Manufacturer's Specifications:
Continuous Power Output: 250 watts per channel, 8 ohms, stereo; 375 watts per channel, 4 ohms, stereo; 750 watts, 8 ohms, bridged mono; 20 Hz to 20 kHz THD Plus Noise: 0.02 percent at rated power, 8 ohms
Frequency Response: 20 Hz to 20 kHz, ±0 25 dB
SMPTE IM Distortion: 0 05 percent.
TIM Distortion: Less than 0.02 percent
S/N Ratio: 105 dB below rated output, A weighted.
Slew Rate: Amplifier, stereo mode, 50 V/µS, supply, stereo mode, greater than 90 V/µS.
Damping Factor: Stereo, greater than 250; bridged mono, greater than 125; both at 50 Hz.
Input Impedance: Stereo, 32 kilohms, bridged mono, 16 kilohms.
Input Sensitivity: Stereo, 1.2 V; bridged mono, 1.0 V; both for rated output.
Stability: Stable with any load two ohms or above.
Dimensions: 19 in. (48.3 cm) W x 7 in. (17.8 cm) H x 15 in. (38.1 cm) D.
Shipping Weight: 55 lbs. (25 kg).
Price: $949.00.
I was introduced to the concept of "Class-H" circuitry by Soundcraftsmen's chief engineer, Paul Rolfes, nearly four years ago, shortly after he had invented and incorporated it in one of the Soundcraftsmen amplifiers. The concept is still very attractive and innovative, as evidenced by the fact that at least two companies that I know of have used "variations on the Class-H theme" ever since Soundcraftsmen showed everyone how it was done. For reasons which have more to do with marketing, public relations and other non-engineering related pursuits, Soundcraftsmen ended up with patents but very little glory. If you are not already familiar with the very clever and energy efficient Class-H amplifier theory, I'll get to that in a moment, but first let's have a look at the latest powerhouse of an amplifier to use the Class-H circuit, the Model RA7502.
This amplifier is the professional version of Soundcraftsmen's lower priced ($749.00) Model PA5001A, a basic power amp that lacks the front panel displays of the RA7502 as well as the mono bridging and balanced input capabilities of the more ex pensive "pro" version. The lower priced unit does, however, have the same performance specs and circuitry and might there fore be worth considering if the price of the RA7502 seems a bit too high for your budget. The rack-mountable front panel of the RA7502 features a power on/off switch at the left, individual rotary input level controls for each channel, and two vertical banks of LED power indicators (20 LEDs per channel) which show power output levels calibrated from -40 to +3 dB. Calibrated wattage scales are also inscribed along the rows of LEDs for mono or stereo operation into 16-, 8-, or 4-ohm loads. Additional individual LEDs (per channel) illuminate when the Class-H "Vari-Portional" circuitry begins to operate and when actual clipping occurs.
Figs. 1 A and 1B - Multiple traces showing how B+ (Fig. 1A) and B (Fig. 1B)
voltages adjust themselves in anticipation of large signal voltage swings,
Class-H circuitry of Sound craftsmen RA7502 power amplifier.
Stereo or bridged mono operation is selectable at the rear panel, which is equipped with 1/4 -inch phone-jack inputs wired in such a way that either balanced or unbalanced inputs may be selected depending upon how mating phone plugs are wired.
Output terminals are five-way binding post types on standard 3/4 -inch centers. The rugged chassis of this hefty amplifier is a 16 gauge steel main-frame structure with modularized construction throughout, which provides easy access to all component parts and p.c board surfaces. The amplifier is supplied with a parchment-like individualized certificate of performance, on' which is noted the serial number of the amplifier and major measured specs for that actual unit, signed off by the final inspector who tested it.
Fig. 2-Peak-to-peak voltage in bridged mono mode measured 240 V under actual
music signal conditions.
Fig. 3-Power output at 1 kHz vs. THD.
Fig. 4 -Distortion vs. frequency.
Circuit Highlights
For those unfamiliar with Class-H operation, the following description, largely supplied by the inventor, may be helpful: The Class-H amplifier has two positive and negative power supplies. The ratio of the voltages in these power supplies is arranged such that the low-voltage supply is 2 / 3 of the high voltage supply. The amplifier behaves like a conventional Class AB amp at low-volume output. As the signal level approaches the limit of the low-voltage supplies, what Soundcraftsmen refers to as their "Vari-Portional" circuit anticipates the sine wave's approach to the lower voltage supply level and begins to in crease the B + voltage to allow for additional headroom. This process continues until the system reaches its maximum, which is the limit of the higher voltage supply.
The advantage of such a system is that the amplifier is operating at the lower voltage most of the time. This not only saves energy but also reduces dissipation within the amplifier substantially. Even when the supply voltage is "turned on" to its maxi mum value, it is only at that level for the short period during which the signal peaks call for it.
Figures 1A and 1B show very dramatically how the system works. The basically horizontal upper and lower traces in each of the displays represent the low-voltage supply levels (positive and negative), while the continuously varying waveform in the center is an actual music waveform being amplified by the RA7502. Notice that as the signal waveform approaches the value of the B+ lower supply level, that supply voltage rises before clipping can take place (Fig. 1A). A sharp peak in the signal waveform, this time in the negative polarity (near the center of the display in Fig. 1B), causes the B- voltage to increase (become more negative) in value to prevent downward clipping of the waveform.
To be sure, this extra headroom does not go on forever. As can be seen in the 'scope photo of Fig. 2, a point is eventually reached at which final and irreversible clipping does take place.
When the RA7502 was operated in the bridged mono mode, that point occurred when output voltage reached a peak-to-peak value of approximately 240 volts across an 8-ohm load--which corresponded to an average power of around 900 watts! Protection circuitry in the amplifier is of four kinds. Short circuit protection is provided by a dual-purpose auto-crowbar circuit with automatic rest and front panel LED indication. There is also a thermal sensing protection circuit with automatic reset and front panel indicator, as well as external fusing protection.
The RA7502 also features an unusual patent-pending "Auto Buffer" circuit which is deigned to provide automatic internal electronic compensation for continuous 2-ohm operation without actuating any of the protective circuitry and without current limiting. Details of this circuit refinement were not provided in schematic form, so I can only comment to the effect that it worked effectively when I lowered the load impedance to 2 ohms for a brief period and pushed the amplifier into overload.
Fig. 5-Spectrum analysis (see text) shows IM components near high end of audio
spectrum. Sweep is linear from d.c. to 20 kHz, 2 kHz per linear division. IM
percentage equivalent works out to be 0.06 percent.
Measurements
Figure 3 is a plot of power output, at 4- and 8-ohm loads, versus harmonic distortion for a 1-kHz test signal. For its rated distortion of 0.09 percent at this frequency, the amplifier delivered in excess of 300 watts per channel into 8-ohm loads, and nearly 450 watts per channel into 4-ohm loads. Backing off to rated values of power output (250 watts per channel at 8 ohms), I measured a THD of 0.015 percent and an SMPTE-IM figure of only 0.035 percent. Figure 4 is a plot of distortion versus frequency for rated output (8-ohm loads). CCIF twin-tone IM distortion measured a very low 0.0097 percent, while IHF-IM was only 0.06 percent (measured using the twin-tone method, with worst case observed using 9- and 10-kHz equal amplitude test tones, as shown in the 'scope photo of Fig. 5). Damping factor measured in excess of 200 (that's about as high as I can measure, even with short leads connecting my test fixture and with the amplifier output contributing a tiny fraction of an ohm of resistance to the actual measurement), while dynamic headroom, measured with reference to rated outputs, was 1.94 dB. With 8-ohm loads, that means that the amplifier, operated in the stereo mode, could deliver short term peaks of as high as 390 watts per channel! Signal-to-noise ratio, measured with respect to 1-watt output and with 0.5 volt of signal input (level controls suitably adjusted) was 85 dB. If measured with the level controls wide open and with respect to rated output (the way Soundcraftsmen and others continue to quote S/N despite Standards-conscious re viewers' pleadings), the reading increases to 113 dB, well above the manufacturer's claimed value of 105 dB. Frequency response for this amplifier measured flat within -1.0 dB from 5 Hz to 50 kHz.
Use and Listening Tests
Although the RA7502 is billed as a professional amplifier (and, according to its makers, was specifically designed for use by touring musical groups or other professionals requiring high- power sound reinforcement amplification), I would have no reservations in recommending it or its less expensive first cousin, the PA5001A, for use in any high-quality home music system.
Soundcraftsmen makes an interesting point in calling this amp (and others in their line) "digital/PCM ready." What they mean, very simply, is that the future PCM/digital program sources are going to have so much dynamic range that you are going to need more powerful amplifiers, or at least amplifiers that offer a great deal of headroom (the RA7502 has ample power and ample headroom), if you hope to be able to reproduce such wide dynamic range program sources without compression or clipping. Indeed, I played all manner of demanding source material through this amplifier and it just refused to quit. Furthermore, even at peak levels which caused the 0-dB LEDs to light repeatedly, the RA7502 remained unusually cool, confirming Sound craftsmen's contention that their Class-H circuitry eliminates the need for any cooling fan, even for amplifiers with such high power ratings used under the most difficult conditions. If you need a full horsepower of audio power (at 4 ohms it actually puts out a bit more than 1 HP) and don't want to give up accurate sound reproduction and ultra low TIM, THD and IM of all forms to get it, I would urge you to audition this rugged amplifier. Don't let its "industrial" look fool you: it's a true high fidelity product in every good sense of that much abused pair of words.
by Leonard Feldman
(Audio magazine, Jul. 1981)
Also see:
Soundcraftsmen Pro-Power Ten Amplifier (Equip. Profile, Sept. 1989)
Soundcraftsmen Pro-Control Four Preamplifier (Equip. Profile, Feb. 1988)
Spectro-Acoustics Model 217 Preamplifier (Feb. 1978)
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