Simple but impressive overdrive circuit

[DDD]

Hi men,
there's an overdrive circuit picture below.
It has very dynamic sound and some kind of "chaw" artifact at the attack portion of a signal.
Battery consumption is abpout 90 uA at all (!), but the circuit produces a big enough sound.
*** Please do not pay any attention at "+" and "-" marks near ceramic capacitors (my old sim thinks that all of the caps are electros))).
*** Also please note that Q1 is BC860C and Q2 is BC849C (I am very sorry for the mistakenly marked trannies on the diagram).

[MaxPower]

Based on the title I expected a circuit containing one transistor and a couple of diodes (or a fuzz face knock off).

I don't understand the fat switch (the schematic). What is it doing? Connecting/disconnecting C_add? Connecting/disconnecting C_add and C7? In any case, looks interesting. I may have to give it a shot.

[DDD]

Well, 2-transistor overdrive circuit seems to be a simple one  

FAT Switch is a 3-position ON-OFF-ON toggle switch. It connects C7 or C_add in parallel to C8. Hence it affects resonant frequency of a tank formed by pickup inductance and C8/C7/C_add capacitors.
To get the most impressive result one can adjust C8, C7 and C_add caps for approx. 750/900 Hz and 1kHz resonance.

The Mids Toggle switch (the same ON-OFF-ON) changes mid frequency notch.

[midwayfair]

The D3/D4/D5 stuff is pretty clever.

It's actually a lot simpler than it looks, could just be how the schematic is drawn and that it takes several components to do some parts of it. But it's basically a BJT-based mu amp with some feedback loop stuff and three sets of clipping diodes.

The output is pretty heavily loaded. You might want to toss in a simple buffer at the end of the circuit.

[DDD]

Yes, the circuit is poorly drawn and looks too much more complicated than it is.
But it's not a mu-amp at all. It's just a two-stage amplifier with each stage utilizing 1/2 of battery voltage (there is about 4.5 VDC on the "+" terminal of C13 electro.
C13 virtually blocks AC interference between Q1 and Q2 stages (except of very low frequencies aka temporary DC shift that gives some dynamics to the output signal).
D1, D2, D3 and D4 except of clipping prevent heavy overloading of the Q1 and Q2 trannies.
D6 and D7 dynamically clip Q2 collector signal, D5 is intended to make Q1 bias conditions equal to that of the Q2.
Total (peak) gain measured on the upper terminal of R11 is about 66 dB. It's quite enough to get long sustain and decent distortion. So we can assume that the Q1 stage is not loaded heavily.
The output signal is audibly louder than dry (bypassed) humbucker pickup signal, so I think there's no need to use any recovery stage e.t.c.
*** I've tried the gadget with Roland Cube-60 combo which has 250-kOhm input impedance.

[knutolai]

looks kinda like the harmonic percolator with that virtual ground between the transistors

[ggedamed]

This is what I could make up in Tina-TI:

and the simulated response:

[petemoore]

  Shark Tooth

[DDD]

Thank you ggedamed for the beautiful and useful schematic diagram improvement.
The picture is OK. At the same time there's no need to use "high-current" 1N4007 for D3 since its maximum current is limited @30 mA (approx.) by R2 (360 Ohm). 1N4148 is to be more than enough for D3.
-----------------
The circuit differs from Harmonic Percolator since its "common supply point" is connected to collector terminals of Q1 and Q2 trannies, but not to emitters as in the Percolator. Hence the stages interact in the considerably different way.

[antonis]

This is what I could make up in Tina-TI:

Would you please post (or write) VG1 properties (like dc level, amplitude, frequency, etc) because I take some very odd AC transfer characteristics...
(maybe I've done some shematic missmatch, but..)

[DDD]

The small-signal AC graph is in the link below.
The FAT and Mids switches are in the same positions as per diagram by ggedamed
Just click the link and then press СКАЧАТЬ (DOWNLOAD)
http://zalil.su/456107

[ggedamed]

Would you please post (or write) VG1 properties (like dc level, amplitude, frequency, etc) because I take some very odd AC transfer characteristics...
(maybe I've done some shematic missmatch, but..)

DC level: 0
Signal: sine wave, 1000Hz, 100mV amplitude.

[midwayfair]

and the simulated response:

Well, that looks different! okay, I want to hear this one now