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Input Test Signal
Figure 14
Without Power Guard
Figure 15
Top Inside View of the MA6900
Figure 12
Figure 13
ance points of two ohms, four ohms and eight ohms. The
unequaled expertise of McIntosh in the design and manu-
facturing of autoformers is legendary in the high fidelity
industry.
The high efficiency circuit design of the MA6900 con-
tributes to low operating temperatures. More than 621
square inches of heat sink area keep the MA6900 operating
safely with convection cooling. No fans are needed.
Autoformers
All solid state power amplifier output circuits work best
into what is called an optimum load. This optimum load
may vary considerably from what a loudspeaker requires.
In the case of more than one loudspeaker connected in par-
allel, the load to the
power amplifier may
drop to two ohms or
even less. A power am-
plifier connected to a
load that is lower than
optimum, causes more
output current to flow,
which results in extra
heat being generated in
the power output stage.
This increase in tem-
perature will result in a
reduced life expectancy
for the amplifier.
The Autoformer creates an ideal match between the
power amplifier output stage and the loudspeaker. Refer to
figure 12. There is absolutely no performance limitation
with an Autoformer. Its frequency response exceeds that of
the output circuit itself, and extends well beyond the au-
dible range. Its distortion level is so low it is virtually im-
possible to measure. In the rare event of a power amplifier
output circuit failure, the McIntosh Autoformer provides
absolute protection from possible damage to your valuable
loudspeakers.
Protection Circuits
The MA6900 incorporates it’s version of the McIntosh
Sentry Monitor output transistor
protection circuit. Refer to figure
13. There is absolutely no compro-
mise in sonic performance with
this circuit, and it ensures safe op-
eration of the amplifier under even
the most extreme operating condi-
tions. The different types of pro-
tection circuits incorporated in the
MA6900 insure a long and safe operating life.
The MA6900 also includes the unique patented McIn-
tosh Power Guard circuit.
Power Guard eliminates the
possibility of ever overdriving
the amplifier into clipping. Re-
fer to figures 14, 15 and 16. An
overdriven amplifier can pro-
duce both audible and inaudible
distortion levels exceeding
40%. The audible distortion is
unpleasant to hear, but the inau-
dible ultrasonic distortion is
also undesirable, since it can
damage valuable loudspeaker
system tweeters. You will never
experience the harsh and dam-
aging distortion due to clipping.
The Power Guard circuit is a
waveform comparator, monitor-
ing both the input and output
waveforms. Under normal oper-
ating conditions, there are no
differences between the shape
of these waveforms. If an am-
plifier channel is overdriven,
there will be a difference between the two signal wave-
forms. When the difference exceeds 0.3% (equivalent to
0.3% harmonic distortion), the Power Guard activates the