Soundcraftsmen Pro-Power Four Amplifier Review

Soundcraftsmen’s Pro-Power Four power amplifier was designed to meet the extended dynamic-range requirements of compact disc players. Its ability to deliver high short-term current outputs (over 50 amperes, according to the manufacturer) into low-impedance speaker loads makes it compatible with many “difficult” speakers (such as the Acoustat or Apogee) that cannot be driven properly by most conventional amplifiers. The high-frequency response of its fully complementary mosfet output stages extends to beyond 10 MHz. The amplifier uses no current-limiting circuits.

The Pro-Power Four also features the proprietary Soundcraftsmen Phase-Control-Regulation (PCR) power supply first used in the company’s PCR800 amplifier. When an amplifier is called upon to deliver a high audio power output, the large current drain normally causes the power-supply voltage to drop, reducing the maximum available power. The PCR error-sensing circuits vary the conduction time of the power supply’s silicon-controlled rectifiers, maintaining a nearly constant voltage under widely varying load conditions. This regulating action also minimizes the effects of reduced power-line voltage, which can seriously limit the maximum power output of an amplifier using an unregulated power supply.

The Pro-Power Four has differential input stages, for low noise and distortion, and high-gain, fast-slewing Class A amplifying stages. Instead of using large, heavy, and costly external heat-sink fins to dissipate the heat from the output transistors at high power levels, the Pro-Power Four uses a two-speed fan to keep them at a safe temperature.

During normal operation the fan turns slowly and quietly, drawing outside air into the left rear of the amplifier, passing it over the internal heat sinks, and exhausting the warm air at the right rear. An internal temperature sensor switches the fan to its high-speed mode when necessary.

The Soundcraftsmen Pro-Power Four is rated to deliver 205 watts per channel into 8-ohm loads from 20 to 20,000 Hz with no more than 0.05 percent total harmonic distortion. It is also rated for 300 watts into 4 ohms and 450 watts into 2 ohms. The black-finished amplifier measures 19 inches wide, 11-3/4 inches deep, and 5-1/4 inches high, and it weighs about 27 pounds.

The front panel contains two rows of led instantaneous-power indicators, twenty for each channel. The led’s, which are green up to 200 watts and red from 200 to 400 watts (into 8 ohms), are spaced at logarithmic intervals, with one-third of them assigned to power outputs under 1 watt. Calibrations are also provided for 4- and 2-ohm loads. Each channel also has a red Truclip indicator that lights only when the amplifier is driven into nonlinearity. In addition to the power switch, the front panel has two pushbuttons that connect the two sets of speaker output binding posts to the amplifier outputs. Price: $699.

Lab Tests

During the standard 1-hour preconditioning period at one-third rated power, the amplifier’s fan switched to its high speed in about a minute, and it cycled between high and low speeds for most of the following tests. Although the top of the case became moderately warm under this harsh treatment, it was never uncomfortable to the touch.

The amplifier delivered 236 watts per channel into 8-ohm loads, 324 watts into 4 ohms, and 406 watts into 2 ohms. We found that these maximum power readings (especially that into 2 ohms) were difficult to repeat because of their apparent dependence on the internal temperature of the amplifier, whose output was considerably greater when cold than when fully heated.

Although we did not receive any instruction manual or circuit details with our test sample of the Pro-Power Four, a check of our file on the Soundcraftsmen PCR800, which we tested a little over two years ago, showed that it used thermal feedback to reduce the power-supply voltage (and thus the power) to safe values as the output transistors heated up. Apparently a similar system is used in the Pro-Power Four, though with much less drastic effects on the amplifier’s power capability. Like the PCR800, the Pro-Power Four’s had nearly identical dynamic- and clipping-power outputs, with slightly less power available during tone bursts of 50 to 100 milliseconds than with bursts of 20 milliseconds or longer than 200 milliseconds. This appears to be a characteristic of its thermally controlled power supply.

The 1,000-Hz harmonic distortion was less than 0.013 percent from 1 to more than 200 watts into 8 ohms. With 4-ohm loads the distortion was less than 0.032 percent up to 300 watts, and with 2-ohm loads the distortion was between 0.02 and 0.07 percent from 1 to 300 watts. Closer to the clipping power, more than 400 watts, the amplifier’s operating conditions and the distortion readings were changing too rapidly to measure. At its rated power of 205 watts per channel into 8 ohms, the distortion was less than 0.01 percent from 20 to 300 Hz, rising smoothly to between 0.02 and 0.03 percent from 3,000 to 20,000 Hz. At half-power and less, the readings were generally lower than at rated power.

The Pro-Power Four required an input of 72 millivolts to develop an output of 1 watt into 8 ohms. The A-weighted hum and noise level was —84 dB referred to 1 watt. Its frequency response was flat within + 0.1, —0.3 dB from 5 to 20,000 Hz, down 3 dB at 140 kHz, and down 6 dB at 270 kHz. The slew factor was greater than 25. The amplifier was stable when driving complex simulated speaker loads, and its El A reactive-load factor was 0.82 dB at 63 Hz. The led power indicators were much more accurate than most such devices, corresponding closely to the measured output at high levels and matching their “steady-state” readings during the 20-millisecond tone bursts of the dynamic-headroom test.

Comments

The inevitable comparison between our test results with the Pro-Power Four and the PCR800 shows a close similarity between the two when driving 8- and 4-ohm loads, but the Pro-Power Four has a far greater power output into 2 ohms. In its normal low-speed mode, the fan of the Pro-Power Four is so quiet that we were unable to hear any sign of its operation from the front of the amplifier. When it switched to high speed, however, the noise was quite annoying, though under normal operating conditions it’s doubtful that this would occur.

The nomenclature of the Soundcraftsmen Pro-Power Four implies that it is a professional product, though it is not a part of the company’s extensive line of professional amplifiers. It might well be suitable for that exacting service, since we were unable to overheat it or damage it under any operating conditions, and even the noisy fan would not be audible in an environment with high ambient sound. However, we prefer to think of it as a very good, powerful, and well-protected home music amplifier.

Despite our knowing little of the design details of the Pro-Power Four, our measurements leave no doubt that it is a close relative— with enhanced performance—of the PCR800, which is still in the Soundcraftsmen line. It has a substantially greater current-output capability, greater high-frequency power capability, a quieter cooling system, and the additional features of switchable outputs for two sets of speakers and an excellent output-level display. The price difference between the two amplifiers is modest and easily justified by the Pro-Power Four’s added performance and features.

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