Technical Description - McIntosh C33 Owner's Manual

Technical Description

The simplified block diagram on page 3 and the
detailed block diagram on page 24 show how the C 33
internal circuits are arranged. The C 33 uses separ-
ate program channels (referred to as "program lines"
in this manual) for "listening" and "recording". This
allows you to listen to any program source while you
record from another or the same program source.
The mode selector, volume control, loudness con-
trol, balance control, and LF and HF filters are in the
Listen program line only. The compandor, the five
equalizer frequency controls, and the monitor ampli-
fier can be individually switched to the Listen pro-
gram line or the Record program line. A description
of the C 33 circuits follows.
PHONO AMPLIFIER
Four separate phono amplifiers are provided; two
for Phono 1 and two for Phono 2. Each amplifier uses
a high technology integrated circuit operational
amplifier. Its differential input stage has been op-
timized for low noise and low distortion perfor-
mance. Open loop gain of this integrated circuit is
100,000 times. With high open loop gain a large
amount of negative feedback is used around the
phono amplifier to further reduce noise and distor-
tion. The feedback network also provides precision
RIAA frequency compensation. The network uses
1 % metal film resistors and 5% poly film capacitors.
To achieve low noise performance it is essential that
the feedback network have very low impedance. As a
consequence, the preamplifier must be capable of
operating as a power amplifier to drive this impe-
dance. The actual power output capability of this
preamplifier stage is more than 100 milliwatts, a
great margin beyond that which is required.
Input sensitivity of the phono amplifier is 2.2 milli-
volts. The gain of the amplifier is 42 dB at 1000 Hz.
The phono amplifier has a very wide dynamic range.
At 1000 Hz the phono input circuit will accept 100
millivolts without overload, a voltage far greater
than the output of any magnetic phono cartridge pre-
sently available.
LISTEN AND RECORD SELECTOR SWITCHES
Input switching is accomplished electronically.
Signals from the 7 inputs connect to two arrays of
Field Effect Transistors (FET) to perform the switch-
ing. Control signals from the Listen and Record
selector switches turn on the appropriate FET
switches to pass the selected input signals. The
other input signals are blocked by the "off" FET
switches. Each FET switch uses two cascaded FET
transistors to provide the required isolation and pre-
vent cross talk between inputs. The FET "on"
resistance is very low which prevents distortion. The
switching transistors are located right at the input
jacks, so that signal wiring is kept to a minimum.
This eliminates cross talk and noise problems.
LISTEN PROGRAM LINE
The listen program signal from the Listen input
switch goes to the Compandor control switching
where the Compandor can be inserted in either the
listen or the record program path. The Compandor
circuit is described later.
The listen program then goes to the Listen Exter-
nal Processor jacks. These jacks have switching
contacts so that the signal passes through when
there is no plug in the jacks. Therefore, if an External
Processor is used, the Listen program leaves the C 33
through the External Processor "to" jack and returns
via the "from" jack. If no processor is used, the sig-
nal passes directly through the jacks.
The volume control is the next step in the listen
path. The volume control is a step attenuator which
has left to right tracking accuracy within 1 dB
throughout its entire range. Such extremely accur-
ate matching is achieved through electronically con-
trolled trimming of the resistance material depos-
ited on pairs of printed circuits. Since the switch
commutator touches only contact pads and not the
actual resistance element, tracking accuracy is not
degraded with use as in ordinary volume controls.
The loudness control and its amplifiers follow the
volume control. In the past, loudness controls have
typically used simple passive circuits connected to
a tap on the volume control. As a consequence, com-
pensation accuracy was dependent on many vari-
ables such as volume control position and differ-
ences in the input level. The C 33 loudness control
uses active circuits. An integrated circuit opera-
tional amplifier is used here. It has two feedback
loops. One has flat frequency response. The other
has response conforming to the Fletcher-Munson
equal loudness contours. A potentiometer is placed
between these two feedback loops making it possi-
ble to select any combination of the two, from a flat
response to full loudness compensation. The overall
gain of the stage is 20 dB at mid-frequencies and the
listening volume is not affected by the position of
the loudness control.
The listen program signals next pass to the bal-
ance control and then to the Equalizer Amplifier. This
amplifier uses a low noise operational amplifier and
has flat response and unity gain. Five other opera-
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