Technical Description
tributes to low operating temperatures. More than 2,000 square inches of heat sink area keep the MC402 operating safely with convection cooling. No fans are needed.
Autoformers
All solid state power amplifier output circuits work best into an optimum load. This optimum load may vary consid- erably from what a Loudspeaker requires. In the case of more than one Loudspeaker connected in parallel, the load 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 more output current to flow, which results in extra heat being generated in the power output stage. This increase in temperature 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. A McIntosh amplifier with an Autoformer can be used to safely drive multiple speakers
without reducing the life ex- pectancy of the power ampli- fier. Refer to Figure 21.
There is absolutely no per- formance limitation with an Autoformer. Its frequency re- sponse exceeds that of the out- put circuit itself, and extends well beyond the audible range. Its distortion level is so low it is virtually impossible to mea- sure.
In the rare event of a power amplifier output circuit fail- ure, the McIntosh Autoformer provides absolute protection from possible damage to your valuable Loudspeakers. The unequaled expertise of McIntosh in the design and manu- facturing of Autoformers is legendary in the high fidelity industry. McIntosh engineers know how to do it right.
Protection Circuits
The different types of protection circuits incorporated in the MC402 insure a long and safe operating life. This is just one of the many character-
istics of McIntosh Power Am- plifiers that make them world famous. The MC402 incorpo-
rates the McIntosh Sentry Monitor Output Transistor Pro-
tection Circuit, which protects the amplifier in the event of a shorted Loudspeaker Connec-
tion. Refer to Figure 22.
The MC402 also includes the unique patented McIntosh Power Guard circuit. Power Guard eliminates the possibil- ity of ever overdriving the amplifier into clipping. Figures 23, 24 and 25 are actual Spectrum Analyzer measurements of a McIntosh Power Amplifier,
with a test signal consisting of 14kHz and 15kHz sine waves. An overdriven amplifier can produce both audible and inaudible distor- tion levels exceeding 40%. The audible distortion is unpleasant to hear, but the inaudible ultrasonic distortion is also undesirable, since it can damage valuable Loudspeaker system tweeters. You will never experience the harsh and damaging distortion due to clipping.
The Power Guard circuit is a waveform comparator, monitoring both the input and output wave- forms. Under normal operating conditions, there are no differ- ences between the shape of these waveforms. If an amplifier chan- nel is overdriven, there will be a difference between the two signal waveforms. When the difference exceeds 0.3% (equivalent to 0.3% harmonic distortion), the Power Guard activates the PG light and a dynamic electronic attenuator at the amplifier input reduces the in- put volume just enough to prevent any further increase in distortion.
The Power Guard circuit acts so fast that there are abso- lutely no audible side effects and the sonic purity of the music reproduction is perfectly preserved. The MC402 Power Amplifier with Power Guard is not limited to just the rated power output, but will actually produce distortion free output well above its rated power due to the McIntosh philosophy of conservative design.
Power Supply Circuits
To compliment the fully balanced design of the MC402, there are huge main filter capacitors that guarantee an ex- cellent signal to noise ratio and the energy storage neces- sary for the wide dynamic range that “Digital Audio” de- mands.
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