McIntosh MR 74 Owner's Manual page 16

without filtering. With the SELECTIVITY switch in
NARROW position, the active filter cutoff frequency
is lowered. The filter then effectively suppresses the
5,000 Hz whistle from nearby television receivers.
The AVC (automatic volume control) system was
designed to prevent blasting when the AM is tuned
through a strong signal. Distortion at low audio fre-
quencies is minimized by using two AVC filter sec-
tions instead of the conventional one.
AUDIO AMPLIFIER
The audio amplifier increases the level of the pro-
gram adequate to drive a preamplifier or other ac-
cessory equipment. It consists of two, two-transistor
amplifiers, one for each channel. The design uses
considerable negative feedback to help achieve low
distortion, wide frequency response, and excellent
stability. Each audio amplifier delivers 2.5 volts to
the FIXED OUTPUT jacks. A second pair of outputs
are available where level can be varied by the VOL-
UME control.
POWER SUPPLY
Special design attention has been given to the
power supply section. Three separate power supply
circuits are used. The first is a 24 volt regulated sup-
ply. This 24 volt regulator is very elaborate in design,
using a specially selected transistor and associated
circuit. The regulator uses electronic filtering to in-
sure the lowest possible background hum level,
maximum stability and extremely good regulation. All
signal stages are powered from this regulator.
The second circuit is a half wave rectifier and
filter for the DC high voltage needed for the anode
of the multipath indicator. The third power circuit is
a half wave rectifier which supplies DC to the AM
section.
14