Technical Description - McIntosh C27 Owner's Manual

Technical Description

The accompanying BLOCK DIAGRAM indicates the
various signal paths within the C 27. Circuits represented
by each block are discussed below.
INPUT SELECTOR SWITCH
The INPUT SELECTOR routes the various input signals
within the C 27. The switch is robust in construction and
has large double wiping silver contacts. Isolating networks
at each high level input and shorting switch action ground
all unused inputs preventing undesired program feed-
through. The same isolating networks block any DC that
might be present on a high level input to eliminate pops
or clicks when changing inputs.
PHONO PREAMPLIFIER
Each channel of the phono preamplifier uses three
discrete transistors especially selected, and circuit op-
timized, to produce low noise and distortion free operation.
Of particular benefit is the use of low impedance com-
ponents in the RIAA feedback equalization networks. The
network drive is provided by the final stage transistor
which operates at an adequate current level to drive the
low impedance without slew rate limiting.
MONITOR AND TAPE COPY SWITCHES
Tape switching uses pushbutton switches. The switches
are mounted directly to a printed circuit board that in-
corporates the necessary inter-connection wiring. Isolating
networks are included at each tape input and output
which eliminates the possibility of DC leakage from asso-
ciated tape recorders which prevent pops or clicks when
switching. The pushbutton switches have reliable push-
push action ratchets to prevent switch hangup. Light
emitting diode (LED) lamps are activated by the switches
to indicate the particular switch function being used.
MODE SWITCH
A seven-position MODE SELECTOR switch allows routing
of the left and right channel signals in various combina-
tions for system balancing and operation. The MODE SE-
LECTOR functions only in the signal path directed through
the MAIN OUTPUT jacks and does not control the TAPE
OUTPUT.
VOLUME CONTROL
The VOLUME control is a precision step attenuator
designed and manufactured for Mclntosh Laboratory. It is
a 32-step control with a 70 dB range, plus volume off.
Left and right channel tracking are within 1 dB. This ex-
treme accuracy is obtained through special electronically
controlled resistance element trimming. Since the switch
commutator touches only contact pads and not the preci-
sion resistor elements, tracking accuracy is permanently
maintained and is noise free.
VOLTAGE AMPLIFIER
A voltage amplifier with 14 dB gain follows the VOLUME
control. This amplifier plus the following amplifier with
6 dB gam constitute the main amplifier section of the C 27.
The amplifier has high input impedance, low output im-
pedance, very low noise level and wide dynamic range.
HIGH FREQUENCY AND LOW FREQUENCY FILTERS
The HF and LF FILTER networks are switch controlled.
The high-frequency filter network reduces treble response
above 5,000 Hz. The low-frequency filter reduces bass
response below 50 Hz. The slope of the filter frequency
response is selected for maximum rejection of objectionable
noises. Careful design has kept the loss of usable program
material to a minimum.
LOUDNESS CONTROL
The LOUDness control is continuously variable. Full
volume and flat frequency response is obtained in the
counterclockwise or FLAT position of this control. As the
control is advanced clockwise the volume level reduces
but loudness compensation is introduced which maintains
the low-frequency output at a higher level than the mid
and high frequency output. This action complements the
human ear response characteristics, the Fletcher Munson
effect. Loudness compensation is provided for a wide range
of listening levels.
BALANCE CONTROL
Rotating the BALANCE control to the left or right allows
you to balance the level of the two channels. At the de-
tented center position the gain of the two channels is equal.
BASS AND TREBLE CONTROLS AND ASSOCIATED
AMPLIFIER
A high gain three stage linear amplifier is used in the
bass and treble control circuits. Negative feedback reduces
the gam to 6 dB when the controls are set flat. The feed-
back path incorporates the control response shaping net-
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