McIntosh MR 73 Owner's Manual page 5

curve has a nearly flat top with linear phase characteristics. The
skirts of the response curve are very steep. The maximum width is
240 kHz at -3.0dB and 400 kHz at -60dB. The response curve is
symmetrical each side of the center frequency. The crystal filters are
permanently sealed and do not require adjusting. The IF cannot
drift nor vibrate out of adjustment.
Each of the two integrated circuits used in the FM-IF of the MR 73
contain 16 transistors, 3 zener diodes, 5 diodes and 23 resistors, all
on a single monolithic silicon chip.
The exceptionally high gain of the two integrated circuits assumes
"hard limiting" at very low levels of input signals.
A "phase" or "Foster Seeley" discriminator has been designed
to complement the integrated circuit IF section. The IF section has
exceptionally high gain and hard limiting characteristics, yet the
capture ratio is quite low. The detected output signal of the dis-
criminator is extremely low in distortion content.
De-emphasis of the discriminator output restores the frequency
amplitude characteristics to the same level they were before trans-
mission.
C. FM Stereo Multiplex Section. Mclntosh Laboratory has de-
veloped a special detecting circuit in the multiplex section. A par-
ticular advantage of this circuit is the elimination of the critical
adjustments necessary with commonly used matrixing circuits. The
circuit detects the L-R sidebands, then automatically matrixes the
recovered information with the L + R main carrier signal. This yields
the left and right program output with maximum separation.
The 19kHz pilot signal is filtered from the composite stereo input
signal, amplified by a special limiting amplifier, doubled to the 38kHz
carrier frequency, and then amplified again by a limiting amplifier.
The composite signal minus the 19kHz pilot is combined with the
38kHz carrier signal. The new combination of signals is fed to the
special detector circuit mentioned above. Balanced full wave de-
tectors are used to cancel the 38kHz components in the output.
The SCA (Subsidiary Communication Authorization) signal must
be removed from the composite output. This is accomplished by the
use of a new "Image Parameter" band elimination filter that has
been computer designed. The SCA filter rejects SCA signals without
impairing stereo performance.
FM muting in the MR 73 operates by detecting ultra-sonic noise
which is present when tuning between stations or when receiving
a weak station. The muting circuit can be activated or defeated by
the use of the muting switch on the front panel. The level of muting
desired can be adjusted by the muting level control on the top panel.
Varying the muting control adjusts the threshold at which the muting
takes effect.
When the 19 kHz carrier of a stereo signal is received, the auto-
matic FM stereo switching circuit activates the multiplex decoding
circuit. This lights the stereo indicator. The circuit switching is all
done electronically with no clicks. The automatic stereo switching
can be defeated by turning the mode selector switch to FM MONO.
(In this position the stereo indicator will still light to indicate the
presence of a stereo signal.) On monophonic transmissions the
stereo switching is inactive at all times, assuring a greater signal to
noise ratio. The stereo switching circuit has been designed so that
noise will not activate it.
AM SECTION:
The "state of the art" in electronic technology has been incorpo-
rated by Mclntosh into the MR 73 with the use of metal oxide silicon
field effect transistors (MOSFET's) in the AM section. An AM-RF
amplifier circuit includes a three section variable tuning capacitor
in the metal enclosed shielded module, which also houses the
FM-RF front end. The AM-RF amplifier uses a dual-insulated gate
MOSFET to obtain more overload protection against strong local
stations than can be obtained with conventional bipolar transistors.
The AM-RF mixer is also a dual-insulated gate MOSFET. The use
of MOSFET's in the RF amplifier and mixer provides the MR 73 with
very little cross modulation (spurious response) and the image re-
jection is very good.
Three double tuned IF transformers are used to obtain a high
degree of selectivity yet still allow good AM fidelity. A 10kHz whistle
filter has been incorporated at the output of the AM detector. Its
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