4. The heaters of the 12AX7 tubes used in the C-4 are returned to
ground through a hum reducing potentiometer in the power amplifier.
Heaters of tubes used in the C-4P are returned to ground through a hum
reducing potentiometer in the power supply. This control requires an
initial adjustment for minimum hum, and should be readjusted each
time one or more tubes are replaced.
5. Grounding the turntable motor frame to the Audio Compensator
chassis near the input jacks may reduce the hum level on the phono-
graph channels.
OUTPUT CONNECTIONS
Three outputs are provided: one tape and two main outputs.
The tape output may be used for recording tape from any source connected
to the Audio Compensator. The Selector Switch, and Compensation Switch are
effective at this output. Adjustment of the Bass and Treble tone controls and
Volume control may be made without affecting the recorded signal. The tape
output delivers a signal of one volt.
All controls are effective at the main outputs, which are available at the
octal socket labeled "INTER-UNIT CABLE" between pins #1 and #2 (pin #1 is
ground), and at the RETMA pin jack labeled "MAIN OUTPUT". The inter-unit
cable connecting the C-4 to any McIntosh power amplifier uses the octal socket
output. The pin jack output may be used for driving a second power amplifier
if so desired.
The main and tape outputs are fed from cathode followers. The input im-
pedance of devices connected to these outputs should be 50, 000 ohms or
greater, and the capacitive reactance of audio cables connecting these devices
should not be less than 8, 000 ohms at 20, 000 cycles. This is the reactance of
a capacity of 1000 mf. Audio cable having a capacity of 25 mmf per foot may
be 40 feet long.
TAPE and TUNER Channels: These channels are each terminated by a
potentiometer mounted on the back panel which should be used to reduce signal
input if in excess of . 1 volts. High impedance sources, such as crystal
microphones and the detector output of tuners, may be connected directly to
these channels since they have an input impedance of 660, 000 ohms.
MIC Channel: This channel is terminated by a 100,000 ohm potentiometer
mounted on the back panel which should be used to reduce signal input if in
excess of 10 MV. Low impedance microphones in conjunction with an input
transformer, such as McIntosh M-107, may be used on this channel.
The TAPE, TUNER and MIC channels provide flat amplification from 20
to 20, 000 cycles. The Bass and Treble tone controls may be used to alter the
response as desired.
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