McIntosh MR 74, Owner'S Manual, Page: 15 / 20

tions for both 300 ohm twin lead and 75 ohm co-
axial cable are made with push type terminals.
FM-IF AND DISCRIMINATOR
The MR 74 uses linear-phase IF filters. The IF fil-
ters have equal time delay in their pass band region.
Any error in time delay causes FM distortion. All
other IF filters have delay distortion, some as much
as 100% of the 10.7 MHz transit delay. The MR 74
has less than 1.0% delay distortion from antenna
input to discriminator output. This makes possible
the overall low distortion performance limit for the
FM tuner and multiplex section.
Amplification of the IF signal is provided by two
high gain integrated circuits, each containing 16
transistors, 3 zener diodes, 5 diodes, and 23 resis-
tors, all on a single monolithic silicon chip. The ex-
ceptionally high gain of the integrated circuit assures
"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 detected output signal of the dis-
criminator is extremely low in distortion content.
Paralleling the main signal path through the FM-IF
and detector module, a secondary amplifying and
de-modulating process is used to activate the signal
strength meter and to provide test point TP1 with a
signal to be used with the Mclntosh Maximum Per-
formance Indicator.
FM STEREO MULTIPLEX
The 19,000 Hz pilot signal, broadcast by an FM
station, is filtered from the composite stereo input
signal, amplified by a special limiting amplifier,
doubled to the 38,000 Hz carrier frequency, and then
amplified again by a limiting amplifier. The compos-
ite signal minus the 19,000 Hz pilot is combined with
the 38,000 Hz carrier signal. The new combination
of signals is fed to the special detector circuit men-
tioned above. Balanced full wave detectors are used
to cancel the 38,000 Hz components in the output.
The SCA (Subsidiary Communication Authorization)
signal is removed from the composite output. This is
accomplished by the use of a new "Image Param-
eter" band elimination filter that has been computer
designed. The SCA filter rejects SCA signals with-
out impairing stereo performance.
When the 19,000 Hz carrier of a stereo signal is
received, the automatic FM stereo switching circuit
activates the multiplex decoding circuit. This lights
the stereo indicator. The circuit switching is all done
electronically. The automatic stereo switching can
be defeated by setting 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, assuring optimum signal to noise ratio.
The stereo switching circuit has been designed so
that noise will not activate it.
13
FM muting in the MR 74 operates by detecting
ultra-sonic noise. When noise is present the muting
circuit can be activated or defeated by the use of
the muting switch on the front panel.
The STEREO FILTER is provided to reduce noise
when listening to weak stereo stations. Careful de-
sign permits an ideal compromise between channel
separation and noise rejection.
AM
The AM-RF amplifier circuit includes a three sec-
tion variable tuning capacitor in the metal enclosed
shielded RF module which also houses the FM-RF
front end. A three section variable capacitor is used
for greatest spurious rejection. The RF amplifier is
unique. The circuit has constant sensitivity, constant
selectivity, and high image rejection across the com-
plete AM band. Ordinary AM-RF circuits cannot do
all of these simultaneously. The circuit design
achieves high sensitivity even at the low end of the
band. Spurious, image, and intermediate frequency
rejection are all superior. The same circuit delivers
equal selectivity across the entire band.
In addition, there is no loss of audio frequency
response at the low end of the band, common in AM
receivers. Another advantage of the Mclntosh circuit
is freedom from cross modulation and overloading
by strong local stations.
A high-quality loopstick antenna is provided. It can
be rotated for maximum performance, optimum sig-
nal reception or minimum interference. In each
MR 74, the loopsticks are individually tuned for opti-
mum performance. After tuning, the loopstick is
sealed. This custom matching of the loopstick to the
AM-RF front end maximizes the performance of the
loopstick antenna. The antenna is rotatable through
nearly 180 degrees. With this mobility you will not
suffer loss of sensitivity regardless of the angle at
which the instrument is mounted. A back panel con-
nector is provided for an external antenna. The an-
tenna is coupled to the tuner through a carefully
designed matching transformer which provides opti-
mum performance regardless of antenna length.
To maintain the excellent image rejection and lack
of spurious cross modulation of the AM-RF amplifier
an autodyne converter circuit was used in producing
the AM-IF signal.
AM-IF uses two double tuned IF transformers de-
signed to obtain a high degree of selectivity yet al-
lowing good audio fidelity. With the SELECTIVITY
switch in NARROW, a narrow band ceramic filter is
added to the AM-IF amplifier.
A 10,000 Hz active filter eliminates the 10,000 Hz
whistle and irritating noise caused by an adjacent
station. Because an active filter is used, the output
level at 10,000 Hz, or the whistle frequency, is down
over 20 dB or one hundredth of what it would be