McIntosh C33 Owner's Manual page 25

tional amplifiers are arranged in circuit configura-
tions that are equivalent to five series tuned circuits,
each at one of the equalizer frequencies. These
series tuned circuits are inserted via control poten-
tiometers into either the input circuit or feedback
circuit of the equalizer amplifier. This provides a
boost or cut capability of 12 dB for each equalizer
band of frequencies. A pushbutton switch allows the
equalizer circuit to be switched from the Listen pro-
gram line to the Record program line.
The LF and HF Filters follow next. These filters
are active filters using operational amplifiers. When
switched in, the LF filter attenuates frequencies
below 50 Hz at a rate of 12 dB per octave. The HF
Filter attenuates frequencies above 7kHz at a rate of
12 dB per octave.
The Listen program signal then arrives at the main
output jacks. The main jacks are fed directly. The
output 1 and output 2 jacks receive the same signal
but can be turned on and off by the Speaker/Output 1
and 2 pushbuttons.
RECORD PROGRAM LINE
The record program signal from the Record input
switch goes first through a unity gain amplifier and
then to the Compandor switching where the Com-
pandor can be inserted into the record program path.
The Compandor circuit is described later. The
Record program then goes to the Record External
Processor jacks. These jacks, like the Listen Exter-
nal Processor jacks, have switching contacts so that
the signal passes through when there are no plugs
in the jacks. Therefore, if an External Processor is
used, the Record program leaves the C 33 through
the External Processor "to" jack and returns via the
"from" jack. If no processor is connected the signal
passes directly through the jacks.
An equalizer amplifier with unity gain follows
next. A pushbutton switch allows the 5 band
equalizer controls to be switched from the Listen
program line to the Record program line. This
feature allows equalizing the record signal before it
is fed to a recorder.
The equalizer amplifier feeds to electronic in-
terlock switching and then finally to the record out-
put jacks. The switching is arranged so that if the
Record input selector is positioned to select the out-
put of a recorder, the Record program line will not
feed the record output jack connected to that
recorder. This prevents recorder feedback.
COMPANDOR
The Compandor will expand or compress the
dynamic range of the program material. The Com-
pandor can be switched to the Listen or Record pro-
gram lines or turned off. Program to the Compandor
is applied to a voltage controlled amplifier (VCA)
which operates as a variable gain block. Control
voltages for this VCA are developed from a sample
taken from the left and right channel input signals to
the VCA. The electronic processing of this sample is
detailed. It includes band shaping, logarithmic con-
version, full wave rectification, level setting, expan-
sion or compression ratio regulation, attack timing,
and DC amplification. The resulting processed volt-
age controls the gain of the VCA to cause logar-
ithmic gain expansion when expanding the program,
or the opposite, logarithmic gain compression when
compressing the program.
MONITOR AMPLIFIER
The monitor amplifier is a 20 watt per channel
stereo power amplifier. The monitor input selector
switch allows the amplifier to be driven from the
Listen or Record program lines or from an external
input.This amplifier is a push pull complementary
class AB amplifier using a differential input stage.
The amplifier includes Mclntosh's Power Guard pro-
tection circuit. The Power Guard circuit compares
the amplifier output signal with its input signal. If
there is a difference between the waveforms of
these signals, Power Guard activates an electronic
attenuator at the input of the amplifier which
reduces the amplifier input level. This automatic
control system makes it impossible to drive the
monitor amplifier into clipping. Thus, clipping distor-
tion and loudspeaker damage due to clipping are
eliminated.
AUTO TURN-ON
A turntable plugged into the green outlets at the
rear of the C 33 can control the on-off operation of
the C 33. Current flow to the turntable is sensed and
controls a silicon control rectifier (SCR). When the
SCR conducts a relay closes. This relay turns on
power to the C33 and to the black AC power outlets.
A current sensing sensitivity control allows cal-
ibrating the Auto Turn-On circuit to the turntable
used. Power is also controlled by the C 33 Power On
pushbutton. The Auto Turn-On circuit may be
switched off for manual power control.
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