I finished my Rough Cut compressor build and I'm generally happy with it. It fired first time and it does what does well, even though, it would be great to improve on the Ratio of the compression. This leads me to the question, how would one go to change/determine and calculate the ratio of compression on this circuit or any other compressor by any means?
I have attached some pics of the finished build.
The ratio and threshold are adjusted in tandem, more or less. The Diamond Comp manual actually goes into some detail about this: https://www.diamondpedals.com/wp-content/uploads/Manual-Diamond_Compressor.pdf
I don't think you can calculate the compression amount directly in this setting b/c it depends not only the gain of the op-amp used to drive it, but the response of the optical device doing the compressing. It would probably have to be measured and plotted via amplitude changes of an input signal. But, it would look something like the pic below, I believe. The rise/run of the curve is entirely dependent on the gain applied and opto device used, but should always level off into a kind of asymptote as the volume increases.
You can tweak the ratio response a bit by increasing the value of R10 some. A higher value than 3k3 might give some extra low ratio settings. Or alternatively, a trimmer in place of R17 could be used to dial in the LED response in the opto device. I haven't tried either of these so it's a bit of a guess.
Hi Brian, thank you for responding to this. I do not know much about compressor technologies and I'm becoming aware that there are many ways of designing a compressor circuit. My idea was to make this circuit have the ability to increase it's Ratio from 3:1 to a 4:1 or 6:1, and so on. I understand that, in this case, the compression seems to be in a sort of feedback loop that it is very dependent on the input Gain. I understand now, after your suggestion, how to have a smaller Ratio, but I'm really interested on how to add bigger ratio's for more compression. Is it possible that the Ratio changes as more Gain is applied?