Specifications

but they have more than 40% harmonic distortion. In
this mode, the sound is grossly distorted and the extra
energy content of the clipped signal will damage most
loudspeakers. The Mclntosh Power Guard circuit
protects your ears and your speakers from this kind of
damage.
The Power Guard circuit consists of a waveform
comparator which monitors the wave shape of the
amplifier input and output signals. Normally there is no
disparity between these signals and the comparator
produces no output. When the amplifier is driven
beyond its maximum power capacity a difference will
develop. If the disparity exceeds 0.3% (equivalent to
0.3% total harmonic distortion) the comparator output
causes the amber power guard indicator to light. If
there is a further increase in the disparity the comparator
output controls an electronic attenuator at the amplifier
input to reduce the amplifier gain, thus holding the
amplifier output to a low distortion value. Overdrive
by 14dB is possible before the output distortion exceeds
2%.
SENTRY MONITOR
All power transistors have limits for the maximum
amount of power they can handle. The MC7150 output
transistors and power supply have been designed to
allow very high current flow into properly matched load
impedances. If, however, a short circuit or very low
value of load impedance is applied to the output of the
MC7150, destructive current levels could be reached
if it was not controlled by the Sentry Monitor circuit.
This circuit senses the dynamic operating time, voltage,
and current of the amplifier output stage and controls
the current flow confining it to nondestructive limits.
Sentry Monitor does not limit the power output available
from the amplifier.
THERMAL CONTROL
All power transistors have limits for the maximum
amount of heat they can tolerate. The MC7150 uses
a highly efficient amplifying circuit which produces
relatively little heat for the output power produced. The
amplifier has 4 oversized heatsinks to dissipate transistor •
generated heat. Natural convection airflow is sufficient
for cool operation. Should the cooling air be blocked
or should the amplifier operating temperature become
too high, thermal cutouts within the amplifier will turn
off the power to the amplifier. When the amplifier has
cooled, it will automatically turn on again.
TURN-ON DELAY
The MC7150 has a turn-on delay circuit that delays
amplifier operation for about 2 seconds after power turn
on. This prevents pops or thumps generated in other
equipment from causing annoying noises or damaging
your loudspeakers.
DIRECT CURRENT FAILURE PROTECTION
The autotransformer protects speakers from
damage in the event of amplifier failure. Should a direct
current component appear in the output it is shunted
by the autotransformer and DC cannot damage the
speaker.
POWER LINE INRUSH PROTECTION
Turn on inrush current is cushioned by
thermistors in the power transformer primary circuit.
A soft start is achieved that eliminates component stress
during turn-on.
CIRCUIT OPERATION
The audio input passes through the gain control
to a preamplifier. The output amplifier is driven by the
preamplifier.
The power output amplifier uses two stages of
voltage amplification followed by three stages of current
amplification. All stages are complimentary balanced.
Even number harmonics are canceled by the balanced
circuits. This means that the amplifying stages have
less total harmonic distortion and less negative feedback
is required to achieve ultra low distortion.
The signal is fed to one input of the balanced
differential stage. Feedback from the amplifier output
is applied to the other input. The differential amplifiers
drive a balanced cascode connected voltage amplifier
stage. Current mirrors are also used to improve
bandwidth and linearity.
The cascode voltage amplifier output feeds
complementary Darlington connected driver transistors.
These supply the signal to 6 complementary connected
output transistors per channel. Ancillary components
for Power Guard, Sentry Monitor, Power Output Meters
and other protection circuits interconnect with the
amplifier circuits. The power supply uses a massive
power transformer, full wave bridge rectifiers. Large
filter capacitors having 58 joules of energy storage.
Four large heatsinks provide cooling for the 12 output
power transistors.
The mechanical and electrical design of the
MC7150 is the result of the many years of engineering
and manufacturing experience held by the staff at
Mclntosh. This "know how", the meticulous attention
to design and production details, makes the MC7150
MClNTOSH LABORATORY INC., 2 CHAMBERS STREET, BlNGHAMTON, NEW YORK 13903
MC7150 Product Preview/Page 6