Forssell opto compressor calibration

[scott2000]

Was wondering if anyone could explain how best to adjust the trimmers in the Forssell opto compressor. I remember doing it in the past and not being thrilled with the results and since then have swapped for some newer vactrols.
some various quotes on the subject are vague and it would be nice to know a method using a meter, are there any particular settings that are better when trimming or do I just get gain reduction however way it happens? how to trim the balance of the cells?..,etc. I think I understand but wanted to hear anyone else's take. I've asked over on GDIY in the past with no responses. Pretty old project. I'm attaching the relevant files.


Here are a couple of quotes of Fred regarding this in no particular order.

So, if you plan on using a variable ratio control (highly recommended, imo) you should use a 5k rev log (who groaned out there?) pot in this position (VR4). The current limiting resistor should be set for the max current rating of the LED, typically 100-200 ohm. I personally do not like using VU type meters for GR reduction, finding them too slow for attack indication. I typically use a LM3916 type IC with a 1k resistor to ground on the input (usually a pot). This shows GR from the Lowest to the highest LED, not from "zero" down like a VU meter, so you do not need VR5. And it's fast.

Also a 500 ohm trim pot (wiper to the output of R7) and a cell's LED connected to each end of the pot will allow you to trim the balance of the cells. Typically I drive the circuit into a measured 5 dB of GR and use the 500 ohm trim pot to get 5 dB GR on the meter. I hope that make sense. Fred

VR1 can be a fixed 1k, but typically I use a trim pot so that I have the option to tweak the meter response for a given opto cell. This happens less than 10% of the time. However, I hate having to grade and select LED/LDR cells and the trim pot in the meter circuit and on the main circuit (opto balance) make it very easy to use ungraded/unmatched optos.

If one were to check the tracking on other opto compressor design (as a function of ratio control setting), you'd find that almost none of them (none of them?) track properly. It does on my design, if you use my metering circuit (once you get it calibrated), at least well enough to be useful.