McIntosh MC1201 monoblock power amplifier Page 3

How low can you go? "Output distortion is so low," claims McIntosh, "it defies measurement, even with the finest distortion analyzers. At mid-frequencies, 8 ohm load, the distortion meter reads the residual distortion of the test oscillator (0.0002%) with or without the MC1201 in the circuit. This means the amplifier distortion is lower than the analyzer is capable of measuring."

The signal is fed to the positive inputs of the balanced differential stages. Feedback from the amplifier outputs is applied to the inputs, and the differential amplifiers drive a balanced Darlington-connected voltage amplifier stage. The voltage amplifier's output feeds complementary Darlington driver transistors. These supply the signal to 24 complementary transistors per side for a total of 48 output transistors. Ancillary components for features such as Power Guard, Sentry Monitor, the meter, and protection circuits "interconnect with the amplifier circuits."

The power supply uses a massive toroidal transformer, full-wave bridge rectifiers, and large filter capacitors with 600 joules of storage. The four large heatsinks that bristle on the back of the chassis provide more than 2800 square inches of surface area with which to cool the power-output devices.

Power Guard and Other Sentries
I think we can all agree that an amp driven into clipping, especially a solid-state one, can sound harsh. Clipping, McIntosh explains, looks and acts like the nonmusical squarewaves produced when an amplifier is asked to put out more power than it's designed to deliver.

"When an amplifier is driven into clipping, it can deliver up to 40% harmonic and intermodulation distortion. In this mode, the sound is grossly distorted and the extra energy content of the clipped signal will damage most loudspeakers. The McIntosh Power Guard circuit protects your ears and your speakers from this kind of damage." This has been a public service announcement...Power Guard is a McIntosh design (US patent 4048573).

The Power Guard circuit is a comparator that monitors the input and output waveforms. As McIntosh explains it, normally there's no disparity between these signals, so the circuit produces no output. When the MC1201 is overdriven and a difference develops, and if that difference exceeds 0.3%, the comparator output turns on the amber Power Guard indicator. If there's a further increase in distortion, the output of the Power Guard circuit engages an attenuator at the amplifier input. Reducing the amplifier gain simply holds the amp's output to a low-distortion value. However, Power Guard operates "only when the amplifier is asked to deliver more power than it was designed to produce"—a situation that I can't imagine. I never saw any amber light go on, I can tell you that, and I played 'em loud! McIntosh claims that the Power Guard acts so fast that there are "no audible side effects and the sonic purity of the music reproduction is perfectly preserved."

The Sentry Monitor joins the Power Guard in protecting your investment. Per McIntosh, all power transistors are limited in the maximum amount of power they can handle. "The MC1201's output transistors and power supply have been designed to allow very high current flow into properly matched load impedances. If, however, a short circuit or very low value of load impedance is applied to the output of the amplifier, destructive current levels could be reached. The Sentry Monitor circuit prevents this. The circuit senses the dynamic operating time, voltage, and current of the amplifier output stage and confines it to nondestructive levels. Sentry Monitor does not limit the power output available from the amplifier."

Thermal Control over the output transistors is also in the cards. The MC1201 uses an efficient output stage biased into class-AB2. Larry Fish claims that while the MC1201 runs pretty toasty, it produces relatively little heat for the output power produced. Four large heatsinks dissipate transistor-generated heat. "Natural convection air flow is sufficient for cool operation. Should the cooling air be blocked or should the amplifier operating temperature become too high, thermal cutouts turn off the input signal to the amplifier. When the amplifier has cooled, it will automatically turn on again." Again, it never happened, and I can't imagine provoking them to do so.

Turn-on inrush current is "cushioned" by thermistors in the primary power transformer circuit; the resultant soft start eliminates stress. There's also a turn-on delay circuit that holds back amplification for about two seconds after power is applied, to prevent pops or thumps from insulting your loudspeakers.