Chauffeur Overdrive Circuit Question - VREF and Bipolar Supply?

[MattG]

Referring to the schematic for the Chauffeur Overdrive (Shnobel Tone Daily Driver). If I'm reading the schematic correctly, this circuit uses a 7660SCPAZ charge pump to generate a negative power rail. And then all the RC4558 opamps are powered with a bipolar supply. So far so good.

Assuming I'm right so far, the question is, why does the circuit need a VREF, i.e. half of VCC? It looks like wherever the 4558s are used in non-inverting mode, the input is biased with VREF; and whenever the opamps are used in inverting mode, the positive input is tied to VREF. Isn't the VREF biasing unnecessary when we have a symmetric bipolar supply for the opamps?

For comparison, look at the @Chuck D. Bones-designed Emperor of Tone, where he uses a bipolar supply, and doesn't use any VCC/2 VREF biasing.

 

[lowpitch]

Interesting circuit. If you want to maximize clean headroom, biasing an op amp to the mid-point of the positive and negative rails is the way to go, but there's no rule that says you have to do that. Maybe they wanted to increase even order harmonics through asymmetrical clipping for more amp like distortion™, since the output waveform will be offset when misbiased like this and one half of it is going to hit the power rails (and clip) sooner than the other, although I'm not sure if hitting the rails is really coming into play with those zeners in the feedback loop. That being said, there might be better ways to achieve this and there are some other weird design choices in that circuit, so it might also just be a case of they didn't know any better.

 

[jwin615]

That's the only dumb thing in this circuit you have questions about?

I'm convinced this circuit was a joke that got out of control.

 

[owlexifry]

That's the only dumb thing in this circuit you have questions about?

I'm convinced this circuit was a joke that got out of control.

https://forum.pedalpcb.com/threads/new-offerings-from-shnobel-tone.17696/#post-288222

 

[MattG]

That's the only dumb thing in this circuit you have questions about?

I'm convinced this circuit was a joke that got out of control.

As someone who's coming at this from a "weekend warrior"/pure hobbyist perspective, i.e. without any formal training in electronics, circuits like these are kind of a landmine: while it does appear to be a novel circuit, I think all the questionable design choices make it a poor study for someone like me (for whom the wacky things aren't immediately obvious).

I built one a while ago, though it is long since sold, I remember it sounded pretty good (just not good enough to justify its physical size necessitated by the amount of components). So more recently I thought I'd try to understand the circuit to see if anything compelling could be borrowed from it... but then I see things like what I brought up in my original question, and I know enough to recognize it as unconventional; but I don't know enough to confidently say if that's advanced circuit wizardry or a mistake or deliberate obfuscation (or like you said, a joke).

Sounds like the right call then (at least for me) is to file it away under interesting curiosities, but definitely not under benchmark circuits for serious study.