Toshiba SA-2500 Stereo Receiver Review

In the past, stereo receivers with list prices under about $300 have usually been rather basic in their operating flexibility and control features, and their electrical performance was rarely comparable to that of higher-price models. Although power ratings of 30 watts or so were not uncommon, their distortion levels were typically several times the minuscule values more expensive products could boast.

Toshiba’s Model SA-2500 receiver represents a sizable step toward closing the gap between low and medium-price receivers, both in performance and in features. It is a very convincing demonstration of the manner in which technology has overcome inflation, at least in the high-fidelity-component field. The SA-2500 is “more” receiver, as well as better in almost every way, than any comparably priced unit we have tested. Certainly five or ten years ago one could not buy a receiver nearly as good as this one for $250 or anything close to that figure.

The power ratings of the SA-2500 are in line with what we would expect at its price (25 watts per channel into 8-ohm loads from 20 to 20,000 Hz), but its distortion rating of 0.05 per cent rivals the performance of most far more expensive receivers. Similarly, although the FM-tuner section has good, if not outstanding, ratings for sensitivity and selectivity, its distortion in both mono and stereo is only a fraction of what we used to get in receivers before the development of specialized integrated circuits.

Other features of the SA-2500 that are not usually found in its price class include a d.c.-coupled power-amplifier section, a relatively high phono-overload rating of 180 millivolts, a switchable infrasonic filter, and an effective (as opposed to a pretentious) use of LEDs as signal and tuning indicators.

The silver-colored front panel of the Toshiba SA-2500 has a clean, open look that will not discourage a non-technical listener from using it to full advantage. The slide-rule dial scales for FM and AM bands have an illuminated traveling pointer. Below the scales, but still within the dial cutout on the panel, are the LED indicators. Correct FM tuning is shown by a green light, which is flanked by red arrows that come on as one approaches a signal to show which direction the dial pointer should be moved to reach the center of the channel. Five green LEDs come on sequentially to form a horizontal line whose length is proportional to the signal strength, for both FM and AM, and a red stereo light shows the reception of a stereo-FM transmission.

The dial cutout is flanked by a rectangular power button and a large knob that operates a flywheel tuning mechanism. The other controls form a single line across the bottom of the panel. In the center of the group are four small knobs for the bass, treble, balance, and volume controls. The balance control is center-detented, and each tone control has eleven detented settings.

To the left of the knobs are three small pushbuttons that separately activate the two sets of speaker outputs and switch in the “16-Hz Subsonic” filter. Three similar buttons to the right of the knobs control the loudness compensation, tape-monitor functions, and the FM muting/mono. The remaining knob is a function (input) switch with positions for am, fm, phono, and aux.

On the rear apron of the SA-2500 there are binding posts for 300-ohm and 75-ohm FM antennas and a wire AM antenna, as well as a hinged and pivoted ferrite-rod AM antenna. The two pairs of speaker outputs are spring-loaded connectors, and there is a single unswitched a.c. convenience outlet. The Toshiba SA-2500 is furnished with walnut-grain wooden side panels on its metal cabinet. Its overall dimensions are 17-3/4 inches wide, 13-3/4 inches deep, and 4-5/8 inches high. It weighs 17 pounds, 10 ounces. Price: $249.95.

Laboratory Measurements

Because of its relatively low power rating in proportion to its size and weight, the SA-2500 ran quite cool during all our tests. With both channels driving 8-ohm loads at 1,000 Hz, the output waveform clipped at 34 watts per channel. Into 4-ohm loads, the power at clipping was 46 watts. The maximum continuous power output could not be measured with 2-ohm loads because the receiver’s internal protective fuses blew before we reached the clipping level. 'The total harmonic distortion with 8-ohm loads rose smoothly from under 0.01 per cent at less than 1 watt output to 0.022 per cent at 30 watts. The distortion was slightly higher when we drove 4-ohm loads, rising from 0.02 per cent at less than 1 watt to 0.09 per cent at 35 watts. Although actual waveform clipping did not occur until we reached 46 watts, some high-frequency oscillation appeared on the waveform at outputs between 35 and 46 watts, preventing measurement of the harmonic distortion at those levels.

At outputs up to the rated power of 25 watts into 8 ohms, the distortion of the amplifier was less than 0.05 per cent from 20 to 20,000 Hz (it was typically in the range of 0.01 to 0.03 per cent at most frequencies and power outputs). Two-tone intermodulation-distortion (1M) measurements using 19,000- and 20,000-Hz input signals of equal amplitude with a combined peak power equivalent to a 25-watt sine-wave signal resulted in second- and third-order products (at 1,000 and 18,000 Hz, respectively) at —72 and —75 dB relative to 25 watts. These distortion levels are too low to be in any way significant.

We measured the dynamic-power output of the SA-2500 with the 20-millisecond tone bursts of the standard IHF test signal. Into 8 and 4 ohms, the short-term clipping power was, respectively, 50.4 and 81.6 watts. Because of the short duty cycle of the test signal, we were also able to measure the maximum unclipped dynamic power into 2-ohm loads, finding it to be an impressive 98.9 watts per channel. Referred to the receiver’s rated 25 watts, the IHF clipping headroom was 1.34 dB and the dynamic headroom was 2.65 dB (both are 8-ohm ratings). When the amplifier was overdriven by 10 dB with the same pulsed test signal, its recovery time was a negligible 20 microseconds. The amplifier rise time was 3 microseconds, and its IHF slew factor was 3. It was stable with capacitors as great as 3 microfarads across its resistor loads.

For a reference output of 1 watt, an input of 23 millivolts (mV) was required at the auxiliary or 0.37 mV at the phono input. The respective A-weighted noise levels, referred to 1 watt, were —80.4 and —76.4 dB. The phono input overloaded at 175 mV at 1,000 Hz and at the equivalent of 185 and 157 mV at 20 and 20,000 Hz, respectively. The phono input has a measured resistance of 47,000 ohms shunted by 15 picofarads of capacitance.

The preamplifier tone controls had typical frequency-response characteristics, with the bass-turnover frequency varying between 200 and 400 Hz and the treble curves hinged at 2,000 Hz. The loudness compensation boosted both low and high frequencies as the volume-control setting was reduced. The “16-Hz” filter, which has a rated slope of 6 dB per octave, affected the response as high as 100 Hz and at 20 Hz had reduced it by about 3 dB. The RIAA phono equalization was very accurate, varying by only 0.5 dB overall between 20 and 20.000 Hz. There was a very slight loss of high-frequency response when the phono equalization was measured through the inductance of a typical magnetic phono cartridge in our test setup.

The FM-tuner section of the SA-2500 had an IHF usable sensitivity in mono of 11.8 dBf (2.1 microvolts, or μV), and in stereo it was set by the automatic switching threshold at 23 dBf (7.5 μV). The respective 50-dB-quieting sensitivities were 15 dBf (3 μV) for mono and 37 dBf (39 μV) for stereo. The distortion at 65 dBf (1,000 μV) input was 0.067 per cent in mono and 0.13 per cent in stereo, and the noise levels were -74 and -70 dB, respectively.

The two-tone intermodulation distortion with FM modulating signals of 14,000 and 15.000 Hz was -57 dB for the third-order (13,000-Hz) component relative to each tone, and —75 dB for the 1,000-Hz difference tone relative to 100 per cent modulation. The capture ratio was 1.38 dB and AM rejection was 56 dB, both at 45 dBf (100 μV) input. The average alternate-channel selectivity was 62 dB, and the adjacent-channel selectivity was 6 dB. The measured image rejection exactly matched the rating of 60 dB. The 19-kHz pilot-carrier leakage was a low —67 dB. The tuner hum level was —68.5 dB, and it consisted of a large number of power-line harmonics whose spectrum indicated a 1.25-millisecond pulse at a 120-Hz repetition rate (the hum was certainly not audible, however).

The stereo frequency response was flat from 30 to 1,000 Hz, rising slightly at higher frequencies to a maximum of + 1.7 dB at 10.000 Hz. The channel separation was 39 dB through the midrange and exceeded 25 dB from 30 to 15,000 Hz. The frequency response of the AM tuner section was down 6 dB at 70 and 2,400 Hz.

Comment

In almost every respect, the Toshiba SA-2500 comfortably surpassed its specifications, which were already better than one would normally expect to find in a $250 receiver. In fact, we would consider it to be an exceptional product in its price class. Its sound matched that of other components costing many times its price— which came as no surprise, since it had already matched many of them just as easily on the test bench. Apparently the output stages are protected by fuses rather than current-limiting circuits, and this gives the receiver an impressive short-term power output into 2-ohm loads of about 100 watts per channel.

The LED indicators were completely functional, and we especially appreciated the precision with which the center-of-channel light came on; only at the exactly correct tuning adjustment did it give its “OK” signal. We would expect that the average owner, using only this light as a tuning guide, would realize very nearly the same very low FM distortion that we measured in our tests even though we had a distortion meter to set the tuning. The same cannot be said for most tuning indicators, either meters or LEDs, that we have used.

Some listeners might miss the possibility of interrupting the signal path between the preamplifier and power amplifier, or the lack of any switched a.c. outlets, but they might remind themselves that this is a very inexpensive receiver by 1981 standards. Rather than pick nits, we would prefer to give the unit its due: the Toshiba SA-2500 is an outstanding value in every respect, as attractive as it is functional, and with a real (that is, audible) performance that cannot be surpassed (except for power) at less than several times its price.

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