McIntosh MR 78, Руководство пользователя

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The MR 78 has the narrowest IF bandwidth ever used
in a stereo tuner. It is the correct width to let just one
FM station through.
The excellent selectivity of the MR 78 {210 kHz
wide at 60dB down) permits tuning stations that are
impossible to receive on ordinary tuners.
LINEAR-PHASE BRIDGE DISCRIMINATOR
The advantages of the Rimo filter would be lost if
it had to work into an ordinary FM detector. A new
detector had to be developed. It is the linear phase
bridge discriminator. The linear-phase bridge dis-
criminator uses a balanced transmission line bridge
in conjunction with a differential voltage-doubling
rectifier to acheive nearly distortionless demodulation
of FM signals. Mclntosh has a US patent on this new
FM detector.
The resistors R1 and R3 form a Wheatstone bridge
in combination with the two transmission line sections.
The RF voltages E1 and E2 at the junctions of the re-
sistors and the transmission lines will vary with fre-
quency in such a manner that their RMS arithmetic
difference (E1 - E2) is a straight-line function. Thus
these RF voltages are peak rectified by an amplitude
detector, and the difference taken, producing a dis-
tortionless recovery of the FM information.
Ordinary stereo filters use a single capacitor. The
MR 78 uses an elaborate twin-T bandpass filter. This
circuit is much more effective in suppressing noise
and still preserving the stereo image.
AUDIO PREAMPLIFIER SECTION
The audio amplifier increases the level of the pro-
gram adequate to drive a preamplifier or other acces-
sory equipment. It consists of two, two-transistor amp-
ifiers, one for each channel. The design uses consider-
able negative feedback to help achieve low distortion,
wide frequency response, and excellent stability. Each
audio amplifier delivers 2.5 volts to the FIXED OUT-
PUT jacks. A second pair of outputs are available
where level can be varied by the VOLUME control.
POWER SUPPLY
Special design attention has been given to the power
supply section. Two separate power circuits are used.
The first is a 24 volt regulated supply.
The 24volt regulator uses electronic filtering to insure
the lowest possible background hum level, maximum
stability and extremely good regulation. All signal
stages are powered from this regulator.
The second is a half wave rectifier with electronic
filter which supplies DC to the multiplex decoder.
BASIC LINEAR PHASE BRIDGE DISCRIMINATOR
Performance of the MR 78 bridge discriminator has
reduced distortion to nearly the theoretical zero. In
addition to its excellent distortion-free performance,
the bridge discriminator also exhibits a capture ratio
close to 0 dB.
STEREO DECODER
An integrated circuit audio amplifier with over 120
dB of negative feedback in two loops is used to drive
the stereo demodulator. This new circuit keeps the
distortion low in the stereo mode of operation. Highly
stable pilot recovery circuits are used to minimize
19,000 Hz intermodulation beats. MUTING, STEREO
ONLY, and AUTOMATIC STEREO/MONO switching
are functions performed within the stereo decoder.
The STEREO FILTER is connected in the audio amp-
lifier to reduce noise when listening to weak stereo
stations. Careful design of the STEREO FILTER circuit
permits an ideal compromise between channel sepa-
ration and noise rejection.
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