Pioneer SX-8 Stereo Receiver Review

The new Pioneer SX-8 AM/FM stereo receiver heads that company’s receiver line. It is completely microprocessor controlled, with a power-amplifier section rated to deliver 100 watts per channel to 8-ohm loads from 20 to 20,000 Hz with no more than 0.005 per cent total harmonic distortion. The styling of the SX-8 matches that of the other current Pioneer audio components, with pale-gold, satin-finish panels flanking a dark-brown center section in which are contained a number of displays that completely define the unit’s operating conditions.

The receiver’s most unusual external characteristic is its total lack of knobs.

Everything is controlled by flat rectangular buttons of different size and shape. These operate voltage-controlled amplifiers and solid-state switches to adjust gain, frequency response, and signal routing through the amplifier circuits.

The SX-8 has all the usual receiver functions, including bass and treble tone controls, loudness compensation, audio muting, a low-cut (infrasonic) filter, and switching for two pairs of speakers. The phono-preamplifier gain and input impedance can be switched to accommodate moving-magnet (MM) or moving-coil (MC) cartridges. The controls are operated by holding in the specified button until the desired condition is reached. The final setting is retained (even with the power off) by the receiver’s memories, and the SX-8 returns to its previously set operating condition each time it is turned on.

The actual setting of each control is shown clearly by luminescent numerical displays in the center panel section. For example, a two-digit readout with numerals from 0 to 31 shows the volume setting. When power is applied, the numerals blink for several seconds before the audio outputs are connected so that one can see the previous setting and change it if necessary before any sound is heard. The actual volume change is continuous and very smooth. The numbers take the place of the numerical calibrations sometimes located around a control knob, although they are far more readable.

Another window with two single-digit readouts joined by a line pattern shows the tone-control settings. A “0” indicates a flat response, and when both controls are flat the digits are joined by a horizontal line. Each control has a calibrated adjustment range of plus or minus seven digits, and any change from flat is accompanied by an up or down “jog” in the line between the numbers to show either a boost or cut of response. Like the volume, the tone-control circuits respond smoothly and almost imperceptibly, depending on the discrete digits to show the equivalent settings of nonexistent knobs.

Unique as this control system is, it is further enhanced by dual memories for the major variable controls, permitting the storage of two volume and two tone-control settings for instant recall (tone-control responses can be returned to flat at any time by pressing a button between the “up” and “down” control buttons). The volume memories also store the status of the loudness compensation and muting buttons as well as the volume setting. The muting button reduces volume smoothly by about 25 dB a second or so after the button is pressed and restores it equally smoothly at a second touch.

The digital-synthesis tuner section of the SX-8 is in many respects similar to other digital tuners we have tested. It is tuned by “up” and “down” buttons, with the frequency displayed by four large digits on the center panel (this is also the readout of a digital clock when the receiver is off or when any input other than the tuner is selected). There are eight memory buttons, each of which can store one AM and one FM frequency in the memory for instant recall. A sliding plastic strip, inserted from the left edge of the panel, holds the frequencies of the memorized stations so that they can be seen through small windows above the memory buttons. This eliminates the guesswork that is associated with the more common unmarked preset buttons.

The tuner has the usual tuning and scanning modes, selectable by small buttons at the bottom of the panel. It can be set to advance one channel in either direction, depending on which button is pressed, to scan to the next station it encounters and stop there, or to scan to each station, stop for 5 seconds, and resume scanning unless stopped by touching another button. The tuning increments are 100 kHz for FM and either 9 or 10 kHz for AM (selectable by a switch in the rear of the receiver). The input source is selected by a vertical row of large buttons at the right of the display panel. In addition to the FM and AM tuner inputs, there are phono, aux/video (high level), tape 1, and tape 2 inputs (the last named is also identified for use with external signal processors).

Across the bottom of the display panel are two sets of LED power-output indicators. Occupying much of the rest of the center panel is a signal-flow diagram whose symbols are illuminated to show the various control settings. It includes a three-LED signal-strength indicator and two arrows that show the direction of any balance-control movement from center (both are lit when it is centered, and it can be centered instantly by pressing in both the balance buttons together). The letters A and B under the tone and volume displays indicate which memory has been selected for each particular function.

The rear panel includes preamplifier-out-put/main-amplifier-input jacks with jumper links, three a.c. outlets (one switched), and an am stereo output jack for use with a yet-to-be-developed stereo-AM adaptor (the FCC has just decided to let the market determine which system it will be). The Pioneer SX-8 has a metal cabinet that matches the color of the front panel.

Laboratory Measurements

Since the power and distortion ratings of the Pioneer SX-8 are specified only for its power-amplifier section, we first measured that part separately (after preconditioning) by driving it through the main-amplifier inputs in the rear of the receiver. Then we repeated those measurements through the preamplifier inputs in the normal manner and also made measurements of the preamplifier section alone through the pre out jacks.

Through the power amplifier alone, the 1,000-Hz distortion was virtually unmeasurable (it was typically 0.0002 to 0.0003 per cent) for all power outputs from 1 to 100 watts. It was only 0.00063 per cent at 120 watts—just at the clipping level into 8-ohm loads. A power-amplifier input of 84 millivolts (mV) produced a reference output of 1 watt. The slew factor was 2.5, and the amplifier rise time was about 2 microseconds. It was stable with simulated reactive loudspeaker loads.

At the rated 100 watts output and at all lower power levels, the distortion rose only slightly at low frequencies to about 0.003 per cent at 30 Hz. (It was not possible to make accurate distortion measurements at 30 and 20 Hz because the distortion components could not be isolated from power-line frequencies at those minuscule levels.) The distortion rose at high frequencies to a maximum of 0.008 per cent at 20,000 Hz and full power, falling to 0.0045 per cent at — 10 dB (10 watts).

Measured through the preamplifier section, the distortion readings were much higher than through the power amplifier alone. The distortion also depended to a degree on the volume-control settings, indicating some nonlinearity in the low-level, voltage-controlled circuits of the preamplifier.

We measured the distortion and clipping level at the preamplifier output, driving a standard IHF load of 10,000 ohms in parallel with 1,000 picofarads, at different frequencies and with various volume settings. At the IHF reference-gain setting (0.5-volt output for 0.5-volt input at a high-level jack) the distortion was low—less than 0.05 per cent from 20 to 20,000 Hz at a 0.7-volt output. However, at a 1-volt output the distortion increased sharply at about 5,000 or 6.000 Hz, rising to about 1 per cent at 10.000 Hz and 4.5 per cent at 20,000 Hz.

Waveform distortion clearly indicated the onset of slew limiting.

At 1,000 Hz, the distortion increased smoothly from less than 0.01 per cent at 0.15 volt and below to 0.7 per cent at 1 volt and about 1 per cent at 1.2 volts. We measured the preamplifier section’s voltage output at clipping at frequencies of 1, 10, and 20 kHz as a function of volume-control setting. The IHF reference-gain setting corresponded to a reading of 18 on the front-panel numerical display. At that point the clipping output was 1 to 1.1 volts for frequencies under 10,000 Hz. The level at which clipping occurred increased to 1.7 to 2 volts at maximum gain (a reading of 31 on the panel). However, reducing the volume to a reading of 15 (which is — 11.5 dB referred to standard gain) reduced the available output before clipping to 0.63 volt at 1.000 and 10,000 Hz and to 0.45 volt at 20.000 Hz. Since the power-amplifier section requires 0.85 volt for its rated output of 100 watts, it is evident that it cannot be driven to that level, even at middle frequencies, when the volume is set below about 17. At 20,000 Hz a volume setting of at least 20 is needed for full output without distortion.

The distortion of the complete amplifier rose from 0.01 per cent at 1 watt to 0.07 per cent at 100 watts and 0.08 per cent at 120 watts using 8-ohm loads at 1,000 Hz. When we drove 4-ohm loads, the amplifier performance was not very different, with the distortion readings ranging from 0.01 per cent at 1 watt to 0.1 per cent at 130 watts (the output clipped at 132 watts). Even 2-ohm loads did not degrade the amplifier performance significantly. The 2-ohm clip-ping-power output changed during our tests, possibly due to the extreme heating of the amplifier when delivering high power into low-impedance loads (though it was never shut down by its thermal protection system or damaged in any way). Immediately after the normal preconditioning period, the 2-ohm output clipped at about 120 watts, but after extended operation the maximum power fell to about 72 to 75 watts. This is still much more than many higher-rated amplifiers can deliver to such a low load impedance. The 2-ohm distortion was 0.014 per cent at 10 watts and 0.215 per cent at 75 watts.

The 8-ohm clipping headroom of the Pioneer SX-8 was 0.72 dB. The short-term power available during dynamic-power and headroom tests was essentially identical to the steady-state clipping output—120 watts (0.72 dB) into 8 ohms, 134 watts into 4 ohms, and 130 watts into 2 ohms. The rise time through the preamplifier measured 4 microseconds, and the slew factor varied from 2.5 at maximum gain to 0.75 at the IHF standard gain setting (0.5-volt input giving 1 watt output).

At full power output and with the preamp in the circuit, distortion measured between 0.08 and 0.1 per cent from 20 to 5,000 Hz, increasing to 0.36 per cent at 20,000 Hz. At lower power levels the curve was similar, but with lower readings (typically 0.06 per cent at half power and 0.03 per cent at one-tenth rated power). The IHF intermodulation distortion (IM) was measured with equal-amplitude inputs of 19- and 20-kHz signals whose combined peak value was equal to that of a single-frequency 100-watt signal. The third-order IM (18 kHz) was — 72 dB and the second-order component at 1.000 Hz was —78 dB, both referred to 100 watts.

The tone-control frequency-response curves were conventional, with a maximum control range of about ±12 dB. The “15-Hz” filter reduced the output by about 0.5 dB at 40 Hz and 2.5 dB at 20 Hz. The RIAA phono equalization was unusually accurate, deviating less than 0.5 dB from the ideal response in the 20- to 20,000-Hz range. When measured through the inductance of a phono cartridge, the phono response decreased very slightly and gradually to — 1 dB at 10,000 Hz and —1.3 dB at 20.000 Hz relative to the 1,000-Hz reference level.

The FM-tuner section of the SX-8 had a mono IHF usable sensitivity of 11.6 dBf (2.1 microvolts, or μV). The stereo-switching threshold was 31.2 dBf (20 μV). The 50-dB quieting sensitivity was 15 dBf (3.1 μV) in mono and 37 dBf (39 μV) in stereo. The respective noise readings at 65 dBf (1,000 μV) input were —75 and —69.5 dB, and the distortion was 0.077 and 0.165 per cent in mono and stereo, respectively. The IM distortion (using 14- and 15-kHz test frequencies whose peak level corresponded to 100 per cent modulation of the signal generator) was — 74 dB (below 100 per cent modulation) for the second-order component at 1,000 Hz and — 55 dB for the third-order distortion at 13,000 Hz (relative to 100 per cent modulation at that frequency). In stereo, the distortion readings were —68 and —55 dB, but there were also a number of additional beat frequencies in the output at levels between — 60 and — 80 dB (typical behavior for an FM tuner).

The stereo-FM frequency response was almost perfectly flat, within ±0.1 dB from 30 to 15,000 Hz. The channel separation was also unusually uniform, between 40 and 45 dB over virtually the entire audio range. The capture ratio was 1.5 dB, and AM rejection was 54 dB at a 45-dBf (100-μV) input. The image rejection was a good 86 dB, alternate-channel selectivity was a reason-ably good 67 dB, and adjacent-channel selectivity was 3 dB. The muting and stereo thresholds were identical at 31.2 dBf (20 μV), and the 19-kHz pilot carrier leakage in the audio outputs was a low —70 dB. Tuner hum was —68 dB. The only measurement made of the AM-tuner section was its frequency response, which was down 6 dB at 100 and 2,400 Hz.

Comment

Our tests show that the Pioneer SX-8 has an excellent power amplifier (using their “Non-Switching” circuit) coupled with a much less distinguished preamplifier section and a good FM-tuner section that is on a par with those of other top-quality receivers.

The distortion readings we obtained through the preamplifier section, and the manner in which they depended on volume setting, suggest some design limitations, as evidenced by the unit’s inability to deliver a 1-volt output at high frequencies with IHF standard gain settings without severe wave-form distortion. Fortunately, the preamplifier can drive its own power amplifier to full output without difficulty up to perhaps 8,000 Hz, and at outputs only a decibel or so below rated output its distortion remains negligible even at the highest frequencies.

While the numerical results of our overall distortion measurements were not too impressive, it must be understood that they were not reflected in the sound of the amplifier in the slightest. With the distortion reading exceeding 0.1 per cent only at high power levels and very high frequencies, it is difficult to imagine that anyone could hear their effects under home-listening conditions. In short, we never heard anything but first-rate sound from the SX-8.

Any judgment on the SX-8 must take into account its unique control features. It is handsomely styled, and it makes a most attractive combination with, for example, one of Pioneer’s matching cassette decks. At first glance, the array of controls and displays may seem overwhelming. Nevertheless, it is so easy to use, so free of human-engineering flaws, and so utterly smooth in every operation that we speedily came to appreciate its advantages and even to feel that this is the proper way for such a highly complex unit to operate.

Admittedly, it takes a bit of practice to become familiar with everything this receiver can do and to use it to full effectiveness. Once one becomes accustomed to pressing instead of turning to produce a result, everything happens naturally. Although the control memories will be retained for up to a week with the receiver unplugged, the clock has no such back-up. It is hard o think of a more versatile or elegant receiver presently available at anywhere near the price of this one. The Pioneer SX-8 is a fine unit, worthy of its place at the top of the new Pioneer line of microprocessor-con-trolled components, and a reasonable value for the money.

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