Specifications

PRODUCT
ANALYSIS
THE McINTOSH MR78
FM/FM STEREO TUNER
Fig. 1 — Front view
MANUFACTURER'S PUBLISHED
LIMITS (SPECIFICATIONS):
PERFORMANCE
Tuning Range: 88 to 108 MHz. Antenna Inputs: 300 ohms
balanced; 75 ohms unbalanced. Intermediate Frequency
(IF): 10.7 MHz Sensitivity: 2 mV for 35 dB of quieting;
2.5 mV at 100% modulation (± 75 kHz deviation) for 3%
total noise and harmonic distortion. Signal to Noise Ratio:
75 dB below 100% modulation. Harmonic Distortion: 0.2%
20 Hz to 15,000 Hz, mono or stereo at 100% modulation.
Typically, 0.05% at 1000 Hz. Drift: 25,000 Hz for the first
two minutes; thereafter 5,000 Hz at 25° C in 24 hours.
Frequency Response: MONO ± 1 dB 20 Hz to 20,000 Hz
with standard deemphasis, (75 mS). STEREO ±1 dB 20 Hz
to 15,000 Hz with standard deemphasis, (75 mS). Capture
Ratio: 0.25 dB detector only; 2.5 dB complete tuner. Selec-
tivity (IHF): Adjacent Channel: set switch to: normal, 7
dB; narrow, 22 dB; super narrow, 55 dB. Alternate Chan-
nel: set switch to: normal, 55 dB; narrow, 90 dB; super
narrow, 90 dB; super narrow, 90 dB. Spurious Rejection:
100 dB IHF. Image Rejection: 100 dB, 88 to 100 MHz,
(IHF). Intermodulation Distortion: 0.2% mono or stereo
for any combination of frequencies from 20 Hz to 15,000
Hz with peak modulation equal to 100% or less. Typically
0.1%. Maximum Signal Input: 12 volts across 300 ohms an-
tenna terminals will not increase harmonic or intermodula-
tion distortion. Audio Hum: 75 dB down from 100% modu-
lation. Muting: 70 dB noise reduction between stations.
Muting Threshold: (Typical): Distant position 5 mV; Local
position 20 mV. SCA Filter: 50 dB down from 67 kHz to
74 kHz; 275 dB per octave slope. Stereo Separation: 40 dB
at 1,000 Hz. Stereo Filter: (Typical): 10 dB noise reduc-
tion in Position 1; 20 dB noise reduction in Position 2.
Audio Output: Front panel controlled: 2.5 volts into
47,000 ohms. Fixed output: 2.5 volts into 47,000 ohms;
1.0 volt into 600 ohms. (All tuner performance limits were
measured with SELECTIVITY switch set at NORMAL, un-
less otherwise stated.)
GENERAL SPECIFICATIONS:
Power Requirements: 120 volts, 50/60 Hz, 35 watts. Semi-
conductor Complement: 3 J FET's. 2 MOS FET's, 17 Bi-
polar Transistors, 43 Diodes, 4 Integrated Circuits.
MECHANICAL SPECIFICATIONS:
Size: Front Panel: 16 inches wide (40.64 cm) by 5-7/16
inches high (13.81 cm); Chassis: 15 inches wide (38.1 cm)
by 13 inches deep (33.02 cm), including PANLOC shelf
and back panel connectors; Knob clearance 1-1/2 inches
(3.81 cm) in front of mounting panel. Weight: 27 pounds
(12.25 kg) net, 39 pounds (17.69 kg) in shipping carton.
Finish: Front panel; Anodized gold and black with special
gold/teal panel nomenclature illumination; Chassis: Chrome
and black. Mounting: Mclntosh developed professional
PANLOC.
Most of the semi-technical literature intended tor audio-
phile-consumer education deals with the specifications of
an FM or a stereo FM tuner as though each were mutually
independent of the others. For example, we are at once
told that "good selectivity" (the ability to tune to stations
closely spaced in frequency without encountering inter-
ference from nearby stations on the dial) and wide band-
width as well as low distortion are mutually exclusive para-
meters. We are also led to believe that a tuner "should have
ultra low harmonic or intermodulation distortion", but we
are hardly ever told that such low distortion can only be
achieved if the IF bandwidth of the tuner is sufficiently
broad (and linear in phase) to accommodate the upper
modulation sidebands of the incoming signal which may
well extend well beyond the nominal 150 kHz which is
supposed to represent the "maximum bandwidth" of a given
FM channel. But if bandwidth is broadened to accommo-
date such extreme sidebands (which occur particularly dur-
ing stereo FM transmissions), how is it possible to achieve
high orders adjacent channel (or even alternate channel)
selectivity? Excellent stereo separation at high audio fre-
quencies is also dependent upon adequate bandwidth, so
that again, such high orders of separation are in conflict
with high orders of selectivity. What's to be done? Most
tuner manufacturer's content themselves with a series of
trade-offs. Selectivity figures are made high enough (without
sacrificing low distortion and good separation) so that in
most listening areas adjacent-channel or co-channel inter-
ference will not pose much of a problem most of the time.
Mclntosh has attacked the problem in a more logical
and direct method — by providing variable selectivity on
their MR 78 tuner. But more of this shortly.
The front panel of the Mclntosh MR 78 maintains the
traditional "Mac" look of "black glass" and features that
company's well known PANLOC mounting method (shelves
and shelf mounting brackets, together with necessary
mounting hardware are, as usual, supplied) as pictured in
Fig. 1. The large, illuminated dial area at the upper center
of the panel has a linear dial scale, calibrated at every half
MHz, as well as a 0-100 logging scale for easy referencing
of favorite stations. The smooth traveling dial pointer is
Printed With The Permission Of Electronic Laboratories
28
Electronic Laboratories'