Specifications

works a bit differently from that contained in the C 32.
The user is instructed to set the volume control for
maximum (lifelike) listening levels while the loudness
knob is set to its "flat" (counterclockwise) position.
Then, for lower listening levels, the loudness control is
rotated (rather than the volume control) and proper
Fletcher-Munson compensation is introduced as listen-
ing levels are lowered.
Completing the panel layout is a group of push-
buttons and a headphone jack at the lower left. The
buttons, five in all, take care of the two tape monitor
circuits and provide tape dubbing from either of two
connected tape decks to the other.
The final button turns on power to the unit and has
an illuminated rectangle just above it to indicate that
power is on.
Fig. 2 - View of back panel
The rear panel of the C 27 is shown in Fig. 2. Adja-
cent to the four switched and one unswitched conven-
ience AC receptacles are three sets of spring-loaded
terminals which require only that the stripped ends of
power amplifier and speaker connecting cables be in-
serted in small holes that are exposed when the ter-
minal keys are depressed. It is this arrangement which
brings speaker switching capability to the front panel of
the C 27, and a handy feature it is, too, since most high
quality basic power amplifiers are normally equipped
with only one set of speaker terminals. The right portion
of the rear panel is equipped with the necessary phono-
tip jacks for signal inputs, tape in and tape out circuits,
two pairs of main output jacks (in case you want to
feed two separate power amplifiers) and a "center
channel" output for feeding the sum (L+R) of both
channels to a separate monophonic power amplifier for
background music in an alternate location or for power-
ing a center-channel speaker system in the main listen-
ing area. A 0.5 ampere line fuse holder completes the
rear panel layout.
Circuit Highlights
Internally, the C 27 major circuit board contains all of
the audio signal handling active circuitry, including the
preamplifier-equalizer low level stages. Smaller p.c.
modules include the pushbutton switch circuits, input
terminal boards, a switch indicator circuit board (for il-
luminating the front panel LED's that light up when
tape monitoring or tape copying is employed) and a
power supply module. Signal paths can be traced by
consulting the block diagram of Fig. 3. The input selec-
tor routes the different input signals within the C 27.
Isolating networks are present at each high level input
and shorting switch contacts ground unused inputs.
The isolating networks also block any DC voltages that
might be present, preventing pops or clicks when
changing inputs. The phono preamp section uses three
selected transistors per channel. Low impedance com-
ponents are used in the RIAA equalization network,
drive for which is provided by the final stage which
operates at an adequate current level to drive the low
impedance without slew rate limiting. Isolating net-
works are included at each tape output and input, The
volume control is a step attenuator type having 32
steps and 70 dB of range plus a total attenuation posi-
tion. A voltage gam stage follows the volume control
and provides 14 dB of gain. High and low cut filters
follow, with the signals then passed along to the loud-
ness and balance control and thence to another gain
stage for an additional 6 dB of amplification. This last
gain section is used for the bass and treble control cir-
cuits and consists of a three-stage linear amplifier with
tone controls included in a negative feedback circuit us-
ing precision capacitors and resistors for shaping the
desired frequency response characteristics depending
upon the setting of the tone controls. The output of this
amplifier is fed to the two pairs of main output jacks.
The C 27 power transformer uses a grain oriented
silicon steel core plus copper and magnetic steel
shields to eliminate radiated magnetic hum fields. The
power supply uses zener diode reference voltage regu-
lation and electronic filtering. Turn-on and turn-off time
characteristics of the power supply are controlled to
prevent switching transients. Total semiconductor
complement of the C 27 includes 18 silicon planar
transistors, 4 silicon diodes and five light emitting
diodes.
Laboratory Test Measurements
All of our measurements were made with respect to
rated output (2.0 volts from the main output terminals)
unless otherwise stated. We first studied the perform-
ance of the phono-equalizer section. Input sensitivity
for rated output measured 2.3 millivolts for both phono
inputs. Though Mclntosh does not provide a "limit
spec" for phono overload, we consider this to be an
important specification. The phono inputs were able to
handle signal levels of 1 35 millivolts at 1 kHz before
noticeable first-stage distortion occurred — considera-
bly more than is likely to be delivered even by a high
output cartridge tracing the most heavily modulated
record grooves. Signal to noise in phono, referred to a
10 mV input measured 82 dB unweighted, increasing
to 89 dB when an "A" weighting network was insert-
ed in the patch between the output and the measuring
instruments. A word is called for regarding the play-
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