McIntosh MR 73 Owner's Manual page 6

purpose is to supress hetrodyning that occurs between adjacent
AM carriers.
To increase the "pulling power" of the AM section of the MR 73,
connect an external antenna at the rear apron of the unit by using
a push connector. An internal transformer matches the external an-
tenna to the input impedance of the AM front end. An optional highly
sensitive loop stick antenna is also provided for local or strong
stations. A switch on the back pane! selects either loopstick or
external antenna.
An AM sensitivity switch has been provided to decrease both the
gain of the RF amplifier and the noise between stations.
AUDIO PREAMPLIFIER SECTION:
An audio amplifier has been incorporated in the MR 73. It in-
creases the audio level from the AM or FM multiplex outputs to a
level sufficient to drive a preamplifier or other accessory equipment.
The audio amplifier consists of 2 separate amplifiers, each amplifier
having three transistors. The design uses considerable negative
feedback. Negative feedback helps to achieve low distortion, wide
frequency response, and unexcelled stability. Each audio amplifier
has two low impedance outputs. One is a fixed output set at 2.5
volts. The second output is variable by the volume control on the
front panel.
By cross-coupling a small portion of one channel into the other
in the audio amplifier, the residual alternate channel information
can be cancelled, thereby further increasing the stereo channel
separation.
Also included in the audio section is a STEREO FILTER switch.
With the STEREO FILTER turned on and weak stations are being
received, stereo noise is reduced, but still allows ample stereo
listening performance.
POWER SUPPLY
Special design attention has been given to the power supply
section of the MR 73. Two separate rectifier circuits are used in
the MR 73.
4
The 16 volt regulator is the power heart of the MR 73. All signal
stages throughout the unit are powered from this regulator. The
16 volt regulator is very elaborate in design, using a specially se-
lected transistor and associated circuit. The regulator uses elec-
tronic filtering to insure the lowest possible background hum level,
maximum stability and extremely good regulation.
A half wave rectifier and filter, supply the DC high voltage needed
for the anode of the multipath indicator. The second power supply
is a full wave rectifier which supplies DC to the multiplex indicator
and to the voltage regulator.