Page 1
Mclntosh research is a continuous program of exploration for new
technology that permits performance improvement and greater value
for you. This intensive research program developed the new
technology necessary for the realization of these two new design
concepts for the MR 77.
1 THE LINEAR PHASE IF FILTER
Even since the beginning of FM, research engineers have
realized that constant delay, or linear phase, IF filters were necessary
for low distortion reception. Crude approximations to constant delay
have always been used in FM tuners - with disappointing results.
So-Called "Butterworth" or "Legandre" filters offer only a fair
compromise with respect to delay error. Crystal and ceramic filters,
usually based on the "Chebychev" model, work fairly well and give
good selectivity, but none of these are constant delay (linear-phase)
filters. Commercial tuners using these filters can show 5% stereo IM
distortion at 100% modulation, and have delay errors up to 100%. In sharp
contrast, the linear phase filter allows a tuner to show only .1% IM
distortion at 100% modulation and has a delay error of only 1.2% in
the useful bandpass. In its useful bandpass, it is a true mathematical
approximation to linear-phase - the world's finest selective,
linear-phase, minimum-phase shift filter.
A FORTRAN computer program using an algorithm that took six
years to develop was used in its design. The mathematical complexity
of the filter design procedure is almost beyond belief. Using a
process called "numerical integration in the complex S-plane," a
high speed IBM 1130 computer spent eighteen minutes on the design
of the MR 77 IF filter. A human engineer, working twenty-four hours a
day and seven days a week, would have taken 300 years to finish
this work . . . assuming he made no mistakesl
2 LINEAR PHASE BRIDGE DISCRIMINATOR
The linear phase filter would be useless if it had to work into an
ordinary FM detector. Thus a new detector with suitably low distortion
had to be developed. A search of the available literature revealed a
little-known bridge circuit with a theoretical distortion of zero!
However, designing a practical working circuit for a commercially
feasible stereo tuner took some doing. A U. S. patent is pending
on this circuit.
Distortion performance of the bridge detector is close to the
theoretical zero. In addition to its excellent distortion performance,
the bridge detector also exhibits capture ratio close to 0 dB. The
linear-phase bridge discriminator uses a balanced transmission line
bridge in conjunction with a differential voltage-doubling rectifier to
achieve 99.9% accuracy demodulation of FM signals. The only
adjustments in the MR 77 detector are two simple variable resistors.
These resistors center the tuning meter and set the transistor bias.
Complex tuning for minimum distortion is not needed.
YOU ARE PROTECTED WHEN YOU BUY MCINTOSH
Every Mclntosh instrument - every one - is tested to be equal to
or better than the performance limits advertised. When a performance
limit of 0.2% harmonic distortion is established for an instrument,
Mclntosh means that every, each, all of the instruments manufactured
must be capable of performance to that limit - or better - or your
full purchase price is refunded.
One of the reasons Mclntosh can make this promise is that we
100% test each of the instruments for maximum performance. We are
not content knowing that 10% of our instruments are tested and
meet the performance requirements established by our engineering
group. We must know that all of the instruments meet the requirements.
This vigorous pursuit of excellence takes time. At Mclntosh more
time means more care, more protection for you. There's no
production rush at Mclntosh.
"What does this mean to
investment in research, an
continuous testing; throug
urement can we promise
life, performance, highest
Mclntosh offers you a F
TRACT. During the first
dime for service. Mclntosh
guarantees labor. It costs
of a Mclntosh, the conserv
ticated appearance make
greater value when you a
to Mclntosh now.
Page 2
$699.00 Walnut Veneer Cabinet. $29.00
me?" you ask. Only through an impressive
d professional test instruments, through
product analysis; and endless meas-
and deliver to you reliability, long
value, and freedom from service.
REE THREE YEAR SERVICE ON-
hree years you can't spend one
guarantees parts and Mclntosh
you nothing. The extended life
ative ratings and the sophis-
a Mclntosh instrument a
e ready to trade. Step up
