THE SSL G384 Compressor Circuitry

The two compressor stages are identical, and component references for the right and left hand channels differ only the use of an “R” and “L” suffix respectively. The diagram below shows the audio signal path in simplified form.

The balanced input signal passes via input bypass relay RL1 through H1 [only at the G384], providing an unbalanced output. This signal feed the input of the main VCA, H2 via gain trim preset VR1. The output of H2 drives hybrid H3 to provide a balanced output which feeds via output bypass relay RL2. VR2-3 are used to set VCA symmetry and output signal balance respectively; these are factory set and should not require further adjustment.

The sidechain VCA, H4, is fed with an unbalanced audio signal either from H1 (see above) via TR1 or from the rear panel KEY connector via H4 and TR2 [only at the G384], depent upon the setting of the External Side-chain IN switch. The output of H4 is buffered by IC1.

Most of the control circuitry is common to both and left hand channels. The diagram below shows the control signal path in simplified form.

The output from the sidechain VCA buffer IC1drives the Ratio circuitry based around IC2-3. This is essentially a precision reftifier circuit whose limits of linear operation may be altered by switching different voltages (derived from potential divider network R29-35) into the inverting input of IC3 via the RATIO switch. The output from IC3 is then fed to the Attack/Release circuitry. The resulting signal is finally buffered by IC53, and is used to drive the meter directly. This singal is available on the REMOTE connectro, allowing a similar external meter to be driven.

The Autofade circuitry gnerates a variable rate voltage change (with a sharper “knee” characteristic towards the end). When Autofader is pressed, the output of IC58 goes low, causing the output of IC63 to gradually fall. This will affect the VCA Control Voltages after D54 is biases on. When C54 is sufficiently charged, IC63 switches low. OVerall Gain Trim provided by VR50.

IC54-55 act as summing amplifiers to produce the Sidechain and Main VCA Control VOltages respevtively. IC54 sums the signals from IC53 and the THRESHOLD control, and IC55 sums the singal from IC53, the MAKEUP control, and fron the Autofade circuitry output from IC52. If LK5 is inserted, VR4 can be used to adjust VCA tracking between left and right channels.

Both “hard” and “soft” bypass options are available, configuration being via LK1 and LK2. In “hard” bypass mode, when either the Compressor is not IN or power fails, the audio signal bypasses the compressor via relays RL1-2. In “soft” bypass mode, switching the compressor IN switch off isolates all VCA control voltages and the external sidechain audio path via TR57-58 and TR3, reducing the unit function to that of unitiy-gain amplifier.