"Decimal" (A distortion circuit)

[juanro]

I was reading about bipolar amplifier topologies and came across the "npn/pnp feedback pair".
As anybody knows, any amplifier type, properly tweaked and overdriven, can be turned into a fuzz / distortion. Hell, the fuzz face is a microphone amp... so I said, let's give it a try.
Here's what I have so far:

Just the simulation for the time being. R7 would be the gain control and R9 the volume.
Look at the component values ("carefully chosen") and you'll get where the name came from... I'll be off a week or so, but I will breadboard it and record how it sounds when I come back. In the meantime, if you have some spare bench time, give it a try...

Regards!
Juanro

[BIGSMITTY]

let er rip see what happens!

[juanro]

I breadboarded it last night. Observations:

1- Low to medium gain circuit. I expected (from simulations) a bit more. Nice sounding, mix between a crunch and a fuzz.
2- Get rid of D1 / R6. It doesn't provide more apreciable clipping (the reason I put that in the first time) and it tends to shift bias (from charging C2) giving a choked, splatty sound. Perhaps that's right for someone? I didn't liked it so I took it out.
3- Changing Q1 for other, higher Hfe transistors, didn't provided more gain (I tryied 2n3904, BC109, 2N5089). Didn't have other PNP to try for Q2, I'll try that later.
4- C2 value gives more or less bass response. 10uF seems a bit high (perhaphs for a thin strat it would be fine). 0.1uF gives good response with a fat humbucker. 1uF it's a good compromise. Good place for a switch.
5- At lower "gain" settings (lower R7 values) it tends to sound trebbly. Not harsh, but with more trebble than bass. Still havn't decided if this is a good thing or not.

Will do some more tweaking, record something and post the final version soon.

Regards!
Juanro