Specifications

the de-emphasis switches, an electronically switch-
ed filter circuit is used to reduce out-of-phase noise
when receiving weak signal stereo. The filter is ac-
tually a twin-T bandpass that blends high and low
frequencies, but leaves separation unaffected at
mid-frequencies for improved stereo imaging when
the filter is required. LC notch filters further reject
any residual 19 kHz or 38 kHz sub-carrier output
products. A separate headphone amplifier, capable
of driving low impedance phones, also serves as the
main output amplifier.
Tuning, Scanning and Control Circuitry
A detailed explanation of the touch sensor, preset
scanning, control logic, scan circuit and lock circuit,
as well as the frequency counter and power supply
circuitry is provided in the excellently written
owner's manual supplied with the MR 80, and full
schematic diagrams are also included, for the benefit
of the technically oriented user or for possible servic-
ing needs. The descriptions of the remaining non-
RF/IF related circuits are too lengthy for even an ab-
breviated treatment here, but we cannot leave this
subject without providing a brief explanation of the
unusual "lock" circuitry designed into this unusual
tuner. This new circuit wilt be correctly tuned even if
the station (or, more likely, the cable company) is not
on its correct frequency. Two operational amplifiers
are used. A deviation signal from the detector is fed
to the first amplifier, which produces an output
voltage proportional to the logarithm of the DC com-
ponent in the detector output. A second amplifier,
connected as a switched gain low-pass filter, re-
moves any audio signals present. The filter output,
which is a correction voltage, is fed into a scaling cir-
cuit that compensates for the tuning diodes' non
linear frequency to voltage response. Both amplifiers
operate with more than 50 dB of gain at DC. So,
with a closed loop gain of more than 1 00 dB, tuning
error (when locked) is less than 1 kHz at 1 00 MHz.
This error is less than that obtained with most fre-
quency synthesized circuits and provides the addi-
tional benefit of correct tuning even if the station or
cable signal is not on proper frequency.
Since this circuit will "track" a station even if it
drifts by more than 1 MHz in either direction, the
user must be able to defeat the lock easily. The
touch sensor switching arrangement on the manual
tuning knob takes care of this and, to prevent the
tuner from locking onto a strong signal next to a
weak signal, a circuit is used to sense strong adja-
cent channel signals and to inhibit the lock circuit in
such circumstances. The lock on/off switch on the
top surface of the unit, incidentally, will cancel the
lock only insofar as the manual tuning knob is con-
cerned. The lock circuit continues to work for all the
preset signals and for the scan circuits.
Laboratory Measurements
The multi-purpose graph of Fig. 3 shows the mono
and stereo quieting and distortion (at 1 kHz) charac-
teristics of the MR 80 tuner with the selector switch
set for "narrow" (normal) selectivity. Usable sen-
sitivity in mono measured 12.0 dBf (2.2 uV) while
for stereo, the usable sensitivity was a very low 20
dBf (5.5 uV). The 50 dB quieting point was reached
with input signals of 3.5 uV (1 6 dBf) in mono and 30
dBf (17.4 uV) in stereo, the stereo result being about
the lowest we have ever measured for any stereo FM
tuner. At 65 dBf of input signal strength, signal-to-
noise ration measured 76 dB in mono (as opposed to
75 dB claimed by Mclntosh) and 71 dB in stereo.
Distortion at that strong signal level was the same in
mono and stereo, a low 0.1 2% for 1 kHz signal.
Distortion (harmonic) versus modulation frequen-
cies, for both mono and stereo operation of the
tuner, were with selectivity set to the normal or
"narrow" position. We measured specific values of
0.12% at 100 Hz for mono, and 0.1 5% for that fre-
quency in stereo. At 3 kHz, mono distortion was a bit
higher than that measured in stereo; 0.18% as
against 0.1 3% but at the highest required test fre-
quency of 6 kHz, stereo THD came close to the 0.2%
limit while mono distortion was again a low 0.1 %.
We should point out that when the "super
narrow" selectivity setting is used, distortion does
rise rather significantly, approaching the 1.0%
mark, but this is a tradeoff that was most deliberate-
ly sought by Mclntosh Laboratory's designers. There
is just no other way to achieve adjacent channel
selectivity of 60 dB without increasing distortion in
the stereo mode where sidebands of high frequency
modulating signals extend well beyond the single
channel width of 100 kHz to either side of center
carrier frequency. What is remarkable, in fact, is that
Mclntosh was able to achieve this sort of adjacent
channel selectivity and still keep the distortion level
in stereo under 1.0%! It should be noted, incidental-
ly, that noise performance, or signal-to-noise ratios
remain essentially the same in the "super narrow"
setting as they were in the narrow or normal selec-
tivity mode.
Fig 5 - Frequency response and
separation, McIntosh MR 8O. with
selectivity switch in "Super Narrow"
position.
36
Fig. 3 Mono and stereo quieting and distortion characteristics, FM section.
Fig. 4 Frequency response and
separation, McIntosh MR 80 with
selectivity switch in "Narrow" (normal
position).