Technical Description 17 - McIntosh MC 7270 Owner's Manual

INPUT AMPLIFIER
Each channel input contains a complete seven transistor
low power amplifier. A differential transistor pair provides
high input impedance with low noise. The differential
signals are combined in a current mirror circuit which
drives a class A amplifier stage. The following output
stage is a complimentary pair of transistors with class AB
biasing. The output signal drives the metering circuit and
the high power output amplifier. This discrete transistor
input amplifier design has low noise, low distortion and
freedom from turn-on/turn-off transients.
The INPUT LEVEL selector and GAIN controls (as passive
attenuators) which precede the input amplifier, prevent in-
put overloaded when the controls are correctly set.
In the MONO BRIDGE mode of operation the input
signal feeds only the right input amplifier using the
RIGHT/MONO GAIN control. The output of the right input
amplifier feeds both output power amplifier sections with
the left input amplifier as a phase inverter to the left out-
put amplifier. The phase inversion puts the output
amplifiers 180° out of phase with respect to each other
to provide a bridge output. The channels, of course,
operate in phase for the STEREO mode.
OUTPUT POWER AMPLIFIER
The output of the input amplifier is fed to the power
amplifier input through a junction FET electronic switch.
The control signal to the switch is held for about one se-
cond to eliminate turn-on/turn-off transients and prevent
transients that may originate in the source equipment
from being amplified and fed to the loudspeakers.
The first stage of the output power amplifier is a dif-
ferential transistor pair biased for best linearity. The offset
to the differential pair is adjustable. Correct adjustment
assures the lowest possible distortion at low frequencies.
In each channel, current mirror circuit combines the dif-
ferential outputs into one signal which is amplified by a
class A voltage amplifier. Both the differential transitors
and the voltage amplifier are supplied by active current
sources. The results are lower distortion and cleaner turn-
on characteristics.
The driver stage consists of a complimentary pair of
power transitors biased class AB. Three complimentary
pairs of rugged power transistors mounted on over-sized
anodized aluminum heat sinks, make up the power out-
put stage. A unique design of the bias network permits
the output transistors to operate class B but prevents the
crossover distortion associated with class B operation. The
heat sinks remain cool when there is no output.
The amplifier output signal is fed to the output terminals
through the output autotransformer. The Mclntosh design-
ed interleaved multifilar wound autotransformer is used to

TECHNICAL DESCRIPTION 17

properly match the amplifier to stereo output load taps
for 1, 2, 4 and 8 ohms. The MC 7270 will deliver full
power over the entire audio frequency range at any of
these impedances. The autotransformer also protects
speakers from damage in the event of amplifier failure.
Should a direct current component appear in the output it
is shunted to ground by the autotransformer. DC cannot
damage the speaker.
A Mclntosh patented Sentry Monitoring circuit constantly
monitors the output signal and instantly reacts to prevent
overload of the output transistors. At signal levels up to
design maximum the circuit has high impedance and has
no effect upon the output. Should the power output ex-
ceed design maximum, the Sentry Monitoring circuit
operates to limit the signal to the output transistors. In
the event of a short circuit across the amplifier output or
severe impedance mismatch the Sentry Monitoring circuit
will protect the output transistors from failure. Positive
and negative halves of the output signal are independent-
ly monitored and protected.
POWER GUARD PROTECTION CIRCUIT
The Mclntosh patented Power Guard circuit eliminates
amplifier clipping due to overdrive. The circuit illuminates
amber POWER GUARD indicator lamps when the
amplifier drive trys to exceed the maximum output capaci-
ty. Power Guard prevents loudspeaker damage and
eliminates harsh output distortion caused by amplifier clip-
ping.
In the Power Guard circuit, the output waveform is com-
pared to the input waveform. As long as there isn't any
disparity between these signals, the circuit hasn't any in-
fluence on the performance of the amplifier. Should the
amplifier drive try to exceed the amplifier's maximum
power capacity, a difference will develop. If the disparity
exceeds 0.03% on the average, (equivalent to 0.3% total
harmonic distortion) the difference causes the amber
POWER GUARD indicators to light. A further increase in
the disparity, controls an electronic attenuator at the
amplifier input to reduce the amplifier gain, thus holding
the amplifier output to its maximum undistorted value
regardless of the degree of overdrive to the amplifier.
The difference signal is fed to a specially compensated
operational amplifier integrated circuit. Its output is
detected by a full wave bridge that feeds signals to the
control circuitry for the POWER GUARD indicators and to
the electronic attenuator at the amplifier input. The at-
tenuator is a light emitting diode/light dependent resistor
network selected specially for its low distortion and time
constant characteristics.