Technical Description - McIntosh C34V Owner's Manual

The simplified block diagram on page 5 and the de-
tailed block diagram on pages 20 and 21 show how the
C 34V internal circuits are arranged. The C 34V uses
separate 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 control,
balance control, and LF and HF filters are in the listen
program line only. The compandor, the five equalizer
frequency controls, and the monitor amplifier can be in-
dividually switched to the listen program line or the
record program line.
PHONO AMPLIFIER
This amplifier uses a high technology integrated circuit
operational amplifier. Its differential input stage has
been optimized 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 distortion. The
feedback network also provides precision RIAA frequen-
cy compensation. The network uses 1% metal film resis-
tors 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 pre-
amplifier must be capable of operating as a power
amplifier to drive this impedance. The actual power out-
put capability of this preamplifier stage is more than 100
milliwatts, a great margin beyond that which is re-
quired.
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 milli-
volts without overload, a voltage far greater than the
output of any magnetic phono cartridge presently
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 switching. Control
signals from the listen and record selector switches turn
on the appropriate FET switches to pass the selected in-
put 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

18 TECHNICAL DESCRIPTION

signal wiring is kept to a minimum which 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. When an EXTERNAL PROCESSOR
is used, the listen program leaves the C 34V through the
EXTERNAL processor TO jack and returns via the FROM
jack. If a processor is not used, the signal passes directly
through the switching contacts In the jacks.
The volume control is next in the listen path. It is a
step attenuator with left to right tracking accuracy better
than 1 dB throughout its entire range. Such extremely
accurate matching is achieved through electronically
controlled trimming of the resistance material deposited
on pairs of printed circuits within the control. 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 vol-
ume control. In the past, loudness controls have typi-
cally been simple passive circuits connected to a tap on
the volume control. As a consequence, compensation
accuracy was dependent on many variables such as
volume control position and differences in the input
level. The C 34V loudness control uses active circuits of
an integrated circuit operational amplifier with two feed-
back loops. One has flat frequency response, while the
other has response conforming to the Fletcher-Munson
equal loudness contours. A potentiometer is placed be-
tween these two feedback loops making it possible 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 average listen-
ing volume is not affected by the position of the
loudness control.
The listen program signals next pass to the balance
control and then to the equalizer amplifier composed of
a low noise operational amplifier with flat response and
unity gain. Five other operational amplifiers are arrang-
ed in circuit configurations that are equivalent to five
series tuned circuits, each at one of the equalizer fre-
quencies. These series tuned circuits are inserted via
control potentiometers into either the input circuit or
feedback circuit of the equalizer amplifier to provide a
boost or cut capability of 12 dB for each equalizer band