible I would prefer not to polute the machine with someone elses dodgy software. If it wasn't written for/by Mac, I don't want it. If I had a PC, I would not think about it. They have had so many stability issues in the past, what's one more piece of crap software? May as well. IMHO.

Don
User avatar
dhuebert
KT88
 
Posts: 820
Joined: Thu May 01, 2003 9:26 am
Location: Winnipeg Manitoba Canada


Return to guitar amps

Who is online

Users browsing this forum: No registered users and 14 guests

cron