McIntosh MC 2500, Owner'S Manual

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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 com-
parator produces no output. When the amplifier is
driven beyond its maximum power capacity a dif-
ference will develope. If the disparity exceeds 0.5%
(equivalent to 0.5% total harmonic distortion) the
comparator output causes the red LIMIT indicators
to light. If there is a further increase in the disparity
the comparator output controls an electronic at-
tenuator 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 amplifier
may be overdriven by 20 dB before the output distor-
tion exceeds 2%.
The comparator is an especially compensated
operational amplifier integrated circuit. Its output is
detected by a full wave bridge that feeds signals to
the control circuitry for the LIMIT and NORMAL in-
dicators and to the electronic attenuator at the
amplifier input. The attenuator is a light emitting
diode/light dependent resistor network selected
especially for its low distortion and time constant
characteristics.
METER CIRCUIT
The meter circuit has three basic sections: a loga-
rithmic amplifier, a full wave rectifier, and a DC amp-
lifier. In the WATTS ranges, the logarithmic amplifier
is used. In the DECIBEL ranges, the signal bypasses
this amplifier and goes directly to the full wave rec-
tifiers through an attenuator which is controlled by
the METER RANGE switch.
The logarithmic amplifier consists of a high gain
operational amplifier with a biopolar connected sili-
con diode pair as feedback elements. These diodes
have a uniform logarithmic characteristic over an 80
dB range. Only 60 dB of this logarithmic range is us-
ed in the MC 2500.
The full wave rectifier circuit uses an operational
amplifier with silicon diode feedback networks. This
amplified diode circuit has nearly perfect rectifica-
tion characteristics. One rectifier detects only posi-
tive signals. The other responds only to negative
signals and produces a positive output. The outputs
of the rectifiers are combined at the operational
amplifier output, so the highest signal, either
positive or negative, is the one that is indicated by
the meters. Gate diodes are used to charge a low
leakage capacitor which attains and holds a charge
during signal peaks. The operational amplifier pro-
vides a large amount of current so the capacitor can
charge suddenly. The charge on the peak holding
capacitor is amplified in a two transistor DC
amplifier which is used to drive the meter. From the
output of this amplifier there is a DC feedback net-
work that connects back to the detector to assure
excellent overall linearity and frequency response.
The current drive to the meters has a peaking
capacitor to accelerate the upscale response of the
meter needle. The meters also have a parallel shunt
resistor to correctly damp their action. In the WATTS
mode the discharge of the peak holding capacitor is
controlled by a resistor current source. In WATTS
HOLD, the resistor is disconnected so the peak
reading is retained. The rate of decay is about 6 dB
per minute.
POWER SUPPLY
The power supply is a conventional full wave
bridge rectifier arrangement providing plus and
minus 50 volts DC. Electronic regulators step down
and regulate plus and minus 15 volt sources for low
level circuits. Thermistors are used in the power
transformer primary circuit to limit the turn-on cur-
rent.
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