The Technical Stuff
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33
As shown in Figure 5, this circuit controls the amount of compression as well as the attack and release
times of the limiter. The input to this circuit is taken from the output of the preamplifier section, just
before the volume control potentiometer (R23). The compression ratio push-button switches determine
the level of the signal which is sent to the sidechain. This determines the amount of limiting or
compression. Transistor Q7 acts as a phase inverter which is followed by Q8 , an emitter follower. This
signal then feeds CR3 as well as Q9 and Q10, which comprise another phase inverter / emitter follower
combination. This is then fed to CR2. Note that the signal applied to CR2 is 180
°
out of phase with the
signal at CR3. Since they are out of phase, the combination of CR2 and CR3 act as a full-wave
rectifier. The output of the rectifiers are then filtered by C22 which smooths the signal. This DC voltage
is proportional to the level of the input signal.
A bias level is applied to the diodes by the Compression Ratio pushbutton switches. This controls the
threshold of limiting, and is adjusted for the correct value as determined by the currently selected
compression ratio selection. R55 controls the compressor’s attack time
by regulating how fast C22 is
charged. Likewise, R56 determines the compressor’s release time by controlling the rate at which C22
discharges. The output of this stage is then applied to the gate of the FET in the gain reduction circuit,
which in turn controls the gain in the manner previously described.
