Adding a mid and distortion knob to an existing preamp circuit

[slimjohnny]

I found this preamp circuit for a 30W solid state amp I want to build. The only problem is that it only has a 2 band tone control(bass and treble) and I would like to make it a 3 band tone control, the other problem is that it only has one gain knob,( they call it gain in the website but I'm not sure of they mean overdrive or just volume). Anyways I would like to have a volume and a distortion knob.

Here's the link to the circuit: http://www.zen22142.zen.co.uk/Circuits/Music/guitar_tc.htm



How would I make these changes?

[Elijah-Baley]

You should replace the tone section. You could use another kind of Baxandall tonestack (almost like this) but at three band. I still doesn't try it, I don't know how much good it could be. It should be fine.
In alternative you could use a simple Marshall/Fender tonestack, we can find it in many guitar amps.
For a Master Volume control I guess it's ok add a potentiometer in the output. But I need a confirm about this, in case I'm forgetting some technical issues.

About the one you called gain/distortion knob, I can't imagine how much distortion could have this preamp, but if it sounds clean VR1 in the schematic in the link could work just like a volume pot.

[antonis]

I should also call VR1 Volume pot..
(A high value (500k say) pot could be placed right after tone control, as a Volume pot, instead of present VR1..)

About gain pot, if you are OK with present gain as minimum, place in series with R2 a pot (wired as variable resistor) of 20k.. (you'll be able to raise initial gain from 13 up to 22)

In case you wish lower than initial gain settings, replace R2 with a 50k pot..

U1 stage gain can be set by 1 + R23/R3 formula..
(of course, you may do exactly the same with IC2, but it's "better" to have distortion in front of tonestack/EQ rather than after it..)

A Master pot could be placed on output but you might also need a buffer just right after it..
(the need of a buffer is strongly depended on pot value and power amp input impedance..)

[thetragichero]

ohhhh this is designed to go inside a guitar, not as a stand alone preamp
makes a lot more sense of why it is what it is
you could use this for an amp build but I'm guessing you could also find a lot more suitable circuits of you did some sleuthing