Soundcraftsmen RA7503 Power Amplifier Review

A few years ago, Soundcraftsmen introduced the MA5002, a novel power amplifier using what they termed “Vari-Por-tional" or “Class-H” circuitry to generate a very-high-power output with a higher overall efficiency than conventional power amplifiers of any other class. A new, somewhat modified and improved version of that class-H amplifier has now been announced by Soundcraftsmen. It comprises a series of three models with identical amplifier sections but different front-panel display features. We tested the top-of-the-line Model RA7503 for this report.

The essence of the “Vari-Portional” system is an extremely responsive power supply whose voltages can follow the instantaneous signal level even more rapidly than the amplifier itself. In fact, the manufacturer claims a slew rate of 90 volts per microsecond for the power supply, compared to 50 volts per microsecond for the amplifier (itself a very good figure).

Normally the amplifier operates with relatively low supply voltages, sufficient to support up to about 100 watts output. As the signal level increases to the point where clipping is imminent, the supply voltages (both positive and negative) increase with it, tracking and staying “ahead” of the signal so that the full power of the amplifier can be realized without distortion. The result is an amplifier designed for (and capable of) very high power output, yet one that generally uses little power and thus runs very cool under typical home-use conditions. In other words, it is highly efficient.

The power ratings of the RA7503 are very similar to those of the older MA5002. It can deliver 250 watts per channel to 8-ohm loads from 20 to 20,000 Hz with no more than 0.09 per cent total harmonic distortion. It is one of the few amplifiers carrying a full 4-ohm power rating, in this case 375 watts per channel from 20 to 20,000 Hz with 0.09 per cent distortion. In the bridged mode of mono operation, the RA7503 is rated at 750 watts into 8 ohms.

The RA7503 normally requires no fan, although the manual warns that for operation in a fully sealed environment or when delivering sustained high power into a low-impedance load, a “whisper fan” should be used for cooling.

The Soundcraftsmen RA7503 is a large, heavy amplifier measuring 19 inches wide, 15 inches deep, and 7 inches high and weighing about 50 pounds. Its exterior surfaces are finished in black. The amplifier can be rack mounted, and optional walnut side plates are available for home use. The speaker outputs in the rear employ heavy-duty, five-way binding posts, and there is a third set of outputs for the mono mode (in which neither side of the output can be grounded). The signal inputs are balanced through three-circuit 1/4-inch phone jacks, but adaptors are furnished to accommodate standard phono plugs and convert the inputs to unbalanced (the usual home-music-system practice).

The front panel of the RA7503 contains an impressive array of indicators and controls. For each channel there is a level-adjustment knob, a green light that shows the onset of the Vari-Portional circuit operation, and a red light that glows when actual waveform clipping occurs. In the center of the panel are two vertical rows of LEDs that display the instantaneous outputs of the two channels. They operate in steps of 3 dB from — 40 to — 7 dB (relative to rated power), 2-dB steps from —7 to — 1 dB, and 1 -dB steps from — 1 to + 3 dB. The lights are green up to 0 dB, red at higher outputs.

Two rows of calibrations next to the left-channel indicators show the mono power output, in watts, into 8- and 16-ohm loads. Next to the right-channel lights are calibrations for stereo operation into 4- or 8-ohm loads. Since the light's for 8-ohm stereo operation cover a power range from 0.02 to 500 watts, they are always glowing and informative during operation of the amplifier. A center-bottom red protect light indicates that the amplifier’s protective system is operating.

The left side of the panel is devoted to a hundred-LED spectrum-analyzer display which shows the program levels in ten octave bands over an 18-dB range at 2-dB intervals. Pushbuttons connect the analyzer to read either channel or their sum. A slider control adjusts the display sensitivity to suit the amplifier’s output level.

The protective system of the Soundcraftsmen RA7503 differs from the usual practice. It has no current-limiting circuits (which have been known to cause distortion under certain operating conditions), yet the amplifier is fully protected for operation into 2-ohm loads or against short-circuited outputs. It has an “Auto-Buffer” circuit that senses a lower-than-usual load impedance (less than 4 ohms) and regulates the power-supply voltages to provide the maximum safe power output without unnecessarily interrupting operation.

A dangerous overload, or an output short circuit, is instantly sensed and an “Auto-Crowbar”—a silicon controlled rectifier circuit—shorts the power supply in microseconds, simultaneously disconnecting the a.c. power by means of a triac and lighting the front-panel protect light (there is a separate power supply to operate the protective system even when the main power is off). This action also takes place if the output transistors become too hot. In a few seconds the system automatically resets itself (if the fault condition still exists, the amplifier will not come on, but it will keep trying until safe operation is possible). In addition to its electronic systems, the amplifier is protected by a line fuse.

The Soundcraftsmen RA7503 is priced at $1,149. The optional walnut side panels are $20. The Model RA7502 is identical except for not having the spectrum analyzer, and its price is $949. The Model RA7501, with neither the analyzer nor the power indicators but an identical amplifier, is $849.

Laboratory Measurements

Preconditioning the Soundcraftsmen RA7503 for one hour at 83 watts (one-third rated power) into 8-ohm loads made the heat sinks and top surface only moderately hot. The 1,000-Hz output into 8-ohm loads clipped at just over 300 watts per channel. With one channel driving 4 ohms it was about 450 watts, and driving 2-ohm loads the amplifier delivered almost exactly 500 watts per channel.

The harmonic distortion into 8 ohms was 0.003 to 0.005 per cent from 1 to 50 watts, rising to 0.016 per cent at 100 watts and dropping slightly at higher power outputs. This effect was the result of the Vari-Portional circuit’s starting to operate at about 100 watts (200 watts into 4-ohm loads). At that time the distortion spectrum changed from predominantly second harmonic to a number of low-level, higher-order harmonic products.

The distortion varied between about 0.01 and 0.02 per cent from 100 to almost 300 watts. With 4-ohm loads the THD was 0.005 per cent up to 100 watts, and when the Vari-Portional circuit was operating it was between 0.013 and 0.023 per cent in the 150- to 450-watt range. With 2-ohm loads the distortion was about 0.015 per cent at most power outputs up to 400 watts.

When the RA7503 was driving 8-ohm loads at rated power, distortion fell from 0.02 per cent at 20 Hz to 0.01 per cent over most of the midrange and rose smoothly to 0.064 per cent at 20,000 Hz. At half power the distortion rose to a maximum of 0.064 per cent at 5,000 Hz but decreased at higher frequencies (it rose to only 0.06 per cent at 20,000 Hz). This effect was evidently due to the Vari-Portional circuit, since it was functioning at the 125-watt output level. The distortion was lowest at one-tenth rated power, falling from 0.01 per cent at 20 Hz to 0.005 per cent in the midrange and rising to less than 0.04 per cent at 20,000 Hz.

The IHF clipping headroom was 0.84 dB into 8 ohms and 0.76 dB into 4 ohms. The IHF dynamic headroom was 1.04 dB (318 watts) at 8 ohms and 1.07 dB (480 watts) at 4 ohms. The dynamic power into 2 ohms was 544 watts per channel. The amplifier was stable with a complex simulated speaker load, which caused only a slight overshoot on a high-frequency square wave. The IHF slew factor was greater than 25, and the amplifier rise time was about 4 microseconds. The IHF intermodulation distortion (IM) was measured with 19,000- and 20,000-Hz signals. Very low levels of odd-order distortion products (third through eleventh) were visible on the spectrum analyzer at levels from —88 to —92 dB referred to 250 watts equivalent sine-wave power. The even-order (1,000-Hz) IM distortion was at —85 dB.

The low-level frequency response of the amplifier was flat within ±0.05 dB from 10 to 20,000 Hz (down 1 dB at 50,000 Hz and 0.3 dB at 5 Hz). A 77-millivolt input was required for a reference output of 1 watt, and the A-weighted noise was —83 dB referred to 1 watt ( — 107 dB referred to rated power). The Vari-Portional light came on at about 125 watts, and the clipping lights operated at the point where visible waveform clipping occurred. The power-display lights were surprisingly accurate and, unlike many others, served much more than a purely ornamental function. Typically they came on within a few per cent of the indicated power, and they switched on abruptly and unambiguously. It was not possible, however, to light only the +l-dB light, since the +1-, +2-, and +3-dB lights all came on virtually simultaneously.

When listening to the amplifier before making any tests, we were surprised at how readily the “500 watt” ( + 3-dB) light could be lit without apparent distortion (of course, we used speakers that could handle the full output of the amplifier safely). Still, it was difficult to accept the reality of those high peak levels even when listening to relatively uncompressed material. Our later tests showed that these peaks actually reached about 300 watts, which indicated that this amplifier can be driven into moderate clipping without audible distress.

We made no measurements on the spectrum-analyzer section, which is identical to that built into some of Soundcraftsmen’s equalizers. It appeared to work well. A pink-noise test record is supplied with the RA7503 and can be used with the analyzer to adjust the system’s frequency response using an external equalizer.

The protective circuits of the RA7503 worked to perfection. We always approach the clipping point in a gingerly fashion when testing a high-power amplifier, since all too often (especially with very low load impedances) there is some damage or a blown fuse to replace when that point is reached. Except with 2-ohm loads, the RA7503 took this treatment with aplomb. Excessive drive into 2 ohms shut it off instantly and silently, and it always recovered with equal smoothness.

Comment

Our experience with high-power amplifiers has given us a somewhat different set of priorities (compared with those of the typical audiophile) for judging the value and desirability of an amplifier. Foolproof protection against almost any abuse is a “must” given the cost of the speakers that might be damaged by any amplifier failure. Since most power amplifiers sound essentially alike (I say “essentially” to forestall a torrent of letters from True Believers who condemn my heretical views), it seems to me that reliability is perhaps the most significant criterion for amplifier selection.

The Soundcraftsmen RA7503 is one of the most thoroughly “bulletproof” amplifiers we have had the pleasure of using. In normal operation the heat sinks become only faintly warm and the rest of the amplifier not even that. One soon tends to forget that shorted outputs and very low load impedances (as many speakers as you care to parallel) are normally “no-no’s,” since the RA7503 takes them in its stride.

Since the 7500 series units are all identical amplifiers, a choice between them can be made solely on the basis of their display features (and price, of course). We liked the power readout, which up to 0 dB is very accurate and fast. The spectrum analyzer can be worthwhile for some. The point is that Soundcraftsmen have made a superb amplifier in a choice of formats that should suit any need.

It is good for the soul to hear wide-dynamic-range programs reproduced through speakers and amplifiers that can do justice to them, with no nagging worries about whether the amplifier will survive the experience. The RA7503 provided us with some of the more uplifting listening experiences we have enjoyed in recent times. It is, in our estimation, an unqualified success.

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