Efficiency? Preamp thoughts for LM3886 amp

[didier]

Hi everyone...

i recently finished my amp.
i made:
- an LM3886 poweramp running on +/-35V which should give me about 50W into my 8ohm Celestion G12T-75.
- a preamp on which i can switch between a tubescreamer-a-like + LPB-a-like channel and just a LPB-a-like channel
- a baxendall tonestack like this: http://www.uashem.com/images/audio/untit7855led_small.gif

now my problem is that i know that i can get more out of the amp than what i get now with the controls maxxed
i got the whole preamp section running on 12V and i doubled the clipping diodes (2x1N4148 and 2x1N4007)

my questions:
- would it be best to change the gain of the tonestack or the LPB? i would like to keep the sound i get now from the LPB
- will adjusting the gain on the LPB change the sound?
- any other advice?

[didier]

oh and:

- how do i change the gain for the tonestack stage?

thx

[petemoore]

  I don't know what the schematics would look like, or what order your tonestack fits in.
  so...do you think if you bypassed the tonestack you'd get more usable preamp power? [haven't looked at a bandaxall, this could be as easy as lifting it's ground?[
  If thats the case, you could put a recovery gain stage a'la RDV's BMP type thing...like the last stage of a BMP.
  or?

[R.G.]

That's a pretty old version of the Baxendall. The passive Baxendall (this is one) has a loss of about 20db.  Mr. Simulator says the circuit you show has a "gain" of between -1 and -6db.

A good thing to do would be to change this out for an active Baxendall with an opamp, but if you have too much invested in it to do that, you can try goosing up the gain of the recoverys stage. That 2.2K on the second transistor's emitter - split it into a 1K in series with a 1.2K, and put a 100uF cap across the 1.2K. That will get you to gains of 6db to about 12db depending on the control settings. That might be enough for you.

Seriously, there are some real flaws with that tone control circuit. It's not flat in any position I could find. There's a notch at about 660Hz at all settings.

- would it be best to change the gain of the tonestack or the LPB? i would like to keep the sound i get now from the LPB

Change the gain of the tone stack.

- will adjusting the gain on the LPB change the sound?

Probably so.

- any other advice?

Make the mods I suggested. Also, change C5 to 47nF and R6 to 4.7K and see if you don't like the tonal balance better.

If possible, scrap the circuit and put in an active Baxendall.

[didier]

allright R.G.

again i am a step closer to understanding transistor circuits...
if i want the circuit to have more gain, i should "lift the ground" as said by using a smaller emittor resistor or decoupling a part of it?

anyway as you say, if an active tonestack is better i will give this one a try:
http://www.generalguitargadgets.com/richardo/practiceamp/practicemaneq.gif
i guess thats the one you're recommending?

also, i would like to get to know Mr. Simulator i know of the tonestack calculator by duncan amp tools but i cannot test a baxendall there.

this is how the stages are put together:
http://i112.photobucket.com/albums/n171/the3dmonkey/preamp.jpg

thx

[slacker]

You might want to check out the Colosound Overdriver thats got a baxandall tonestack that has plenty of boost.
http://home.hawaii.rr.com/stompbox/CSOVR.JPG
Or there's an active op amp baxandall here
http://sound.westhost.com/dwopa2.htm#baxandall