McIntosh MC1201 monoblock power amplifier Page 2
Balanced connections are made at the switch-selectable XLR input, which is rather individualistic in not using the universal locking-type XLR connector that snaps the fitting into place. I had no trouble with that, but it did catch my attention. I think it's the small details that define an audiophile design, and there lie the differences in most stark relief. In my opinion. A pair of standard RCA connectors are provided for unbalanced inputs.
The IEC mains-in connector is set horizontally on the left rear of the chassis; some of the larger audiophile power cords I used rose in wide, stiff arcs over the amps before snaking their ways to the AC outlets.
The imposing faceplate is nicely set off with a pair of what McIntosh describes as extruded aluminum anodized handles in "champagne gold." They look like matte silver to me, and are set off nicely against the big, shiny chassis and the hulking black upper structure. Classic McIntosh.
The MC1201 costs $7500 each. (Larry Fish was chuckling about a customer waiting to upgrade the half-dozen MC1000s in his home-theater system with a sextet of 1201s.) Of course, McIntosh makes speakers designed to take full power from these huge amps. The loquacious Mr. Fish then told me that their People's Republic of China dealer had set up—on stage in a concert hall—a triamped system with a pair of McIntosh XR290 loudspeakers powered by six MC1000s, and sold tickets! He said it was a successful event.
McIntosh's goal for the MC1201 was that its every stage of voltage or current amplification would be as linear as possible. To accomplish this, they used the following techniques.
First, "each transistor is selected to have nearly constant current gain (Beta) over the entire range of currents at which the transistor must operate." The load impedance presented to each amplification stage is as uniform as possible for all signal levels. The input impedance of stages is increased and "linearized where possible by using emitter degeneration." Resistors and capacitors in the signal path are carefully selected to have "exceedingly low voltage coefficients—low change of resistance or reactance with applied voltage." Precision metal-film resistors and low-dielectric-absorption film capacitors are used in all critical circuit locations. Output transistors have matched uniform current gain, "high-current gain-bandwidth product," low output capacitance, and "a large active-region safe operating area."
According to the documentation, "These characteristics and the automatic-tracking bias system eliminate crossover distortion. The distortion graphs supplied show clearly that distortion does not increase at low power output levels." Ah-ha—I'd wondered about that. According to the estimable Mr. Fish, only the first watt or two is biased into full class-A operation.
The MC1201 is fully balanced from input to output. Two matched amplifiers operate in push-pull, with their outputs combined at the Autoformer, so each half of the amplifier contains complementary balanced circuitry. McIntosh claims that the resulting "double balanced" configuration cancels virtually all distortion, and that "this circuit is possible only with the exclusive McIntosh Output Autoformer," which provides matching for 2, 4, and 8 ohm loads.
The output transformer coupling, or Balanced Dual Core Autoformer in Mac Speak, is rarely seen with solid-state designs, and provides "low-distortion power transfer and delivers peak output current in excess of 200 amperes." The MC1201 is said to have the output-current reserve to deliver more than 5kW output on tonebursts. Of the Autoformer, McIntosh says, "There is absolutely no performance limitation. Its frequency response exceeds that of the output circuit itself, and extends well beyond the audible range." The Autoformer also protects speakers from damage in the event of amplifier failure. Should a DC component appear in the output circuit, it is shunted by the transformer and thus causes no damage to the speaker.
As laid out by McIntosh, the basic circuit is a time-honored one: two balanced stages of voltage amplification followed by three stages of current amplification. All stages are complementary balanced, as noted. This means, McIntosh points out, that the amplifying stages have less total harmonic distortion and that less negative feedback is required to achieve "ultra-low" distortion.