Specifications

When a DC voltage is applied to the coil, current
flows and creates a magnetic field. The force of the
field causes the leads to bend and contact one another
inside the sealed glass tube. The inert gas eliminates
corrosion of the contacts, which insures a low
resistance, distortion free switch.
All signal switching in the C40 is done with
ELECTROMAGNETIC switches driven from DIGITAL
LOGIC.
PHONO AMPLIFIER
The phono amplifier uses a high technology integrated
circuit operational amplifier. Its differential input stage
has been optimized for low noise and low distortion
performance. Open loop gain of this integrated circuit
is 100,000. With high open loop gain, a large amount
of negative feedback can be used around the phono
amplifier to further reduce noise and distortion. The
feedback network also provides precision RIAA
frequency compensation. The network uses 1 % metal
film resistors and 5% polypropylene film capacitors.
To achieve low-noise performance it is essential that
the feedback network have a very low impedance. As
a consequence, the preamplifier must be capable of
operating as a power amplifier to drive this impedance.
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 2.5
millivolts. The gain of the amplifier is 40dB at 1000Hz.
The phono amplifier has a very wide dynamic range.
At 1000Hz, the phono input circuit will accept 90
millivolts without overload, a voltage far greater than
the output of any current magnetic phono cartridge.
Phono input overload, therefore, is virtually impossible.
A signal level of 10 millivolts at the phono input at
1000Hz will produce 1 volt at the tape output. The tape
output has a source impedance of 100 ohms, designed
to operate into a load impedance of 10,000 ohms or
greater.
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
EXTERNAL Processor jacks. When an EXTERNAL
PROCESSOR is used, the listen program leaves the C40
through the EXTERNAL processor TO jack and returns
via the FROM jack.
The volume control is next in the listen path. It is
a step attenuator with left to right tracking accuracy
better than 1dB through 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.
LOUDNESS AMPLIFIER
At the input to the high level or loudness amplifier,
the signal passes through the mode switch, then through
the volume control, and into the loudness amplifier. In
the past, loudness controls have typically used 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 input level. The C40 uses
active loudness control circuitry. An integrated circuit
operational amplifier is used here. It has two feedback
loops. One feedback loop has flat response. The other
feedback loop has loudness compensation. A
potentiometer is placed between 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 loudness stages
is 20dB and is not affected at mid frequencies 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. Four other operational amplifiers are
arranged in circuit configurations that are equivalent to
four series tuned circuits, each at one of the equalizer
frequencies. 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 12dB for each equalizer band
of frequencies. A passive circuit is used for the fifth
frequency. A pushbutton switch allows the equalizer
circuit to be switched from the LISTEN program line to
the RECORD program line.
After the equalizer section are the MAIN, SWITCHED
1 and 2 and BALANCED OUTPUT connectors.
RECORD PROGRAM LINE
The RECORD program signal from the RECORD input
switch goes first through a unity gain amplifier and then
MClNTOSH LABORATORY INC., 2 CHAMBERS STREET, BlNGHAMTON, NEW YORK 13903
C40 Product Preview Page 6