Theory - D.W. Fearn VLC Operating Instructions Manual

18
THEORY OF OPERATION
Input section
The first stage is a selected 6072 tube configured as a Class A voltage amplifier with a gain of
approximately 30. Negative feedback from the plate of the second stage reduces distortion,
flattens the frequency response, and makes the gain of the first two stages less dependent on
individual vacuum tube characteristics.
Second stage
The output of the first stage is coupled to the grid of the second stage through a polystyrene
capacitor. This stage operates as a Class A voltage amplifier with a gain of approximately 30. The
plate is coupled through a polypropylene capacitor to the top of a conductive plastic rotary
potentiometer (Gain).
Third Stage
The arm of the Attenuation potentiometer feeds the grid of the third stage (a 6072A), which also
operates Class A with a gain of approximately 30. This stage is capacitively-coupled to the grid
of the output stage through a polystyrene capacitor.
Output Stage
The output stage operates as a cathode follower, presenting a comparatively low output
impedance (approximately 800 ohms).
Equalization Section
The passive LC (inductor/capacitor) equalization is inserted at the mic preamp output.
VLC Microphone Preamplifier/EQ/DI HAZELRIGG INDUSTRIES