Specifications

The spectrum analyzer plots of Fig. 4 illustrates
frequency response and separation characteristics
of the tuner measured with selectivity set to the
"narrow" or normal position. We measured a
separation of 50.5 dB at 1 kHz, 48 dB at 100 Hz and
35 dB at 1 0 kHz. The lower trace in the 'scope photo
shows the cross-talk in the unmodulated channel.
The center trace shows what happens when the
multiplex filter circuit is introduced. While separation
at the frequency extremes diminishes, notice that at
mid-frequencies it is actually greater than it was
without the filter over a narrow region of frequencies
around the 500 Hz mark.
As might be expected, when selectivity is switch-
ed to the "super narrow" mode, separation suffers
somewhat, as illustrated in the 'scope photo of Fig.
5. In additon, we note the appearance of some beats
at around 9 kHz and at 1 9 kHz in the unmodulated
channel output. Again, these are some of the
tradeoffs that must be made to obtain the kind of ad-
jacent channel selectivity of which the MR 80 is
capable. And that kind of selectivity can only be ap-
preciated by the listener who, having been unable to
receive a preferred station
because of strong local
station interference, sud-
denly hears the desired
signal with absolutely no
interference.
In Fig. 6 we have
changed the sweep mode
of our spectrum analyzer
so that it is linear (in
previous 'scope photos it
was logarithmic as in-
dicated by the frequency notations at the top of the
display), and printed frequency notations should
now be ignored. The sweep is from 0 Hz to 50 kHz,
at 5 kHz per division. The tall spike at the left is the 5
kHz output from the modulated channel. Contained
within that spike is the opposite (unmodulated) out-
put from the other channel, while to the right of
these are the cross-talk products at harmonics of 5
kHz as well as any residual 1 9 kHz and 38 kHz sub-
carrier output products, all of which are about 60 dB
or more below the level of 100% modulation.
Capture ratio measured exactly 1.5 dB as claimed,
while image rejection was in excess of the 90 dB
claimed at a limit specification. We were unable to
measure alternate channel selectivity in the super
narrow mode (our equipment can only read reliably
to 100 dB) but were able to confirm all other selec-
tivity readings claimed by Mclntosh in both the nar-
row and super-narrow modes of the I-F system.
Summary and Listening Tests
Since our own local distribution of FM stations
was such that we did not run into adjacent channel
problems if we used a good directional antenna in
our listening tests, these tests were divided into two
separate parts. First, we did some off-air testing and
found that the MR 80 picked up more usable signals
than any tuner we have tested over the last three
years. The automatic blend filter action and the
Fig. 6 - Crosstalk components,
5kHz. Mclntosh MR 80
variable separation (which occurs automatically)
resulted in stereo reception of weak signals that was
completely acceptable from a noise point of view but
that would have been too noisy for pleasurable
listening with other tuners we have tested in recent
months. The "lock" circuit always yielded optimum
tuning point, as evidenced by a complete absence of
audible distortion. In short, as we said at the outset,
the MR 80 is designed to cope with the real world of
broadcasting.
As proof that it could respond well to the world of
the laboratory and in order to determine just how
selective the "Super Narrow" I-F setting was, we
conducted closed circuit experiments as well. We
used a sound Technology Model 1100A Signal Con-
ditioner to modulate our primary Sound Technology
Model 1000A stereo signal generator. The Model
1100A provides for direct tape or disc program in-
puts to the signal generator and also supplies re-
quired pre-emphasis. With this arrangement, we
were able to transmit some of our favorite disc and
tape program material both by direct cable hookup to
the MR 80 and by low-power radiation over a
distance of several yards. The experiments involved
received signal strengths ranging from around 50 uV
(39.2 dBf) to over 100,000 uV (105.2 dBf).
Dynamic range capability of the tuner, when
reproducing such non-compressed program source
material was fully up to the task. With stronger
signal strengths, we were literally unable to
distinguish between direct playback of the source
material and playback through the elaborate "closed
circuit" hookup.
To be sure, when listening to off-the-air programs,
sound quality was, in almost all cases, governed by
the less than perfect transmission and studio prac-
tices of most of our locally received FM stations. But
quieting, selectivity and capture ratio were deemed
to be superior even under these circumstances,
since these parameters are not a function of audio
quality but of RF and IF section design which, in the
case of the MR 80, are superb. One application that
may come to mind for this not inexpensive tuner is
as a receiver for FM wireless microphone transmit-
ters used in theatrical work. Since even the best of
these wireless mics tends to drift over a period of
time, the locking and tracking circuitry of the MR 80
would be an ideal tuning arrangement for such mic-
rophones. While we have not had experience in our
area with cable FM, we have been told that in some
instances, frequency accuracy and stability in such
cable FM operations is also less than ideal. In addi-
sion to its many other virtues, the MR 80 might well
provide the solution to steady tuning here as well.
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