McIntosh MAC 1900, Owner'S Manual

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ceived, the automatic 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 depressing the mono
pushbutton. (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. De-empha-
sis of the signal out of the multiplex decoding circuit
restores the frequency amplitude characteristics to
the same level they were before transmission.
AM SECTION
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 greater spurious rejection. The RF amplifier is
unique. The circuit has constant 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 down to 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 superhetrodyne AM receiver.
In addition, there is no loss of audio frequency re-
sponse at the low end of the band commonly known
to AM receivers. Another advantage of the Mclntosh
circuit is freedom from cross modulation and over-
loading by strong local stations.
A high-quality loopstick antenna is provided. It
can be rotated for maximum performance, optimum
signal rejection or minimum interference. In each
MAC 1900 the loopsticks 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 maximizes the performance
of the loopstick antenna. The antenna is rotatable
through nearly 180 degrees in all directions. 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 pro-
vided 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 autodyne converter circuit was used for the AM-IF.
AM-IF uses two double tuned IF transformers
designed to obtain a high degree of selectivity yet
allowing good audio fidelity.
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 roll off begins. Because an active filter is used,
the output level at 10,000 Hz, or the whistle fre-
quency, is down over 30 dB or one thousandth of
what it would be without filtering. With the select
button IN, the active filter cutoff frequency is low-
ered. The filter then effectively suppresses the 5,000
Hz whistle from nearby television receivers.
The AVC (automatic volume control) system was
designed to prevent bursting or thumps when the
AM is tuned through a strong signal. Distortion at low
audio frequencies is minimized by using two AVC
filter sections instead of the conventional one.
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