Top Inside View of the MA6900
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
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