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the recovered information with the L + R carrier signal.
This yields the left and right program output with maximum
separation.
The 19,000 Hz pilot signal, broadcast by an FM station,
is filtered from the composite stereo input signal, ampli-
fied by a special limiting amplifier, doubled to the 38,000
Hz carrier frequency, and then amplified again by a limit-
ing amplifier. The composite 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 mentioned above. Balanced full wave detectors
are used to cancel the 38,000 Hz components in the out-
put. 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.
When the 19,000 Hz carrier of a stereo signal is re-
ceived, the automatic FM stereo switching circuit activates
the multiplex decoding circuit. This lights the stereo indi-
cator. The circuit switching is all done electronically with
no clicks. The automatic stereo switching can be defeated
by depressing the mode selector switch to 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
optimum signal to noise ratio. The stereo switching circuit
has been designed so that noise will not activate it.
FM muting in the MX 113 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 button
on the front panel. The threshold 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.
AM SECTION
The AM-RF amplifier circuit includes a three section
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 greater spurious
rejection. The RF amplifier is unique. The circuit has con-
stant sensitivity, constant selectivity, high image rejection
across the complete AM band. Ordinary AM-RF circuits
cannot do all of these simultaneously. This circuit design
achieves equal 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. The Mclntosh circuit is unique in a
superheterodyne AM receiver.
In addition, there is no loss of audio frequency response
at the low end of the band, common in AM receivers. An-
other 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 signal recep-
tion or minimum interference. In each MX 113 the loop-
sticks are individually tuned for optimum performance.
After tuning, the loopstick is then sealed. This custom
matching of the loopstick to the AM-RF front end maxi-
mizes the performance of the loopstick antenna. The an-
tenna is rotatable through nearly 180 degrees in all direc-
tions. With this mobility you will not suffer loss of sensitivity
regardless of the angle at which the instrument is mounted.
A back panel antenna connector is provided for connecting
an external antenna if desired.
To maintain the excellent image rejection and lack of
spurious cross modulation of the AM-RF amplifier an auto-
dyne converter circuit was used in producing the AM-IF.
AM-IF uses two double tuned IF transformers designed
to obtain a high degree of selectivity yet allowing good
audio fidelity. With the SELECT button IN a narrow band
ceramic filter is added between the AM-IF amplifier and the
detector.
A 10,000 Hz active filter eliminates the 10,000 Hz
whistle and irritating "Monkey Chatter" caused by an
adjacent station. The frequency response of all stations is
nearly flat from 20 Hz to around 3,500 Hz, then rolloff
begins. 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 without filtering.
With the select button IN, 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 de-
signed to prevent bursting or thumps when the AM is
tuned through a strong signal. Distortion at low audio fre-
quencies is minimized by using two AVC filter sections
instead of the conventional one.
PHONO PREAMPLIFIER
There are three transistors in each channel of the phono
preamplifier. The output of the third transistor is connected
by a negative feedback loop to the emitter of the input
transistor. The feedback loop reduces noise and distortion.
It also provides precision RIAA frequency compenation
required for magnetic phonograph cartridges. Feedback
remains in effect even at 20 Hz, where gain is highest.
The negative feedback also provides a low output imped-
ance for the tape output.
Phono input overload is virtually impossible. For ex-
ample, at 1,000 Hz, the phono input can accept 150
millivolts of signal without overload. Ten millivolts of signal
at the phono input at 1,000 Hz will produce 1.2 volts at
the tape output.
The selector switch connects either the output of the
phono amplifier, the FM tuner section, the AM tuner
section or one of two high level inputs (TAPE or AUX)
to the main preamplifier. The input impedance of the high
level input is 250,000 ohms. The high level inputs feed
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