BYOC Comp Threshold Control + Vbias question

eeviac · Post by **eeviac** » 12 Dec 2012, 01:02

Just getting my feet wet here.

I'm looking into controlling compression threshold in a Dynacomp based pedal. Attached is the BYOC Comp schematic with some markings. I'm thinking that threshold could be increased by connecting a pot to ground at those locations. My train of thought: cranking the pot lowers its resistance -> overall sidechain signal level is dropped -> more signal is needed to get the same amount of compression. Voila! Threshold control! Can someone more experienced than I tell me why this would or wouldn't work?

Separate topic: the Dynacomp derived pedals I've looked at have a 4.5V bias at the 3080 input stage, sandwiched between 3.3V bias at Q1's base and the 3080's output. What's the deal there? I changed my pedal to universal 4.5V bias and it didn't seem to sound any different, so it seems to me like an unnecessary complication.

Guilherme · Post by **Guilherme** » 12 Dec 2012, 11:20

Well, the "sustain" pot is already a threshold control. (and btw the "attack" pot is actually a release control - see the emma transmogrifier for proper attack/release controls)

You may want to investigate how to add a "ratio" control to this circuit, this is a mod I've never seen.

eeviac · Post by **eeviac** » 12 Dec 2012, 15:04

Thanks for lead. Hm... the Emma calls the Sustain/Threshold pot, Ratio.

Guilherme · Post by **Guilherme** » 13 Dec 2012, 13:20

Well, the Emma is a Dynacomp with mods.

https://www.freestompboxes.org/viewtopic ... hilit=emma

And the Dynacomp is actually a limiter, not a compressor. The pot changes threshold. There's a plot of the limiting curve of a Dynacomp with the pot on max and min on the trhead about merlin's "engineers thumb", where you can clearly see this.

https://www.freestompboxes.org/viewtopic ... mb#p207519

eeviac · Post by **eeviac** » 13 Dec 2012, 16:56

So, in a Dynacomp-based circuit, Sustain and Threshold are the same thing, and my suggested mod (if it even works) will sound more-or-less the same as what's already being done with the Sustain pot?

eeviac · Post by **eeviac** » 13 Dec 2012, 20:29

Oh man, I just realized how stupid my mod is (beyond the fact that sustain = threshold). Because Dynacomp-likes get their output from Q2's emitter, those pots won't just attenuate the sidechain level - they'll attenuate the output.

Maybe if I added a transistor buffer stage directly in front of Q2, and tapped the output audio from there, then I could mess with the sidechain levels...?