Technical Description - McIntosh MC 502 Owner's Manual

Technical Description

While the MC 502 performs a rather elementary
function as a basic stereo power amplifier, it does
contain an number of rather sophisticated circuits.
Each channel consists of an input preamplifier, a
power amplifier section, three separate protecting
control circuits, and a phase inverter which is in-
tegral with the left channel. The power supply in-
cludes positive and negative electronic regulators. A
description of each circuit follows:
Preamplifiers
Each channel's preamplifier is a high perfor-
mance low noise operational amplifier arranged for
a stage gain of 4.5 times. The input signals to the
MC 502 first pass through the input level switch at-
tenuator, then to the left or right/mono gain controls,
next to electronic attenuators which are part of the
Power Guard circuit to be described later and then
to the preamplifiers. The left channel input path also
includes the mode switch which, in stereo, connects
to the normal stereo signal path or, if switched to
mono, directs the output from the right channel
preamplifier to the inverting input of the left channel
preamplifier. This mono arrangement drives the
following power amplifier stages in phase opposi-
tion so that mono output power is derived as the
sum of the left and right power channels.
Power amplifier sections
Each of the identical power amplifier sections
consist of 4 stages of amplification. At the input is a
differential transistor pair optimised for low noise.
Input and feedback signals are applied to this dif-
ferential amplifier. The two outputs of the differen-
tial amplifier are combined in a current mirror and
the resulting signal drives the second stage, a class
A voltage amplifier. The second stage output feeds
complementary emitter followers which in turn drive
the output transistors which are also complemen-
tary emitter followers. Biasing for the driver and out-
put transistors is accomplished in the base circuit of
the driver transistors using a temperature sensitive
transistor thermally coupled to the output transistor
heat sinks. Cool efficient operation is thereby ob-
tained.
Protection control circuits
Three control circuits are used. The first control
circuit is the Sentry Monitor which senses the cur-
rent flow in the output stage transistors. The sense
characteristic is shaped to confine current and
voltage conditions
characteristics of the output stage power tran-
to the safe operating
sistors. This protection greatly contributes to the
reliable, long life, performance of the MC 502.
Power Guard is the second control circuit.
Amplifiers are capable of delivering large quantities
of power when they are driven to clipping. Clipping
is caused when the amplifier is asked to produce
more power output than it can deliver with low
distortion. A clipped amplifier can have more than
40% harmonic distortion. The extra energy content
of the clipped signal will damage most loud-
speakers, particularly delicate high frequency
tweeters. A new Mclntosh advancement helps pro-
tect your speakers from this kind of damage. The MC
502 has a built-in waveform comparator which com-
pares the wave shape of the output signal to the in-
put signal. If the disparity between the two signals
exceeds 0.5% (equivalent to 0.5% total harmonic
distortion) a red limit indicator illuminates. With any
further increase in distortion the Power Guard cir-
cuit will operate. This circuit limits the input
dynamically so that the amplifier cannot be over-
driven. Power Guard eliminates amplifier output
clipping.
Power Guard does not limit the dynamic range or
the power output of the power amplifier. Clipping oc-
curs when an amplifier is asked to exceed its design
limits and the capacity of the power supply. Since
Power Guard does not begin to work until this point
is reached, the power capability of the amplifier is
never affected.
The third control circuit is the speaker protection
and turn on delay arrangement. In most direct coupl-
ed circuits a failure of any transistor in the power
amplifier will cause a DC potential to appear in the
output. To assure that no damaging or interfering
DC appears across the output terminals, a special
very fast acting protector circuit constantly
monitors the output for DC. If, at any time, a cons-
tant DC level appears, the speakers are discon-
nected. The protective circuit reacts in milliseconds.
Speakers remain disconnected until the cause has
been fixed. Integral with this circuit is the transient
free turn on and turn off controller. The power
amplifier output signals pass through a heavy duty
relay before reaching the output terminals. The relay
is controlled by an electronic circuit which closes
the relay approximately two seconds after the power
switch is turned on and releases the relay almost in-
stantly when the power is turned off.
The Sentry Monitor, Power Guard, and speaker
protector circuits were developed by Mclntosh
engineers and are each covered by U.S. patents
assigned to Mclntosh Laboratory.
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