McIntosh MC 7270, Owner'S Manual

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METER CIRCUIT
The meter circuit has three basic sections: a logarithmic
amplifier, a full wave rectifier, and a DC amplifier.
The logarithmic amplifier consists of a high gain opera-
tional amplifier with a bipolar connected silicon 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 used in the MC 7270.
The full wave rectifier circuit uses an operational ampli-
fier with silicon diode feedback networks. This amplified
diode circuit has nearly perfect rectification characteristics.
One rectifier detects only positive 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 provides a large amount
of current so the peak holding capacitor can charge sud-
denly. The charge on the peak holding capacitor is
amplified in a two transistor DC amplifier which is used to
drive the meter. The output of the amplifier has a DC
feedback network connected to the detector to assure ex-
cellent overall linearity and frequency response. The cur-
rent drive to the meters has a peaking capacitor to ac-
celerate 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 cur-
rent source. In WATTS HOLD, the resistor is disconnected
so the peak reading is retained. The rate of decay in
WATTS HOLD is about 6 dB per minute.
POWER SUPPLY
The power supply is a full wave bridge rectifier arrange-
ment providing plus and minus 52 volts DC. Electronic
regulators step down and regulate plus and minus 15 volt
sources for low level circuits and plus 5 volts for the logic
circuits. Thermistors are used in the power transformer
primary circuit to limit the turn-on current.
18 TECHNICAL DESCRIPTION