Well, I finished my PSup...went overboard, and pre-regulated down to +/-28VDC and then into an CRC network prior to the on-card regulators. With my supply I get -109 dBFS at min gain, and -107 dBFS at max. That will work for me!
I don't think the link is really necessary. Opto isn't really what I would go for when a controlled stereo image is the desired effect.
There's no sidechain filter on the schematic, but would be pretty straightforward to add it, and IIRC there is an unused half of a 2604 available.
To understand the schematic you need to first identify the parts:
1) Sidechain
2) Input buffer
3) Output amp & servo
4) Meter
5) PSup
Start by just looking at the sidechain and come to grips with how it works. Use Fred Forssell's explanation as a guide: http://groupdiy.com/index.php?topic=16016.msg53932#msg53932
Then, draw the connections between the input buffer, audio path opto attenuator, and the output buffer. You should notice at this point that there is no makeup gain control. So you need to make a decision, do you want a fixed gain output stage and an attenuator control (log pot) feeding the output amp, or do you want to vary the gain of the output stage. I opted for the former as it keeps the bandwidth of the op-amp constant and doesn't change the authority limits of the servo. The latter will have less noise, and might be a little easier to implement.
At this point in time you should have a good grip on the most important parts of the schematic. After that, look at the connection of the input buffer to the opto & sidecahin.
Finally, look at the meter circuit. There's lots of options there. I went for a VU because that makes me feel warm and fuzzy for an opto comp. The circuit for a VU is simpler, but hasn't been drawn out in this thread. If you go this route I can let you know what I did.
Here are the things I would change (or design in from the start):
1) Relay bypass. As designed right now the compressor makeup gain is active all the time, which makes A/B comparison between In and bypass harder.
2) Jumpers after the optos allowing them to be more easily calibrated (e.g. drive them with a constant tone and vary the balance pot while monitoring the resistance of each opto). Otherwise its a bit of a circular process to calibrate the meters
3) Either add a coupling cap after the input buffer, or wrap a DC servo around it (my preference, then you can get rid of all the audio path caps). I've got a wee bit of DC offset on the input amp and that results in a small 'woosh' when changing the output gain.
4) Keep the audio path short. Input buffer, audio opto, Output buffer. You should be able to fit this into about 3 inches of PCB space.
5) Consider socketing the resistor feeding the ratio pot...select it for maximum slope. Most folks just put in 100 ohms and were done with it, but at extreme settings the optos can produce negative ratios (i.e. the output actually gets quieter when the input gets louder).
Cheers
Kris
