Realistic STA-112 Stereo Receiver Review

The Realistic ST A-112 is a compact stereo receiver rated to deliver 30 watts to 8-ohm loads from 20 to 20,000 Hz with no more than 0.05 per cent total harmonic distortion. Its AM and FM tuner sections feature digital frequency synthesis.

Except for a large volume knob, all the visible controls of the STA-112 are flat pushbuttons operated by very light pressure. Six of them are preset station selectors, each usable for both an AM and an FM channel, and another is the memory button used when storing a frequency in one of the station memories. Three longer buttons, pivoted at their centers, are used for tuning (up or down in frequency, depending on which end is pressed), selection of either AM or FM reception, and selection of either phono or aux/tv inputs. Small red lights on the input selectors show which source is connected. The remaining controls are mechanically latching buttons, including one for search (which, when pressed after the tuning button, causes the tuning to scan in the selected direction until a signal is encountered), one for the tape-moni-tor function, and a square power switch. There is also a headphone jack.

Much of the front-panel area is devoted to a large blackout window through which can be seen the tuner frequency display, five signal-strength LED’s and a stereo indicator light, and two groups of LED output-power indicators calibrated from 0.01 to 40 watts per channel into 8-ohm loads. Pressing and releasing the upper left corner of the window causes it to hinge downward, revealing a number of small knobs and pushbuttons. They include the tone and balance controls, switches controlling mono/ stereo mode, FM muting, an FM-multiplex filter, a 30-Hz low-cut filter, and loudness compensation, and switches for the two sets of speaker outputs.

On the rear apron of the STA-112 are spring-loaded speaker connectors, binding posts for the antennas (300- and 75-ohm FM feedlines and a wire AM antenna), and a hinged AM ferrite-rod antenna, in addition to the various signal jacks. There is a single unswitched a.c. outlet. The Realistic STA-112 is 17-3/4 inches wide, 14-1/2 inches deep, and only 3-1/4 inches high. It weighs about 15-1/4 pounds. Price: $339.95.

Laboratory Measurements

After a one-hour preconditioning warm-up at 10 watts output (which left it only slightly warm), the Realistic STA-112 delivered just over 40 watts per channel (at 1,000 Hz) to 8-ohm loads at the clipping point. The 4-ohm clippping output was 54 watts, and into 2 ohms it was 55 watts per channel. The dynamic power output, using a 20-millisecond 1,000-Hz tone burst, was 56 watts, 78 watts, and 66 watts into loads of 8, 4, and 2 ohms, respectively. The measured 8-ohm clipping headroom was 1.3 dB, and the dynamic headroom was 2.73 dB.

The 1,000-Hz harmonic distortion with 8-ohm loads was less than 0.002 per cent up to nearly 20 watts output, increasing to 0.0034 per cent at 40 watts. The distortion with 4-ohm loads was nearly as low, in the 0.002 to 0.004 per cent range from 1 to 25 watts, rising to 0.054 per cent at 55 watts. Even 2-ohm operation did not faze the STA-112, whose distortion was under 0.0056 per cent up to 10 watts and 0.02 per cent at 40 watts. After sustained high-power testing we could not drive the amplifier into clipping with 2-ohm loads, since its protective relays shut it down before there was visible waveform distortion. When that occurs, the power must be shut off for a few seconds to restore normal operation.

The total harmonic distortion of the STA-112 was extremely low over most of the audio-frequency range regardless of the power output. Readings were typically in the range of 0.001 to 0.01 per cent from 20 to 10,000 Hz and beyond at power outputs from 3 watts to the rated 30 watts. The worst-case condition was with the amplifier delivering 3 watts output at 20,000 Hz, when the distortion was all of 0.025 per cent! The IHF IM (intermodulation) distortion with inputs of 18 and 19 kHz was —90 dB for the second-order (1,000 Hz) component and — 73 dB for the third-order component at 17 and 20 kHz. The amplifier was stable with reactive simulated speaker loads and with the standard IHF reactive load, and its slew factor exceeded our measurement limit of 25.

The input sensitivity for a reference 1-watt output at 1,000 Hz was 30 millivolts (mV) through the auxiliary inputs and 0.95 mV through the phono input. The respective A-weighted signal-to-noise ratios were 81.6 and 67.8 dB referred to 1 watt. The phono input overloaded at very safe levels, ranging from 152 mV at 20,000 Hz to 230 mV at 20 Hz. The measured phono input impedance was 50,000 ohms in parallel with 180 picofarads of capacitance.

The bass tone control had a sliding turnover frequency that shifted from about 100 to 300 Hz as the control was varied. The treble response curves were hinged at about 3,000 Hz. The maximum bass boost of almost 23 dB at 20 Hz was more than anyone could need, especially for a modestly powered amplifier. If used carelessly, this could cause output clipping at low-bass frequencies, so discretion is advised in its application. The “30 Hz filter,” on the other hand, rolled off the response by only 1 dB at a frequency of 30 Hz and by less than 2 dB at 20 Hz. Assuming that its ultimate slope is 6 dB per octave, this filter could not be expected to have much audible effect on either program content or rumble, and in fact we heard no change in the sound when it was switched on.

The loudness contours boosted the low frequencies (under 300 Hz) moderately at reduced volume settings, with a very slight high-frequency boost beginning at about 7,000 Hz and reaching 5 db at 20,000 Hz. The RIAA phono equalization (measured at the tape-recording outputs) was flat within 0.5 dB overall from 50 to 20,000 Hz and down 2.5 dB at 20 Hz. It was not affected significantly by the inductance of a phono cartridge at the preamplifier input.

The tuner’s FM usable sensitivity (mono) was 10.3 dBf (1.8 microvolts, or μV). The stereo sensitivity was set by the switching threshold of 28 dBf (14 μV). The 50-dB quieting sensitivity in mono was 12.8 dBf (2.4 μV), and in stereo it was 36 dBf (34.7 μV). The tuner distortion at 65 dBf (1,000 μV) was 0.11 per cent in mono and 0.16 per cent in stereo, with corresponding noise levels of —76.6 and —69.6 dB. The alternate-channel selectivity was an excellent 77.5 dB, and the adjacent-channel selectivity was 5 dB. The tuner’s image rejection was a good 74.4 dB, its AM rejection was 56 dB at 65 dBf input, and the capture ratio was an excellent 0.97 dB at 45 dBf (100 μV). The muting threshold was 27 to 29 dBf (12 to 15 μV), the 19-kHz pilot-carrier level in the audio was —69 dB, and the tuner hum was — 69 dB. The IHF IM distortion (14- and 15-kHz modulating frequencies) relative to 100 per cent modulation was in the range of — 73 to —92 dB, depending on the specific distortion product and whether the tuner was operating in mono or stereo.

The FM frequency response was flat within ±0.5 dB from 30 to 15,000 Hz. The channel separation was 46 dB from 300 to 2.000 Hz, 35 dB at 30 Hz, and 38 dB at 15.000 Hz. The AM frequency response was flat from 40 to 1,500 Hz and down 6 dB at 30 and 2,600 Hz.

Comment

The designers of the Realistic STA-112 have succeeded in creating a full-featured stereo receiver whose appearance and operation should not intimidate even the least-technical-minded user. Although all the standard receiver amenities are provided, most are out of sight behind the hinged window on the panel. The low-profile design also contributes to the ease with which the receiver can be accommodated in almost any home installation.

The ability of the STA-112 to drive 2-ohm loads at a substantial power level without distortion, damage, or interruption of operation sets it aside from most receivers with comparable power ratings. There has been no sacrifice of protection for the output transistors, as we discovered when the protective relay tripped under full drive conditions with a high-frequency square wave or when we approached clipping levels with 2-ohm loads.

All the control and operating functions of the STA-112 worked as expected, and we found no operating bugs during our use tests. During extended listening tests, the receiver always sounded excellent. It is also worth noting that the capture ratio and selectivity of the FM tuner section were both considerably better than we have measured on most tuners and receivers, including some priced well above the STA-112. Altogether, this receiver represents a lot of performance in a very compact and attractive package.

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