QuoteNow if you want pure studio super clean compression DON'T bother,, but if you are wanting something that comes remarkably close to soft clipped Valve power stage compression then this would be very hard to beat.
There's already the A110 chip and involved circuitry, for "tube like" clipping. It employs "soft clipping" characteristics of the CA3094 OTA and drives Iabc terminal with input signal to achieve dynamically varying degree of asymmetric clipping (and related effects to harmonic distortion).
Compressor part of L5 is actually incredibly simpe. There's a JFET source follower working as a "unity gain" amplifier and a CA3080E OTA gain stage essentially working as its variable "feedback loop". In order to properly feed one of OTA inputs the signal must be heavily attenuated (input range is few millivolts). Varying gain of this stage also varies the gain of the entire follower circuit. The stage can be considered as plain "open loop" amp (with lowish gain), but gain can also be controlled by applying signal to Iabc terminal.
Signal from JFET source advances to three places: aforementioned OTA input, power amp input and circuit that generates Iabc control signal for the OTA.
So other circuitry is there pretty much just to generate that control signal: First there's a plain resistive attenuator for the "input" signal, then signal is amplified. Two diodes and an inverting amp full wave rectify the signal, C139 and R194 make up the RC circuit for "filtering" the rectified signal. This circuit primarily establishes attack, sustain and decay characteristics of the compression. Very "crude" circuit for anything remarkable on that regard, BTW. The result signal drives transistor Q102 and it's collector circuit in turn drives both the OTA Iabc input and another transistor, which simply drives a LED indicator lamp.
If an amp has an OTA-based compressor then the circuit most likely follows pretty much this architecture. Follower, OTA, rectifier + RC filter, BJT drive for OTA. What you have in L5, IMO, is about the most minimalistic example of an OTA based compressor circuit. Really.