McIntosh MC500 Owner's Manual page 14

POWER GUARD
TECHNICAL
The MC500 includes the unique patented Mclntosh POWER GUARD circuit on each chan-
DESCRIPTION
nel. POWER GUARD eliminates the possibility of ever overdriving the amplifier into clipping.
Clipping occurs when an amplifier is overdriven past its output design capabilities. An over-
driven amplifier can produce both audible and ultrasonic distortion levels approaching 40%.
The audible distortion is certainly unpleasant, but the ultrasonic distortion is also undesirable,
since it can damage tweeter loudspeakers.
The Mclntosh POWER GUARD circuit acts as a waveform comparator, monitoring both the
input and output signal waveforms. Under normal operating conditions there are no differences
between these waveforms. When an amplifier is overdriven beyond its maximum distortion
free output, there will be a difference between the two signal waveforms. If the difference
exceeds 0.3% (equivalent to 0.3% harmonic distortion), the amber POWER GUARD indicator
will light. If the difference continues to increase, the POWER GUARD circuit controls an elec-
tronic attenuator at the input of the amplifier to reduce the gain just enough to prevent any
further increase in distortion. Distortion will not exceed 2% with as much as 14dB overdrive.
Without POWER GUARD, the distortion could easily reach 40% for the same operating
conditions.
A Mclntosh power amplifier with POWER GUARD will always deliver its maximum distortion-
free output. This power is always well above the rated power due to the Mclntosh philosophy
of conservative design. You will never experience the harsh and damaging distortion due to
clipping when using a Mclntosh MC500 amplifier with POWER GUARD.
THE VERSATILE McINTOSH OUTPUT AUTOFORMER
All transistor power output circuits work best into what is called an optimum load. Depend-
ing on the number and configuration of transistors used, this optimum load may vary con-
siderably from what a loudspeaker requires. In the case of more than one loudspeaker con-
nected in parallel, the impedance load reflected to the power amplifier may drop to two ohms
or even less.
A power amplifier connected to a load that is lower than optimum causes extra current
to flow and extra heating of the power transistors. This means the amplifier will run much
hotter, with a corresponding reduction in operating life.
The output signals from the two channels in the MC500 are coupled into the autoforrner
at the point on the windings that is an optimum impedance match for the transistors in the
output circuit. The MC500 amplifier output circuits are always operating at their highest possi-
ble efficiency.
The MC500 autoformers include output impedance taps for exact loudspeaker matches
of 2 ohms. 4 ohms and 8 ohms. Loudspeaker performance will always be at its best Any
Mclntosh power amplifier with an autoformer is especially well suited for stereo installations
that require multiple speakers, since it is possible to have a very close impedance match.
The frequency response of a Mclntosh autoformer exceeds that of the output circuit itself,
and extends well beyond the audio range. Its distortion level is so low it is virtually impossible
to measure. The Mclntosh Autoformer is quite simply a significant engineering advance in
power amplifier performance and reliability. The unequaled expertise of Mclntosh in the design
and manufacturing of autoformers is legendary in the high fidelity industry.
In the rare event of output circuit failure, the Mclntosh autoformer will conduct any speaker
damaging DC (direct current), directly to circuit ground. This will protect your valuable
loudspeakers from possible damage.
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