very low levels of Input signal.
A Foster-Seeley discriminator has been designed
to complement the integrated circuit IF section. The
detected output of the discriminator is extremely
low in distortion.
FM STEREO MULTIPLEX
The heart of the multiplex section is a new third
generation phase lock loop (PLL) stereo decoder in-
tegrated circuit (IC). This PLL IC incorporates two
special systems, an automatic variable separation
control circuit to reduce background noise when
receiving weak stereo stations, and a tri-level digital
waveform generation which eliminates interference
from SCA signals and from the sidebands of adja-
cent channel FM signals.
The variable separation control is operated from
the IF amplifier's signal strength detector system. A
smooth transition is provided from mono to stereo or
visa versa at weak signal levels to provide the op-
timum signal to noise ratio and best stereo separa-
tion for the prevailing signal conditions. The circuit
operates only during stereo reception, It switches
automatically to monaural if the 19 kHz pilot tone is
absent.
In the PLL the internal oscillator operates at 228
kHz locked to the 19 kHz pilot tone. The 228 kHz
feeds a 3 stage Johnson counter via a binary divider
to generate a series of square waves. Suitably con-
nected NAND gates and exclusive OR gates produce
the tri-level drive waveform for the various de-
modulators in the circuit. The usual square wave-
forms have been replaced in the PLL and decoder
sections by tri-level waveforms. These tri-level forms
contain no harmonics which are multiples of 2 or 3.
This eliminates frequency translation and detection
of interference from the side-bands of adjacent sta-
tions since the third harmonic of the sub-carrier (114
kHz) is excluded and interference from SCA broad-
casts since the third harmonic of the pilot tone (57
kHz) is excluded. Unwanted spurious audible com-
ponents and phase jitter in the PLL with consequent
intermodulation distortion are inherently eliminated
by this technique.
Additional advantages of the phase locked loop
stereo demodulation are the elimination of induc-
tors to minimize drift, integral lamp driving capabili-
ty to indicate the presence of the 19 kHz pilot carrier,
excellent channel separation over the entire audio
frequency range, extremely low distortion, low out-
put impedance, and transient-free mono/stereo swit-
ching.
After multiplex detection, 19 kHz pilot and 38 kHz
carrier suppression circuits are used to prevent tape
recorder interference.
The FM muting circuit is unusual. It operates both
by detecting ultrasonic noise and by sensing correct
tuning of the detector circuit. To 'un-mute' it is
necessary for the signal to have an adequate signal-
to-noise ratio and to be tuned to the center of the FM
carrier. The MUTING circuit can be activated or
defeated by the front panel muting pushbutton. The
switching on and off of the audio signal is done with
FET analog switches.
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
amplifier circuit is unique. It has constant sensitivi-
ty, constant selectivity and high image rejection
across the complete AM band. Ordinary AM-RF cir-
cuits cannot do all of this simultaneously. This
design achieves equal sensitivity even down to the
low end of the band. Spurious, image and in-
termediate frequency rejection are all superior. The
same circuit delivers equal selectivity across the en-
tire band. The Mclntosh circuit is unique in a super
heterodyne AM receiver. In addition, 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 high quality loopstick anten-
na is provided and can be rotated for maximum per-
formance over 180 degrees. Each MAC 4100
loopstick is individually tuned for optimum perfor-
mance. After tuning, the loopstick is sealed. This
custom matching of the loopstick to the AM-RF front
end maximizes the performance of the loop stick
antenna.
A rear panel antenna connector is provided for
connecting an external antenna if desired. To main-
tain the excellent image rejection and spurious
cross modulation of the AM-RF amplifier, an
autodyne circuit was used for the AM mixer. The AM-
IF uses two double tuned IF transformers designed
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 frequency response of all
stations is nearly flat from 20 Hz to 3,500 Hz then
rolloff begins. Because of the active filter the 10 kHz
whistle elements are down over 30 dB or one-
thousandth of what they would be without a filter.
The automatic volume control system was designed
to prevent bursting or thumps when the AM is tuned
through a strong signal. Distortion at low audio fre-
quencies is minimizing by using a two-section filter
instead of the conventional single section.
INPUT SELECTOR SWITCHING
Low level, noise sensitive input selector switching
is done electronically using field effect analog swit-
ches. The front panel selector simply switches small
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