Technical Description - McIntosh C26 Owner's Manual

The C 26 preamplifier functions can be divided
into 5 sections. They are: phono preamplifier,
main preamplifier, power supply, center channel
amplifier, and speaker control.
PHONO PREAMPLIFIER
There are three transistors in each channel of
the phono preamplifier. The input transistor, a
high-gain amplifier, dives an emitter follower. The
emitter follower drives the third transistor which
is another high-gain amplifier. To reduce noise
and distortion the output of the third transistor is
connected by a negative feedback loop to the
emitter of the input transistor. The feedback net-
work also provides precision RIAA frequency
compensation required for magnetic phonograph
cartridges and a low output impedance for the
tape output. Feedback remains in effect even at
20 Hz, where gain is highest.
The phono input sensitivity is 2 millivolts. In the
C 26, phono input overload is virtually impossible.
For example, at 1,000 Hz, the phono input circuit
will accept 150 millivolts of signal without over-
load.
Ten millivolts of signal at the phono input at
1,000 Hz will produce 1.2 volts at the tape output.
MAIN PREAMPLIFIER
There are five transistors in each channel of the
MAIN PREAMPLIFIER. The selector switch con-
nects either the output of the phono amplifier or
a high level input to the main preamplifier.
The high level input impedance is 250,000 ohms.
The high-level input feeds through the volume con-
trol to a pair of transistors connected as high-gain
amplifier. In the left channel the second transistor
is connected in a balanced output arrangement to
provide equal amplitude signals for the phase
switch. With this arrangement the output level
does not vary when the phase switch is changed.
Negative feedback is used to reduce noise and
distortion and to provide the low impedance
needed to drive the highly selective filter networks
which follow.
The filter networks are switch controlled. The
high-frequency filter network reduces treble re-
sponse above 5,000 Hz. The low-frequency filter
reduces bass response below 50 Hz. The slope of
the filters is selected for maximum rejection of
objectionable noises. Careful design has kept the
loss of usable program material to a minimum.
The signal is then fed into the loudness and
balance controls. The loudness control is con-
tinuously variable. It may be used in conjunction
with the volume control. Rotating the loudness
control produces any loudness compensation for
a desired listening level. At 50% rotation, the loud-
ness control will be effective below 150 Hz, gradu-
ally increasing low-frequency output to a maximum
of 6 dB at 20 Hz. At full rotation, the loudness
control will be effective below 1,000 Hz gradually
increasing low-frequency output to a maximum of
15 dB at 20 Hz.
The output of the balance control is connected
to an emitter follower which is the first stage of
the tone control section. The emitter follower pro-
vides a high-input impedance required for loud-
ness and balance circuits and a low-output imped-
ance for the tone control circuits. The remaining
two transistors are connected as a high-gain ampli-
fier stage. Again negative feedback is used to
assure low distortion and, in addition, accurately
shapes the tone-control response curves and pro-
vides a low output impedance for the main out-
puts. Negative feedback is maintained at all fre-
quencies, even with the tone controls turned to
full boost.
CENTER CHANNEL AMPLIFIER (L + R)
The center channel amplifier is a single transis-
tor connected as a voltage amplifier. The main left
and right outputs are fed through mixing resistors
to the input of the voltage amplifier. The center
channel level control is connected in a negative
feedback loop around the voltage amplifier. It per-
mits adjusting the output ±6 dB compared to the
main outputs. Feedback also provides a low out-
put impedance for center channel.
POWER SUPPLY
Careful design of the power supply section in-
sures proper supply voltages for the preamplifier
circuits. A wide variation of line voltage will not
affect the D.C. voltage output of the power supply.
A series regulator transistor acts as a highly effec-
tive filter for A.C. ripple as well. The voltage regu-
lator transistor is stabilized by a zener reference
diode for constant voltage output.
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