Proton 930 Stereo Receiver Review

The Proton 930 stereo receiver combines a high-performance FM tuner section designed by Larry Schotz with an amplifier capable of driving load impedances as low as 2 ohms at high power levels without damage or excessive distortion. It is rated to deliver 30 watts per channel into 8 ohms. There is an “anti-clipping circuit” designed to reduce the audible consequences of waveform peak clipping and a bass eq circuit to correct for the bass-response limitations of many compact speaker systems. Dimensions are 4 x 16-1/2 x 9-3/4 inches, weight 15-3/4 pounds. Price: $360.

LAB TESTS

The Proton 930 demonstrated an exceptional output-current capability. Its measured 1,000-Hz output power at clipping was 44 watts into 8 ohms, 63 watts into 4 ohms, and 66 watts into 2 ohms. Its dynamic power output was even more impressive: about 54, 78, and 130 watts into 8, 4, and 2 ohms, respectively. Clearly, this is no ordinary “30-watt” receiver!

Our test unit, an early production sample, lacked a final instruction manual as well as any of the pertinent performance specifications other than rated output. In a way, this made our tests more interesting, since each measurement showed this to be an even more unusual receiver than we could have expected.

The amplifier distortion of the Proton 930 was not only very low, but it remained low over the full audio range and at any power level up to the clipping point. The noise level was exceptionally low, the phono preamplifier overloaded at the same high level of 210 millivolts over the full audio range, the RIAA equalization was highly accurate and unaffected by cartridge inductance, and so on. The amplifier did shut down as we approached the clipping level when driving 2-ohm loads. An audible click and lapse of a few seconds before the amplifier returned to service suggested that there was a thermal protection system in operation.

We were especially interested in the FM-tuner performance in view of the acknowledged talents of its designer and our previous experience with his products. We were not disappointed. The measured sensitivity was high, and the noise level was as low as we would expect to find in any competitively priced receiver or tuner. The distortion in stereo was very low—lower, in fact, than we can recall having previously measured in an FM tuner. In only one respect did the FM tuner’s performance disappoint us. Its image rejection was so unexpectedly low at 40 dB that we suspect it was only a defect in the early production sample that we tested.

The selectivity of the Proton 930 was among the highest we have ever measured: 96 dB for altemate-channel spacing and almost 16 dB for adjacent-channel spacing. This was even more noteworthy in view of the tuner’s low distortion, since distortion usually must be traded off for high selectivity.

Capture ratio and AM rejection were both much better than average. The stereo channel separation was more than adequate and quite uniform. The front-panel LED signal-strength indicators were well spaced, coming on unambiguously at input levels ranging from 23 to 72 dBf (when at least three lights are lit, the full tuner performance is realized). Even the AM tuner had a better than average frequency response, flat within 2.5 dB overall from 26 to 3,300 Hz and down 6 dB at 4,000 Hz.

COMMENTS

The Proton 930’s anti-clipping circuit causes the waveform to clip more “softly” than is usually the case. Though in theory this is desirable, we are not necessarily convinced of its practical benefits. We prefer to operate an amplifier well below clipping, and the power reserves of the Proton 930 make clipping even less likely than with most receivers of considerably higher continuous-power ratings. But as far as we can tell, the circuit does no harm and has no detectable or measurable effect below the clipping point.

The bass eq is a useful feature that complements the bass rolloff of many compact and bookshelf-sized speaker systems. Its 10-dB peak at about 45 Hz and steep infrasonic rolloff below 30 Hz can greatly enhance the low-bass performance of such speakers (see graph). The bass eq response can give an actual reduction in audible rumble instead of the increase likely to result from the use of a simple equalizer or tone-control circuit for that purpose. The boost frequency is so low that it has no significant effect on most reproduced sound, including voices. But when the program material calls for it, the bass eq gives a welcome solidity to the sound of a typical bookshelf-sized speaker system, though without any boominess.

The Proton 930 is conspicuously free of most of the gadgetry and glitter of other recent receivers, but it more than makes up for this by its stellar performance. Its relatively high short-term output power and high-current capability make it a much more “powerful” receiver than its 30-watt rating would suggest. The Schotz FM tuner is, in its own way, even more noteworthy than the amplifier section. Assuming that the somewhat low measured image rejection was a peculiarity of our sample and not typical of the design, this receiver easily outperforms anything we have seen at anywhere near its price.

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