Specifications

back curve measured via the phono inputs and shown
as photographed from the face of our spectrum ana-
lyzer's CRT tube. Mclntosh has wisely elected to adopt
the new IEC standards for playback which call for an
additional roll-off time constant of 7960 microseconds
in addition to the three time constants normally pre-
scribed for R1AA equalization. This final low-end roll-off
makes a great deal of sense, since it reduces needless
amplification of turntable rumble components which
serve no purpose other than to overload the bass
power output capabilities of modern power amplifiers
(especially those that have response down to "DC")
and to cause speaker cones to fluctuate wildly sending
the voice coils into non-linear regions of operation. The
final low-end roll-off or turnaround of the new curve is
clearly visible in our 'scope photo of Fig. 4 and, al-
Mclntosh does not quote IM distortion figures for the C
27, we nevertheless measured this important
parameter as well and, for rated equivalent output from
the main output terminals obtained a reading of just
Fig. 4 - Playback response of
phono preamp-equalizer section
though our frequency sweep extends only down to 20
Hz, the response continues to roll off below that fre-
quency at a 6 dB per octave rate, exactly as prescribed
by the new IEC standards. If you try to measure the
phono response against the "old" RIAA curve, how-
ever, you will find that at 30 Hz it differs from the old
prescribed point-by-point plot by approximately 1 .3
dB In terms of the new and preferred playback curve,
the equalization of the C 27 was so accurate that it
varied by no more than 0.1 dB at any of the remaining
test points measured.
Fig. 5 is a plot of distortion versus frequency (at rated
output) via the high-level inputs, as observed at the
main output terminals. Over the entire spectrum from
20 Hz to 20 kHz, harmonic distortion never exceeded
0.025% and at mid frequencies it measured an ex-
tremely low 0.009%. Since the distortion measured
from the phono inputs to the tape outputs was even
lower, we did not bother to plot these results since the
high-level section would govern actual distortion of the
output signals under actual use condition. Though
Fig. 5 - Distortion versus frequency, for rated
output (2.0 volts) aux input to main outputs
under 0.04%. Maximum output obtained from the
main output terminals was 1 0 0 volts for a rated distor-
tion figure of 0.05% harmonic.
Tone control action of both the bass and treble con-
trols is so precise and well calibrated that we decided to
trace overall response for each of the ten possible set-
tings of each of those controls. The results are dis-
played in the 'scope photo of Fig. 6. Flat response is re-
Fig. 6 - Step-by-step tone control
response characteristics
presented by the center curve of this series and was
measured as flat within 0.5 dB from 1 0 Hz to 23 kHz.
The -3 dB points in response occurred at 4 Hz and 60
kHz. Signal to noise ratio in the high level settings
measured 86 dB below rated output, unweighted, and
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