D.W. Fearn VT15 - Manual (Page 26)

D.W. FEARN
VT-15 Recording Channel
28
response, low phase shift, excellent square wave response, low distortion, and high noise
immunity. The secondary of is connected directly to the grid of the first amplifier stage.
First stage
The first stage is a selected 6072 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 potentiometer
(Preamp Gain).
Third Stage
The arm of the Preamp Gain 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.
Preamp Output Stage
The output stage operates as a cathode follower, presenting a comparatively low output
impedance (approximately 800 ohms). The cathode output is coupled through a proprietary
polypropylene capacitor to either the Equalizer or Compressor section.
Compression Section
Audio from the mic preamp is fed to the input of the pulse-width modulator (PWM) board,
which provides the gain reduction. The audio output of the PWM goes through sidechain cir-
cuitry which determines the control voltage and hence the amount and characteristics of the
compression.
The compressor preamp consists of two stages of a 6072A dual triode, which then feeds
either the Line Amp or the Equalizer circuitry.
Equalization Section
The passive LC (inductor/capacitor) equalization is inserted either at the mic preamp output
or the compressor output. A 6N1P dual triode makes up the gain and then feeds either the
Compressor input or the Line Amp.
Line Amp Section
Either the eq output or the compressor output feeds a 6N1P dual triode with both sections in
parallel to provide line-level output. The output transformer is custom-made for us by Jensen
Transformers, Inc. and is the same transformer we use on all of our products.