Big muff distortion without diodes

[pdhonte]

Hello all,

First of all, thanks for all the info on this forum, it's really helpful.

I'm breadboarding a big muff, triangle version from kitrae site, and I get the usual transistor voltages (c: ~4V for the first 3 stages, b: ~0,6V, e:~0,03V). I do however get a fuzz effect, not gentle overdrive, at Q2 and Q3 collectors, with all diodes taken off. It's already very fuzzy at the first clipping stage, and full blown at the second. So much that I get very little difference in the sound when I put the diodes back in. All in all it produces a fuzz that lacks definition apart for the higher frequencies (the circuit is tested on a strat).

I can tame the effect by increasing the limiting resistors, but I need pretty big values to get a noticeably cleaner signal (about 90K instead of the 8k2).

So my question is this: are the transistors in Q2 and Q3 supposed to participate that much to the fuzz effect? If not, any idea what I'm doing wrong? I've tried to measure the signal amplitudes at each stage, but I can't seem to get anything useful with my DMM.

Thanks,
Pierre.

[antonis]

Without the clipping diodes you have a 2-stage "normal" amplification with maximum - restricted by power supply - voltage..

So, at Q3 Collector you have a distorted waveform of about 4.5 V instead of 650mV. This waveform goes in Q2 base and it's further distorted at Q2 Collector with 4.5 V amplitude again instead of 650mV.

That's why you have to place huge restriction resistors before Q3 & Q2 bases..

edit1: Omitting diodes at NF loop raise circuits gain..

edit2: Wellcome..

edit3: For further reading: http://www.electrosmash.com/big-muff-pi-analysis

[Mark Hammer]

The Supa Tonebender was essentially a Big Muff with diodes only in what we think of as the 2nd clipping stage.  Or rather, the Big Muff was a Supa with diodes in two stages.

In truth, there are plenty of distortions and fuzzes that can yield grind and sizzle without diodes.  Lift the diodes on a Superfuzz, or any of its many cousins, and you still get fuzz.  The EHX Muff Fuzz is basically a silicon Fuzz Face with fixed gain, and if you lift its diodes, you still get distortion.

So, it should not surprise us that a Big Muff, minus diodes, is still far from clean-sounding.  The question is whether the kind of non-clean you get is the kind you want.  And in the case of the BMP, the diodes produce a tone we want.  Not to say that the tone produced without the diodes is undesirable, though.

[pdhonte]

Hello gents,

Thanks for the infos and the link, this helps a lot.

Cheers,

Pierre.