Hi, I'm here to add mess to the soup. I'd like to mention a few already done examples that can be interesting for the topic.
The well known API 525 compresor has an interesting behavior, something like changing the time constant depending on the frequency content of the signal. Heavy LF content signals makes the compressor slower. It has a 600Hz LPF that charges a cap. When the cap isn't charged the selected resistors act as expected, with a second time constant, as many auto release circuits. When the cap is charged Q4 starts to open and increases the release time (up to twice the slowest selectable time in extreme cases)
Symetrix has a few interesting things, I do have a 501 limiter compressor and I really like it. It has a very nice auto timing option, faster attack as the signal rise is greater, (2 diodes in parallel with 1M resistor charging the caps) and a dual time constant release. For the manual setting it uses an emiter follower charging the release cap from the attack cap, getting independent time constants with just a few beans. 8 parts each circuit, plus the pots for the manual settings. Nothing interesting in the limiter side, charges as fast as it cans (opamp, 1N4148 charging a 39nF cap) and discharges with 0.5s time constant. Links to the schemes:
http://www.symetrix.co/kb/501_sch.pdf
The 301 low distortion compressor also has an interesting sidechain, never used one but I'd like to give it a try. It has an integrator, long story short, inhibiting or slugs the release for a fraction of a second after a big attack edge. The discharge mechanism sims odd but is a current source proportional to the voltage (a very expensive resistor) that is there to simulate a resistor that can be changed depending on the previous dynamic history. Scheme:
http://www.symetrixaudio.com/kb/301_sch.pdf
JS
The well known API 525 compresor has an interesting behavior, something like changing the time constant depending on the frequency content of the signal. Heavy LF content signals makes the compressor slower. It has a 600Hz LPF that charges a cap. When the cap isn't charged the selected resistors act as expected, with a second time constant, as many auto release circuits. When the cap is charged Q4 starts to open and increases the release time (up to twice the slowest selectable time in extreme cases)
Symetrix has a few interesting things, I do have a 501 limiter compressor and I really like it. It has a very nice auto timing option, faster attack as the signal rise is greater, (2 diodes in parallel with 1M resistor charging the caps) and a dual time constant release. For the manual setting it uses an emiter follower charging the release cap from the attack cap, getting independent time constants with just a few beans. 8 parts each circuit, plus the pots for the manual settings. Nothing interesting in the limiter side, charges as fast as it cans (opamp, 1N4148 charging a 39nF cap) and discharges with 0.5s time constant. Links to the schemes:
http://www.symetrix.co/kb/501_sch.pdf
The 301 low distortion compressor also has an interesting sidechain, never used one but I'd like to give it a try. It has an integrator, long story short, inhibiting or slugs the release for a fraction of a second after a big attack edge. The discharge mechanism sims odd but is a current source proportional to the voltage (a very expensive resistor) that is there to simulate a resistor that can be changed depending on the previous dynamic history. Scheme:
http://www.symetrixaudio.com/kb/301_sch.pdf
JS