Question on DOD Grunge schematic

[Heywood]

Howdy all.

I noticed a few different DOD pedals (the Grunge specifically) have BJTs as part of (what looks to me like) the feedback biasing setup, and I was wondering what's going on with that.

For reference here is a schematic I found:
http://www.diystompboxes.com/pedals/dodfx69.jpg

Specifically, I'm talking about the arrangement of Q5 and Q7/Q8 with their respective op amps.

Is it like an automatic gain control of some sort? I usually see JFETs for that kind of thing.

Whatever is going on, I'm curious to know how it works. So if anyone can shed some light, that would be spectacular. Thanks!

[J M Fahey]

The light will blind you
Those are gyrators, resonant filters that behave like an LC filter (coil+capacitor) so "peak" some frequency.

FWIW:
Q5= 2166 Hz it will increase bite before distortion.

Q7= 6000Hz will make for bright distortion ... looks too high for me, remember that schematic is incomplete and may have errors. Iīd expect a much lower frequency.

Q8= 324 Hz it provides body. Donīt believe the values either, Iīd expect something around 90/100Hz to simulate 4x12" thump.

But anyway, besides schematioc errors, think of them as single bands from a Graphic Equalizer, compensating pre and post distortion equalization.

[Heywood]

Thanks so much, JM!

[Audiotexan]

Not sure if this will help or not...it's a bit in 'disagreement' with the above schematic.

dodfx69b_grunge_dist.pdf

At least it's easier on the eyes.

[Heywood]

Thanks, Texan.

I saw that one floating around as well, but the "-4.5v" threw me off. Since there is +4.5v, -4.5v, and GND, I wasn't quite sure what to make of it. I've not ever encountered bi-polar virtual grounds. Seems like if you went to the trouble to have bipolar voltage, you'd just not bother with a virtual ground at all.

[J M Fahey]

Itmight be his way of labelling things, but heīs not drawing *how* he gets such voltages.
And by the way the jack grounds are connected to "-4.5V" so itīs another confirmation that names given to rails are not exactly the best.
Neither itīs an official factory schematic.

[Audiotexan]

Yeah, Juan's dead on. It's another 'homebrew' at least as I refer to them..(meaning anything other than original factory/oem drawn). So take it with a grain as well. Just figured at least it's an A/B point, and perhaps between them, they could confirm aspects/similarities.

[Heywood]

Much appreciated Texan and Juan both.

[doctor]

Much appreciated Texan and Juan both.

To whom it may concern. The FX-69 genuine schematic.