AM SECTION
The AM-RF amplifier circuit includes a three sec-
tion variable tuning capacitor in a metal enclosure.
This three section variable capacitor is used for
greater spurious rejection. The Mclntosh AM-RF amp-
lifier circuit is unique. It has constant sensitivity, con-
stant selectivity and high image rejection across the
complete AM band. Ordinary AM-RF circuits cannot
do all of this simultaneously. This design achieves
equal sensitivity even down to the low end of the
band. Spurious, image and intermediate frequency re-
jection are all superior. The same circuit delivers
equal selectivity across the entire band. Thus, there is
no loss of audio frequency response at the low end of
the band. Another advantage of the Mclntosh circuit
is freedom from cross modulation and overloading by
strong local stations.
A loopstick antenna is provided which can be
rotated for maximum sensitivity over 180 degrees.
Each MR 75 loopstick is individually tuned. This
custom matching of the loopstick to the AM-RF front
end maximizes the performance of the antenna. A
rear panel antenna connector is provided for connec-
ting an external antenna if desired.
To maintain the excellent image rejection and spur-
ious cross modulation rejection of the AM-RF ampli-
fier, an autodyne circuit is used for the AM mixer. The
AM-IF uses two double tuned IF transformers design-
ed to obtain a high degree of selectivity yet allowing
good audio fidelity. A 10 kHz active filter eliminates
the whistle and irritating "monkey chatter" caused by
an adjacent station. The automatic volume control
system was designed to prevent bursting or thumps
when the AM is tuned through a strong signal. Distor-
tion at low audio frequencies is minimized by using a
two-section AVC filter instead of the conventional
single section.
OUTPUT/HEADPHONE AMPLIFIER
The output audio amplifier is a push-pull com-
plementary class AB amplifier using a signal inver-
ting differential stage at its input. This amplifier
drives the variable output and the headphone jack.
TURN ON DELAY
A turn-on delay circuit is located ahead of the out-
put amplifier. This circuit uses a light emitting
diode/light dependent resistor network that functions
to transmit no signal until two seconds after the
power switch is turned on and to remove the output
signal almost instantly upon power turn-off. Thus the
turn-on and turn-off is transient and noise free.
POWER SUPPLY
To minimize hum, radiation, and thus improve
signal to noise ratio the MR 75 power transformer is
triple shielded. Shielding includes a copper strap, a
silicon steel strap, and finally a steel outer shell. The
transformer secondary output voltage is fed to a full
wave bridge rectifier and 2,200 microfarad filter
capacitors to provide the +24 volts for powering the
plus regulator and - 24 volts for the minus regulator.
The ± 18 volts needed for low level amplifier stages is
supplied by integrated circuit voltage regulators.
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