Nakamichi SR-3A Stereo Receiver Review

The Nakamichi SR-3A is one of the first receivers licensed to use the Stasis audio-amplifier topology developed by Threshold Corp. The Stasis circuit was designed to provide completely stable, low-distortion output without using overall negative feedback. It does this by combining a very low-distortion, low-output impedance amplifier with a high-impedance, high-cur-rent “bootstrap” amplifier in a single output stage. The high-current amplifier supplies the major portion of the total power output, and the low-current amplifier (which effectively determines the total amplifier distortion) merely supplies the small signal current needed to compensate for nonlinearities in the other amplifier’s output.

The Nakamichi SR-3A is rated to deliver up to 45 watts per channel into 8-ohm loads from 20 to 20,000 Hz with no more than 0.1 percent distortion. Its rated dynamic headroom is 1.5 dB, and it can deliver, according to the manufacturer, a peak current output of 18 amperes per channel. Although many of its operating features are similar to those of other good receivers, it is built to the traditional Nakamichi standards of quality and attention to detail. The multiple-winding power transformer drives separate rectifiers and regulators for the audio, tuner, display, and video circuitry. Considerable effort was devoted to eliminating signal currents in ground circuits to prevent instability or crosstalk resulting from the sharing of a common ground path by more than one part of the receiver. The SR-3A is designed to switch video sources as well as conventional audio tape recorders, turntables, and compact disc players. A video output is also provided to drive a video monitor from the selected video source.

The bass and treble tone-control circuits can be bypassed by pressing a small button on the panel. Another button selects preamplifier inputs for either a moving-coil (mc) or a moving-magnet (mm) phono cartridge. A separate switch on the rear apron of the receiver increases the mc gain by an additional 8 dB for use with low-output cartridges. A subsonic filter (usable only with a phono source) provides a 12-dB-per-octave cutoff below 20 Hz. The tape/vcr monitor knob selects for playback either the regular program source or the output of either tape deck or VCR. It also has positions for audio dubbing in either direction between the audio or the video recorders. The front panel also features controls for volume, balance, and loudness, a headphone jack, and two speaker-output selectors.

The display window, though simple and tasteful in design, conveys all the necessary information, including tuner band and frequency, tuning mode (auto scan or manual), and relative signal strength. Flat keys below the window select the AM or FM band, select the tuning mode, and tune up or down in frequency. The five station-preset keys can be switched to store a total of ten frequencies. Each key recalls the appropriate band as well as the channel frequency. A light on each key indicates the selected preset channel.

The rear apron of the SR-3A contains all of the signal input and output jacks, a 75-ohm Type F antenna jack, and a mounting for the detachable, pivoted am loop antenna. A 300- to 75-ohm coupling transformer is supplied for use with antennas having 300-ohm feeders or as a connecting plug for a 75-ohm coaxial cable. The speaker outputs use insulated binding posts, and there are two AC convenience outlets, both of them switched.

The Nakamichi SR-3A, finished in black with gold markings, measures 17 inches wide, 14-1/2 inches deep, and 4 inches high. It weighs 18-3/4 pounds. Price: $599.

Lab Tests

During the 1-hour preconditioning period at 15 watts output (one-third rated power), the top of the receiver became very hot over the heat sinks, though it was never more than slightly warm during actual use. The 1,000-Hz output power at clipping was 54 watts per channel into 8 ohms, 62.4 watts into 4 ohms, and 72 watts into 2 ohms. With 8-ohm loads, the 1,000-Hz distortion was a nearly constant 0.1 to 0.08 percent from 0.1 to 10 watts, decreasing to a minimum of 0.0135 percent at 50 watts. The shape of the distortion curves with 4- and 2-ohm loads was quite similar, but the readings were somewhat higher— typically 0.15 to 0.2 percent with 4 ohms and 0.4 to 0.5 percent with 2 ohms.

The distortion was equally unaffected by frequency. At rated power into 8-ohm loads it was about 0.015 percent from 20 to 5,000 Hz, rising to 0.047 percent at 20,000 Hz. At half-power and one-tenth power the distortion was similarly constant over the entire audio range, averaging about 0.05 percent at 22.5 watts and 0.09 percent at 4.5 watts. The slew factor exceeded 25, and the reactive-load factor was 0.59 dB at 63 Hz. The 10,000-Hz square-wave response with a reactive simulated speaker load was one of the best we have seen from any amplifier, showing virtually no signs of overshoot or ringing.

The high-level (CD) input’s sensitivity for a 1-watt output was 29 millivolts (mV), and the A-weighted noise level was —85.5 dB. The phono input’s sensitivity was 0.315 mV for the MM input and 0.02 mV for the mc input, with respective noise levels of —81.4 and —75 dB. The mm phono input overloaded at inputs between 100 and 148 mV over the 20- to 20,000-Hz range. The RIAA phono equalization was accurate within 0.03 dB overall from 20 to 20,000 Hz. The subsonic filter introduced a 1.5-dB rise in the 40- to 60-Hz range, decreasing to — 5 dB at 20 Hz (with the rated 12-dB-per-octave slope below that frequency).

The tone controls had very good characteristics, affecting only the frequencies below about 200 Hz and above 5,000 Hz. The loudness compensation boosted both low and high frequencies at reduced volume settings, but to a moderate degree, and did not cause an unduly heavy sound. Its maximum bass boost of 10 dB took place below 50 Hz, and the treble boost was only 6 dB at 20,000 Hz.

The fm tuner section had an unusually flat frequency response, within +0.2, —0.3 dB from 30 to 15,000 Hz. The stereo channel separation was also very uniform, 44 to 46 dB from 100 to 10,000 Hz, decreasing to 37.5 dB at 30 Hz and 40 dB at 15,000 Hz. The tuner’s sensitivity was good, and its distortion and noise measurements were excellent. The muting and stereo thresholds were rather high, in the range of 30 dBf (or 17.4 μV), with the result that stereo transmissions could be received only if they were strong enough to provide listenable quality.

Comments

Our measurements of the Nakamichi SR-3A show it to be an above-average receiver in most respects. Only its 46.5-dB alternate-channel selectivity was slightly disappointing (in view of its general excellence), though even performance equaling the manufacturer’s rating of 55 dB would not be considered especially noteworthy in a receiver of this price. With that minor exception, we found many uncommon qualities in the SR-3A. The tone controls had superb characteristics, the phono equalization and the subsonic filter were as nearly ideal as one can find, and the fm tuner’s noise and distortion levels were excellent.

The audio distortion of the Stasis amplifier section followed an unusual pattern, with nearly constant readings over wide changes of power and frequency and only the typical (and expected) increase when it was driving low-impedance loads.

The ultimate test of any audio component is its actual listening performance. Here the Nakamichi SR-3A really impressed us. For instance, although the measured sensitivity of the fm tuner section was good, it was not especially outstanding. In real-world use, however, connected to the same antennas we use with other tuners and receivers, it delivered a dead-quiet background, and there was a striking absence of the usual aberrations of fm reception (such as multipath distortions). In an A/B comparison with a system of separate components, each of which costs about as much as the Nakamichi SR-3A, it more than held its own.

Nakamichi claims that the rather modestly powered SR-3A can play as loud as much more powerful conventional amplifiers. In our listening tests it came remarkably close to doing just that—it was difficult at times to accept that we were hearing a 45-watt amplifier! Since its headroom and power-envelope measurements, though good, were not particularly unusual, the most probable explanation lay in the ability of the Stasis amplifier to operate smoothly up to its clipping point into real speaker loads, without the various dynamic distortions that are typical of most amplifiers. We were unable to verify this quality with previous Stasis amplifiers (such as Nakamichi’s own PA-7), probably because their 200 watts and more of available power produced volume levels far higher than could be tolerated during prolonged listening. Because it is very easy to drain the reserves of a 45- or 55-watt amplifier, its true performance at maximum output can be assessed more readily.

In addition to its fine performance, the Nakamichi SR-3A appealed to us because of its tastefully understated styling and the good feel of its controls. These are characteristics that Nakamichi has always strived to build into its products, and they certainly contribute to the pleasure of using this one.

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