Specifications

variable output jacks. The power supply section consists of
a 24-volt regulated supply which uses electronic filtering
and supplies power to all signal stages, while a second, half-
wave rectifier supply also equipped with electronic filter
circuits feeds the necessary DC voltages to the multiplex
decoder section.
Laboratory Measurements
Although most listening situations require selectivity
settings of NORMAL on the MR 78, we decided to measure
performance in this setting as well as the NARROW selectiv-
ity setting. In the NORMAL setting, IHF sensitivity meas-
ured 2.0 mV (11.2 dBf) and 50dB quieting was reached with
an input signal strength of 3.0 mV (14.7 dBf) in mono.
Referring back to the published specifications, readers
will note that Mclntosh has not as yet seen fit to bring their
specs into line with the newly adopted IHF/IEEE/EIA FM
Tuner Measurement standards. Thus, no claim is made for
the 50 dB quieting sensitivity, for example, and Mac offers
us a 35 dB quieting point instead. While we recognize that
Mclntosh has, in the past, differed with the rest of the in-
dustry in the matter of publishing specifications, we do feel
that the new tuner standards are worth following and would
hope that if they have not already done so, Mclntosh would
bring their published specs into line so that they might be
easily compared with those of other companies. Be that
as it may, 3.0 mV (14,7 dBf) is a very respectable figure for
50 dB quieting. Stereo sensitivity was 4.5 mV (18.3 dBf),
at which signal strength in stereo was sufficient to cause
nearly 35 dB of quieting. 50 dB of quieting in stereo was
attained with an input signal of 32 mV (35.3 dBf). Ultimate
S/N in mono was an incredibly high 81 dB (we never
thought our signal generator could read that low — now we
know it can), while in stereo, best quieting for strong
signals was 73 dB. Total harmonic distortion in mono was a
low 0.06%, while in stereo, for the same strong signals used,
THD read 0.1% at 1kHz. Curves of results obtained in the
Normal selectivity setting are plotted in Fig. 6.
Switching to the NARROW selectivity setting, S/N read-
ings in both mono and stereo remained virtually the same,
but, as was to be expected, THD readings increased slightly,
to 0.08% for mono and 0.35% for stereo. These results are
shown in Fig. 7.
Returning to the NORMAL selectivity setting, we meas-
ured a capture ratio of 1.8 dB. Alternate channel selectivity
was 57 dB for this setting, while image and spurious rejection
were both in excess of 100 dB (the limits of our test equip-
ment). Maximum deviation from ideally flat frequency re-
sponse, for both mono and stereo, was less than 1.0 dB,
with the deviation approaching that number at 1 5 kHz but
remaining within 0.2 dB at frequencies from 10 kHz down
to 50 Hz. Muting threshold occurred at 7.0 mV for the "dis-
tant" position, 30 mV for the "local" position. Stereo
switching occurred at around 4.0 mV (17.2 dBf).
Stereo separation measured 52 dB at mid frequencies for
the normal selectivity position, decreasing to 43 dB at 100
Hz and 36 dB at 10 kHz, as plotted in Fig. 8. In order to
properly ascertain the THD produced by the tuner at high
modulating frequencies it was necessary for us to employ
our spectrum analyzer and to "sum" the harmonic products
mathematically to arrive at the 0.14% THD figure shown
for a 10 kHz modulating frequency. If this is not done,
super-audible products (not properly identifiable as har-
monic distortion) "cloud" the single-reading measurement
that is obtained on a conventional meter-type distortion
analyzer.
Fig. 9 is a plot of separation versus frequency with the
selectivity control set to the NARROW position. Separation
naturally suffers somewhat when this narrow setting is
used, but remains well above 30 dB for most frequencies
tested. Distortion in stereo also rose somewhat in the narrow
setting, as shown in the curves of Fig. 9. While in mono,
THD remained incredibly low at mid-frequencies even when
the narrow setting of the selectivity switch was used.
Sub-carrier product rejection was so good that it was not
even necessary to use the recommened 15 kHz low-pass
filters for all of our high-frequency noise and distortion
measurements, except as already noted for the 10 kHz
readings. Audio output level was exactly 2.5 volts, as
claimed, for a 100% modulation signal at the fixed output
terminals. Since our own signal generating equipment is
limited to 0.2 volts output, there was no way for us to
verify Mclntosh claims regarding 12 volt antenna input
overload capability, though we have no reason to doubt
the claim, based on subsequent strong-signal listening tests.
We purposely carted the tuner over to a mid-city location
where we have access to a listening room that is just a few
blocks away from several high-powered FM transmitters.
Fig. 7 - FM quieting and distortion characteristics
(with selectivity switch set to "NARROW")
30
INPUT - MICROVOLTS ACROSS 300 OHMS
Mono: (1) Total Harmonic Distortion [THD) (2) Signal To Noise Ratio (S/N)
Stereo. (3) Total Harmonic Distortion (THD) (4) Signal To Noise Ratio (S/N)
Fig. 6 — FM quieting and distortion characteristics
(with selectivity switch set to "NORMAL")
INPUT - MICROVOLTS ACROSS 300 OHMS
Mono: (1) Total Harmonic Distortion (THD) (2) Signal To Noise Ratio (S/N)
Stereo: (3) Total Harmonic Distortion (THD) (4) Signal To Noise Ratio (S/N)