Section 4 Principles of Operation
4.1 Amplifier Circuit Board
4.1.1 Input Transformer Stage
The Input from the Female XLR Input connector feeds the bypass side of the Bypass
switch and the input pad and input transformer. The input pad provides 20 dB of loss and
transforms the 600 Ohm input to the proper 120 Ohm transformer impedance. The input
transformer feeds the grids of the 6BC8 variable-mu gain reduction stage through the
Input Level control in a balanced 200K Ohm arrangement.
4.1.2 6BC8 Variable Gain Stage
The 6BC8 gain is determined by a negative bias applied to the grids through the Input
Level control. This control voltage is derived from the detector and timing circuitry. The
6BC8 stage is powered by a regulated power supply using the OB2. This circuit is
balanced with VR2 across the cathodes and VR3 across the plates.
4.1.3 Interstage Transformer
The interstage transformer separates the gain reduction stage from the amplifier stage. It
provides a distinctive load for the variable gain stage to work into. Alternatively, the
interstage transformer can be bypassed which changes the tone and character of the
compression.
4.1.4 12AX7 and 12BH7 Amplifier Stage
The amplifier stage is a fully push-pull circuit. The first stage, a 12AX7 and the second
stage, a 12BH7 are closely tied through a dual balanced negative-feedback loop. This
provides gain and balance stability and provides low distortion and a low virtual driving
impedance for the output transformer. VR4 trims the balance of this amplifier and serves
as a simple way to verify correct operation.
4.1.5 6AL5 Detector and Ratio Switch
The output transformer has four sets of input taps to accommodate the four ratio settings.
Each set of taps is brought out to the ratio switch. The ratio switch then feeds the 6AL5
full wave rectifier through the Sidechain Highpass filter network and Asymmetry mode
switching. Bias for setting the compression threshold and ratio is derived from a resistive
ladder also on the ratio switch. The 6AL5 detector output feeds the attack/release
controls and timing capacitors before driving the gain reduction stage.
