20Input Test SignalFigure 14Without Power GuardFigure 15Top Inside View of the MA6900Figure 12Figure 13ance points of two ohms, four ohms and eight ohms. Theunequaled expertise of McIntosh in the design and manu-facturing of autoformers is legendary in the high fidelityindustry.The high efficiency circuit design of the MA6900 con-tributes to low operating temperatures. More than 621square inches of heat sink area keep the MA6900 operatingsafely with convection cooling. No fans are needed.AutoformersAll solid state power amplifier output circuits work bestinto what is called an optimum load. This optimum loadmay vary considerably from what a loudspeaker requires.In the case of more than one loudspeaker connected in par-allel, the load to thepower amplifier maydrop to two ohms oreven less. A power am-plifier connected to aload that is lower thanoptimum, causes moreoutput current to flow,which results in extraheat being generated inthe power output stage.This increase in tem-perature will result in areduced life expectancyfor the amplifier.The Autoformer creates an ideal match between thepower amplifier output stage and the loudspeaker. Refer tofigure 12. There is absolutely no performance limitationwith an Autoformer. Its frequency response exceeds that ofthe 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 amplifieroutput circuit failure, the McIntosh Autoformer providesabsolute protection from possible damage to your valuableloudspeakers.Protection CircuitsThe MA6900 incorporates it's version of the McIntoshSentry Monitor output transistorprotection circuit. Refer to figure13. There is absolutely no compro-mise in sonic performance withthis circuit, and it ensures safe op-eration of the amplifier under eventhe most extreme operating condi-tions. The different types of pro-tection circuits incorporated in theMA6900 insure a long and safe operating life.The MA6900 also includes the unique patented McIn-tosh Power Guard circuit.Power Guard eliminates thepossibility of ever overdrivingthe amplifier into clipping. Re-fer to figures 14, 15 and 16. Anoverdriven amplifier can pro-duce both audible and inaudibledistortion levels exceeding40%. The audible distortion isunpleasant to hear, but the inau-dible ultrasonic distortion isalso undesirable, since it candamage valuable loudspeakersystem tweeters. You will neverexperience the harsh and dam-aging distortion due to clipping.The Power Guard circuit is awaveform comparator, monitor-ing both the input and outputwaveforms. Under normal oper-ating conditions, there are nodifferences between the shapeof 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 to0.3% harmonic distortion), the Power Guard activates the
