McIntosh MAC 1900, Owner'S Manual

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plifier. Each gate of the transistor is internally pro-
tected by zener diodes against incoming transients
that may occur due to external conditions. Use of the
MOS-FET greatly enhances the cross modulation
performance over a wider dynamic range. A wider
dynamic range permits the input circuit to accept
extremely strong signals without overload.
For greater signal gain a second tuned RF am-
plifier has been designed in the MAC 1900.
The second RF amplifier consists of a junction
field effect transistor (JFET) in a common gate con-
figuration. Since both first and second RF amplifiers
have negligible internal feedback, external neutral-
ization is not required and a more stable RF am-
plifier circuit is achieved.
A mixer has been designed that uses a JFET for
high sensitivity and freedom from overload. Low
temperature co-efficient components were selected
for the FM local oscillator to prevent frequency drift.
The frequency stability inherent in the local oscillator
makes automatic frequency control (AFC) unneces-
sary. The rate of drift of the local oscillator is less
than ten parts per million per degree centigrade.
From the mixer emerges the FM signal at the 10.7
MHz IF frequency. The first stage of IF amplification
is in an integrated circuit within the RF module. Ad-
ditional gain of the 10.7 MHz IF within the RF shielded
enclosure assures a better signal to noise ratio. A
better signal to noise ratio gives your MAC 1900 the
characteristic that "it listens quieter." The integrated
circuit used as the IF amplifier in the RF front end is
a monolithic silicon differential/cascode amplifier.
The circuit is used in a cascode amplifier con-
figuration.
For optimum signal transfer and lower distortion,
a special matching transformer has been designed to
interface the FM-RF to the FM-IF amplifier. This
matching transformer considerably enhances the
linear phase characteristics of the IF amplifier.
All of the RF circuitry and the AM sections of the
variable capacitor are encased in a metal module.
Within the metal module each FM-RF section is sep-
arated by metal shielding. This extreme shielding
gives protection against radiation or interference.
The RF circuits of the MAC 1900 exceed the FCC re-
quirements for suppression of oscillator radiation.
Antenna connections for either 300 ohm twin lead
transmission line or 75 ohm coaxial cable are pro-
vided on the rear apron of your MAC 1900. The nor-
mal input impedance of the RF amplifier is 75 ohms.
Impedance match to 300 ohms is provided by a Mc-
lntosh designed balun transformer which has negligi-
ble losses. Connections for a 300 ohm line are made
with new push type terminals. No tools are required.
A type F male connector is furnished for 75 ohm
coaxial cable.
The FM-IF consists of two integrated circuits and
two quad-tuned, link coupled, filters. They combine
to give a total gain of over 120 dB (the signal is ampli-
fied to over 1,000,000 times its original level.) Each
integrated circuit contains 16 transistors, 3 zener
diodes, 5 diodes and 23 resistors, all on a single
monolithic silicon chip. The response curve has
nearly linear phase characteristics. The skirts of the
response curve are very steep. The maximum width
is 170 kHz at —3.0 dB and 500 kHz at —60db. The
response curve is symmetrical each side of the
center frequency. The filters are permanently sealed
and do not require adjusting. The IF cannot drift nor
vibrate out of adjustment. The exceptionally high
gain of the two integrated circuits 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 discrimi-
nator is extremely low in distortion content. De-
emphasis of the discriminator output restores the
frequency amplitude characteristics to the same level
they were before transmission.
FM STEREO MULTIPLEX SECTION
Mclntosh Laboratory has developed a special de-
tecting circuit in the multiplex section. A particular
advantage of this circuit is the elimination of the
critical adjustments necessary with commonly used
matrixing circuits. The circuit detects L - R side-
bands, then automatically matrixes the recovered
information with the L + R main carrier signal. This
yields the left and right program output with maxi-
mum separation.
The 19,000 Hz pilot signal is filtered from the com-
posite 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 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 com-
ponents in the output.
The SCA (Subsidiary Communication Authoriza-
tion) 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 MAC 1900 operates by detect-
ing 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 pushbutton on the front panel.
When the 19,000 Hz carrier of a stereo signal is re-
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