The Solid State Logic E-Series Dynamics Module for API 500 Series Rack is a single-unit API 500 Series lunchbox-style compressor/limiter and expander/gate module. The device utilizes a similar circuit design and components as the original SSL E-Series channel strip, including a true RMS converter in the sidechain and a gain element that replicates the original Class A VCA chip.
Half of the module consists of a compressor/limiter with a ratio control that allows you to select from no compression to full limiting. You can change the curve from over-easy to hard-knee style compression with the push of a button. A threshold knob allows you to set the threshold at which point compression occurs between +4 and -26 dB. A release knob allows you to set the release time between 0.1 and 4 mS. A linear release button allows you to switch between a linear and a logarithmic release curve, and a fast attack button allows you to change the attack time from 3 ms for a 20 dB gain reduction to 30 ms for a 20 dB gain reduction. For metering, the unit has a 5-segment LED meter that shows the Compressor/Limiter activity in red and yellow and the Expander/Gate activity in green.
The Expander/Gate component of the module features an EXP button for switching between a 20:1 gate and a 2:1 expander. You can set the range of gating or expansion between totally inactive to up to 40 dB. A threshold knob sets the level at which the gate or expander engages between +4 dBu and -26 dBu. The threshold circuitry is designed with variable hysteresis which increases in relation to the threshold being lowered. This allows the sound of a musical instrument to decay below the threshold and before being gated or expanded by the module. The Expander/Gate section of the unit generally has a linear attack time of 1.5 ms per 40 dB, but this can be changed to a faster attack time of 100μs per 40 dB with the push of the FAST ATT button. A release knob allows you to adjust the time between 0.1 and 4 seconds at which the Expander/Gate will reduce the signal level after it is under the threshold. There is also a dynamics "In" button that allows you to put the entire module in or take it out of the circuit in your API 500 lunchbox-style rack.