McIntosh MC2255 Power Amplifier Review

The McIntosh MC 2255 basic power amplifier is rated to deliver its output into loads of 1, 2, 4, or 8 ohms, from 20 to 20,000 Hz, with no more than 0.02 per cent harmonic or intermodulation distortion. Its stereo outputs may be paralleled or bridged to drive a mono load with a maximum output of 500 watts at 0.02 per cent distortion. Depending on the connection used, the mono load impedance can be from 0.5 to 16 ohms.

The unusual load capabilities of the MC 2255 derive from the use of large autotransformers to match the output transistors to their loads. Like vacuum-tube amplifiers, the MC 2255 has output terminals designated for 1, 2, 4, or 8 ohms. Thus, regardless of the speaker impedance, the output transistors are optimally loaded and can deliver their full power without excessive distortion or overheating.

The output stages of the MC 2255 operate in class-B, but a unique biasing system completely eliminates the crossover distortion usually associated with class-B operation. This being the most efficient mode of linear amplifier operation, the total power consumption of the MC 2255 from the 120-volt a.c. line is only 0.7 ampere at idle (or normal playing volume) and 12 amperes at full output. The input and driver stages form a complete class-AB low-power amplifier which drives the front-panel headphone jack as well as the power stages. Switches connect the input sections for mono operation. In the mono/parallel mode the right-channel input drives both output sections in phase, and for the mono bridge mode the other input section is used as a phase inverter so that the outputs can be driven 180 degrees out of phase.

The power stages are protected by a novel Power Guard circuit that makes it impossible to clip the amplifier output by overdriving it. A waveform comparator monitors the input and output signals of the amplifier, and if the output waveform differs from the input by an amount corresponding to about 0.5 per cent harmonic distortion, a red limit light glows on the panel (there are separate lights for the two channels). Any further increase in the drive level causes the signal to be attenuated ahead of the output section. This prevents the output from ever exceeding its linear operating range (according to McIntosh, the amplifier can be overdriven by 20 dB before distortion reaches 2 per cent).

Internally, the McIntosh MC 2255 is a very complex amplifier, containing some eighty-five transistors, forty-seven diodes, and fourteen integrated circuits. Many of its components are involved in the protective systems and in its novel self-test feature. Each time the amplifier is turned on, an automatic seven-step test sequence checks the key operating voltages for correctness. As each step is executed, the corresponding numeral lights up on a front-panel display and a green light signals that it has been passed. If any stage of the test is not satisfactory, its number remains lit to indicate the problem area. Two different test speeds can be selected, and one can choose to have a “beep” sound after each step or to have the tests proceed in silence.

Two large meters are calibrated logarithmically from less than 2.5 milliwatts to 500 watts output (because of the output transformers, these readings are equally applicable to any of the load impedances for which the amplifier is designed). Another scale reads in decibels from —20 to +2 (the latter corresponding to about 250 watts output). Knobs below the meters control left GAIN, RIGHT/MONO GAIN, METER RANGE ( — 20 dB, —10 dB, 0 dB, watts, hold), the speakers outputs, and power. The hold position of the meter range switch causes the meters to retain their highest readings. The meter-driving circuits allow them to respond to very short program peaks, although they are calibrated in average watts.

At the right side of the panel are the two indicator groups. The power guard display shows the number of the system test sequence step as it is executed, and pairs of red and green LEDs show either that the limit (of output power) has been exceeded or that the amplifier operation is normal. Above this group, a meter group illuminates the words watts, hold, or DECIBELS, according to the setting of the meter range switch.

On the rear of the chassis are two sets of barrier terminal strips for the speaker outputs, a single unswitched a.c. outlet, and the holder for the 15-ampere line fuse. A three-position mode switch selects stereo, mono bridge, or mono parallel operation. Next to the two input phono jacks is a switch that sets the input sensitivity to either 0.75 or 2.5 volts for full output depending upon the associated equipment. (The latter is the normally preferred setting for most setups.)

The MC 2255 is a handsome and rugged amplifier, following a long-standing McIntosh tradition in its styling and construction. The panel and most of the top metal-work are finished in black, with front accents of silver and softly lit blue-green meters. The chassis is chrome-piated. Also furnished with the amplifier are side brackets and hardware for the McIntosh Panloc system for custom installations.

Laboratory Measurements

Preconditioning the MC 2255 at one-third rated power made the heat sinks very hot, but the rest of the amplifier remained comfortably cOol throughout our tests. In normal operation the MC 2255 is no more than faintly warm and has no need of a cooling fan or any unusual ventilation precautions.

With both channels driving 8-ohm loads at 1,000 Hz the distortion was undetectable (well below the noise level) until we reached 10 watts output, when it measured 0.00056 per cent. It increased very gradually with higher power to 0.0032 per cent at 250 watts and 0.0045 per cent at 300 watts. The maximum power (corresponding to “clipping power,” except that the waveform could not be made to clip) was about 357 watts, with distortion reading 0.24 per cent at the limiting point. The output into 4 ohms (using the appropriate output terminals) was also 357 watts, and we were able to develop 420 watts per channel into 2-ohm loads.

At the rated 250 watts output into 8 ohms, the maximum distortion was 0.01 per cent at 20 Hz. Over most of the audio range it was about 0.004 per cent, rising to 0.009 per cent at 20,000 Hz. At lower power outputs the distortion was substantially lower. The amplifier sensitivity (using the 2.5-volt switch setting) was 0.15 volt for a 1-watt reference output, and the A-weighted noise and hum level was 86 dB below 1 watt. The frequency response of the amplifier was within +0, —0.1 dB from 20 to 20,000 Hz and was down 0.9 dB at 5 Hz and 3 dB at 150 kHz.

The amplifier rise time was about 3 microseconds, and its IHF slew factor was about 10. The IHF intermodulation distortion, measured with 18- and 19-kHz signals, was — 94 dB for the second-order component at 1,000 Hz and — 67 dB for each of the third-order products at 17 and 20 kHz, all being referred to a 250-watt level.

The clipping headroom of the amplifier was 1.55 dB for 4- and 8-ohm loads and 2.55 dB for 2-ohm loads. The dynamic power output was 455 to 466 watts, depending on the load impedance, giving dynamic-headroom ratings of 2.65, 2.7, and 2.5 dB for loads of 8, 4, and 2 ohms, respectively.

The meters read quite accurately (about 0.2 dB high at full power) and responded to very brief transient signals. They are driven from the class-AB input amplifier instead of from the output stages as in most amplifiers, so the meter readings are unaffected by switching off the speakers. We found the headphone volume to be only marginally useful with 600-ohm phones. It was adequate with conventional-impedance phones.

Comment

McIntosh (one of the oldest names in hi-fi, and perhaps the only firm from its time to remain under the original ownership) has earned an impressive reputation for their continued support of their products, their exceptionally conservative design and specifications, and generally outstanding quality. The MC 2255, the first McIntosh product we have reviewed in many years, is a perfect example of the continuation of those policies.

In its circuitry and operating features, the MC 2255 is quite unlike any other basic power amplifier we have seen. By using autotransformers to match the load impedance to the transistor requirements, McIntosh has made an amplifier capable of delivering its full potential performance into almost any load impedance it might encounter. That performance, as our tests have shown, is absolutely first-rate. It is difficult to imagine any home installation needing more power than the MC 2255 delivers with such apparent ease. Its noise, distortion, stability, and any other quality one might name are quite literally “state of the art.”

The Power Guard system is most effective in making it impossible to hard-clip the output of the amplifier. Regardless of how hard, it is driven, it simply cannot develop an audible amount of distortion on musical program material (2 per cent is well below the probable threshold of audibility of distortion in a music system being driven to 350-plus watts). This feature should also mean a greatly reduced likelihood of blowing out a speaker, since clipping is a common cause of tweeter damage. For the nontechnical user, the self-test feature is mostly a “security blanket,” although we can appreciate that it would also simplify troubleshooting and servicing.

Clearly, no effort has been spared in the design and construction of this amplifier. This sort of perfectionism carries a considerable price, both in dollars and pounds (avoirdupois, not sterling!). Considering the probable long life of the MC 2255, that price does not seem at all unreasonable.

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