Fluffy Fuzz

[antonis]

(or how to turn a low-noise high-gain audio preamp into a wannabe fuzz..)

It's part of a low noise - high gain audio preamp (on breadboard for many years.. ) with proper modifications for +9V single supply (originally +/- 15V)..
There is nothing special except the NFB (series voltage) loop moved from Q2 Collector (conventional configuration) to Q3 Emitter, both for the amount of applied feedback independency (output  as well as input terminals are outside the feedback loop, hence they are independent of load and source impedances) and the possibility to apply a second (independent) feedback loop (parallel voltage feedback) for reasons beyond the present scope (not applied here..)



How it works (brute analysis):
Feedback is taken from Q3 Emitter decreasing the distortion (* see below) of the voltage waveform at that point..
Q3 stage gain magnitude is approximately R11/R10 (21dB) (practically linear – low distortion)..
As the voltage gain from Q2 Collector to Q3 Emitter is almost unity, the gain from input to Q3 Emitter should be equal to the one of a conventional feedback pair (Q2-Q1 series feedback)..
So, the total voltage gain is [(RF +RG) / RG] X R11/R10, where RG= R2 and RF=R12+GAIN pot..
Q2 Collector resistance is split and bootstrapped, via C9, for higher open-loop gain (73dB) hence more precise closed-loop one (MAX 55dB)..



As for the rest, it's a, more or less, 3 directly coupled stages conventional design..
Quiescent point stabilization is obtained using a DC feedback across  Q1 & Q2 (via R13)..
Q1 & Q2 about 200μA currents are chosen as being the optimum current of BC549 for minimum noise.. (don't forget its origin..)

Although R3, R7+R8 (as well as R9) are decoupled (via 47μF respective capacitors), Q1 & Q2 intrinsic Emitter resistances (about 120Ω) should be seriously taken into account both for particular stage calculation and respective HPF corner frequency..
(their high values, due to low working current, guarantee  a significant non-linearity (*) which NFB is called to heal..)
(*)e.g. Q1 stage gain varies from 22k/(220+120) to 22k/120 – a variation of 280% ..!!

500k input impedance can't be considered OK for any self-respected Fuzz (it can be lowered by either lowering  R17 or R13 value – the later provokes the need for bias and input cap alteration..)
With current setup, it can be used from a low noise linear preamp (X30 gain for signals up to 120mV) to a high distortion fuzz (X560 gain)..



P.S.
(A) and (B) points are "extracted" from the circuit diagram both for reduced visual complexity and making NFB loop path clear..

[antonis]

And, of course, stripboard layout..
(actually, not practically verified..)

[Mark Hammer]

I don't need another fuzz to build, but I have to say, VERY nicely documented.  Great job.

[antonis]

Thanks Mark..!!
(your comments are highly appreciated..)

[Steben]

Ow no!
Low noise well ok, Linear (?) and high input Z for a fuzz  ...
Should be close to distortion 

[antonis]

Linear (?) and high input Z for a fuzz  ...

Almost linear..

Zin can be set according to taste..