21Figure 16With Power GuardPG light and a dynamic elec-tronic attenuator at the amplifierinput reduces the input volumejust enough to prevent any fur-ther increase in distortion. ThePower Guard circuit acts so fastthat there are absolutely no au-dible side effects and the sonicpurity of the music reproductionis perfectly preserved. TheMA6900 Amplifier with PowerGuard is not limited to just therated power output, but will actually produce distortionfree output well above its rated power due to the McIntoshphilosophy of conservative design.Power Supply CircuitsTo compliment the design of the MA6900, there is a highvoltage power supply for both channels. Refer to figure 17.The power amplifiers draw high current from the ACpower line. Therefore, it is important that they plug directlyinto the wall outlet. Turn on inrush current is cushioned bythermistors in the power transformer's primary circuit.This soft start eliminates component stress during turn-on.The MA6900 can provide greater than 50 amperes peakoutput current to drive uneven speaker loads. Some poorspeaker designs have input impedance that dip to 1 or 2ohms at various frequencies and the MA6900 has the out-put current reserve to drive them. The MA6900 has mainfilter capacitors that guarantee an excellent signal to noiseratio and the energy storage necessary for the wide dy-namic range that "Digital Audio" demands.Figure 17Block Diagram of the Power Supply
