|McIntosh MC-240 stereo power amplifier|
AT A GLANCE: The Mclntosh MC-240 is a two-channel basic amplifier, rated at 40 watts per channel. Its published specifications are impressive, and laboratory tests confirm them in every detail. Additionally, the MC-240 features a remarkable degree of flexibility for a basic amplifier. Price: $288 (includes cage).
IN DETAIL: As with previous Mclntosh amplifiers, the MC-240 uses the patented bifilar output transformers and the "unity coupled" output circuit. In this configuration, the output transformer actually has two primary windings: one is connected between the output cathodes, and the other between the output plates. Its purpose, of course, is to reduce distortion caused by leakage inductance, which in turn is caused by loose coupling between transformer windings. With this circuit, it is possible to operate the amplifier in "class B" with a pair of 6L6GC tubes. This makes for a high efficiency stage so that the amplifier draws relatively low power from the AC line under "stand-by" or low power output conditions. The bifilar winding also has been credited with extending the amplifier's power bandwidth to the 100,000 cps region.
Actually, the MC-240 consists of two entirely separate amplifiers built on the same chassis. The output windings of the transformers are ungrounded (a tertiary winding is used for feedback), so that both amplifiers can be series-connected to provide a full range of output impedances (2, 4, 8, 16, 32, 62, and 300 ohms) for various monophonic applications. On stereo or "twin" operation, output impedances of 4, 8, 16, 125 and 600 ohms are available.
In this amplifier there is a difference between stereo and "twin" operation, incidentally. In stereo mode, input sensitivity is 2 volts (controlled by a balance adjustment), which would suggest its use with the Mclntosh preamplifier or any other stereo program source or control unit with a suitable high output.
In "twin amplifier" mode, the MC-240 serves as two distinct amplifiers, each with an input sensitivity of 0.5 volts and each with its own gain control. Thus, the unit can be used as two separate mono amplifiers channeling two unrelated signals, or as a dual amplifier for use with an electronic crossover, or for stereo with signal sources having low output voltages.
On mono, of course, the MC-240 becomes an 80-watt single-channel amplifier, with a gain control. Both the mono and the “twin-amp” modes permit connecting signal sources of up to 30 volts without danger of overload.
The three modes of operation are selected by a three-position lever, and suitable connections are provided for all the various inputs and outputs. The MC-240 also can supply, from a socket on its chassis, operating voltage to power an external pre-amp (that does not have its own power supply).
Initial examination of the MC-240 indicated that this amplifier is built of high-caliber parts and with a remarkably high level of craftsmanship. Laboratory and listening tests simply confirmed this impression and would indicate that the MC-240 should meet the most critical of requirements. Its IM distortion, for instance, is rated at less than 0.5 per cent at peak power output (up to twice rated power). This would mean, per channel, an instantaneous peak power of 80 watts, which corresponds to an equivalent sine-wave power of 40 watts in the IM test, for which less than 0.3 per cent distortion was measured. Harmonic distortion is rated less than 0.5 per cent at full power from 20 to 20,000 cycles. Taking the most severe test, at 20 cps, the lab measured 0.28% distortion in one channel, and 0.47% in the other channel - at 40 watts output.
The MC-240 will deliver its rated power, with a healthy safety margin, from 20 to 20,000 cps at really insignificant amounts of distortion, on the order of a small fraction of a per cent. It was found to be quite stable under all sorts of capacitive loads, including a full-range electrostatic speaker. (In the lab's view, by the way, this represents an improvement over earlier Mclntosh amplifiers.) Power and distortion measurements were made with both channels driven simultaneously. This is a severe way of testing which the MC-240 came through admirably. It also indicates excellent regulation in the power supply.
Listening tests confirmed what the measurements showed. The MC-240, says the lab, "sounded beautiful."
The amplifier has a power line voltage selector switch, for choosing between 117 volts and 125 volts, whichever more closely matches the average line voltage in a given area. Our measurements were made with this switch set at 117 volts. In 125-volt operation, the maximum power output is reduced by about 15 per cent. In view of the conservative ratings of the MC-240, and the beneficial effect on tube and component life of running the unit at lower voltages, the lab recommends setting the switch for 125 volts in most installations . . . except those where the line voltage is typically less than 110 volts.
In sum, the Mclntosh MC-240 is another superb amplifier to take its place among the growing roster of sturdily built, fine-sounding giant powerhouses.