Onkyo TX-51 Stereo Receiver Review

The Onkyo TX-51 AM/FM stereo receiver features a phono preamplifier switchable for either moving-coil (MC) or moving-magnet (MM) cartridges and a built-in CX decoder capable of reducing noise by as much as 20 dB when playing CX-encoded discs. It is rated to deliver 40 watts per channel into 8-ohm loads between 20 and 20,000 Hz with no more than 0.04 per cent total harmonic distortion.

The tuner section of the TX-51 employs digital frequency synthesis for both FM and AM bands, with tuning intervals of 100 and 10 kHz, respectively. Like many other digital receivers, the TX-51 is tuned by pressing either end of a horizontal bar, pivoted at its center, to move upward or downward in frequency. Above the tuning bar (which is in the lower right corner of the panel) are three light-touch pushbuttons, marked MEMORY, MANUAL, and AUTO. In MANUAL the frequency shifts by one step each time the bar is pressed, and the tuner scans rapidly if the bar is held in for more than a second or two, stopping when it is released. In AUTO, a single touch of the bar starts the scanning action, which stops automatically when it encounters a signal strong enough to overcome the muting threshold. The MEMORY button is used with the eight station-preset buttons to store the frequencies of up to sixteen stations (each button serves for one AM and one FM channel).

About half of the front panel is devoted to a display window containing the frequency readout (four large, pale-green numerals), a four-segment signal-strength indicator, and orange stereo and c.x noise reduction lights. The preset buttons, below the right half of the window, are supplemented by illuminated indicators. To the left of this control group are the input selectors: square pushbuttons for TAPE-2, TAPE-1/AUX. phono, and tuner (an FM/AM button next to the last switches between the two bands). These buttons operate with a very light pressure, and illuminated symbols above them show which function is selected.

The left half of the panel contains a number of slim rectangular pushbuttons that activate the two sets of speaker outputs, the CX noise-reduction system, the loudness compensation, and FM muting. Other buttons select mono or stereo operation (with any program source), the MM or MC phono-preamplifier mode, and cither the normal 75-microsecond FM de-emphasis time constant or the 25-microsecond value for use with an external Dolby-FM decoder (connected in one of the tape circuits).

Below the buttons are three knobs, for the bass and treble tone controls and the RFC SELECTOR. The last supplies the TAPE-1 and TAPE-2 outputs with either the tuner or the phono signal, disconnects them from all signal circuits, or cross-connects them for dubbing tapes from either machine to the other without interfering with the normal use of the receiver. These knobs are detented, with panel markings to show their settings, but the setting of each knob is also shown by a back-lit green identifier visible though a small window above the knob. The remaining front-panel controls are the power button, the headphone jack, and a large volume control at the right above the tuning controls. The volume control is concentric with the balance control, a narrow ring close to the panel, which is adjusted by means of a small, protruding tab.

On the rear apron of the Onkyo TX-51 are the usual signal input and output phono jacks, binding posts for the AM and FM (75- and 300-ohm) antennas, and a detachable pivoting AM loop antenna whose 2-foot connecting cable allows it to be positioned for best reception. There are also two CX ADJ controls for matching the cartridge output level to the requirements of the CX decoder circuit. A small flexible disc provided with the TX-5 1 contains 3.54-cm-per-second calibration test tones recorded separately for left and right channels. While playing one channel at a time, the corresponding adjustment is varied until the LED lights. That occurs only within a limited range of adjustment, making this a convenient and accurate means of calibrating the CX decoder system. The speaker output terminals resemble binding posts with holes into which the stripped ends of the wires are inserted. A clockwise rotation of the post makes a solid electrical connection with a firm mechanical grip on the wire. One of the two a.c. outlets is switched. The output transistors of the receiver are protected by fuses in the speaker lines that are readily accessible by removing a plastic protective cover in the rear of the unit. The front panel of the Onkyo TX-51 is finished in silver, contrasting with its grey metal cabinet. It measures 16-1/2 inches wide, 15-1/2 inches deep, and 4-1/4 inches high and weighs about 21 pounds. Price: $400.

Laboratory Measurements

The Onkyo TX-51 amplifier clipped at 52.5 watts per channel when driving 8-ohm loads at 1,000 Hz, for a clipping-headroom rating of 1.18 dB. With 4-ohm loads, the clipping power was 68 watts. When we drove 2-ohm loads, the 4-ampere output fuses blew before the waveform clipped. We temporarily replaced them with 5-ampere fuses and observed a clipping power output of 84.5 watts per channel. The dynamic-headroom test produced clipping outputs (during 20-millisecond tone bursts) of 62.6, 92.5, and 105.5 watts per channel into loads of 8, 4, and 2 ohms, respectively. This corresponds to an 8-ohm dynamic headroom of 1.95 dB. The slew factor of the TX-51 was greater than 25, and it was stable with reactive simulated loudspeaker loads.

The 1,000-Hz distortion into 8 ohms increased smoothly from 0.0013 per cent at 1 watt to 0.0145 per cent at 50 watts output. With 4-ohm loads, the distortion increased from 0.0025 per cent at 1 watt to 0.016 per cent at 65 watts. Driving 2-ohm loads resulted in appreciably higher distortion (though still much less than would be audible in actual use), ranging from 0.005 per cent at 1 watt to 0.036 per cent at 70 watts and rising rapidly to 0.36 per cent at 80 watts. The distortion varied little with power, over the audio frequency range, typically reading between 0.001 and 0.003 per cent from 20 to 10,000 Hz at power outputs-from 4 to 40 watts and rising slightly to 0.007 per cent at 20,000 Hz. All distortion readings were far below the very conservative 0.04 per cent amplifier rating. The IHF intermodulation (IM) distortion, using mixed test signals of 18 and 19 kHz whose peak level was equal to that of a 40-watt sine wave, was — 88 dB at 1,000 Hz and — 61 dB at 17 and 20 kHz (relative to 40 watts).

Through the high-level (aux) input, 24.5 millivolts (mV) was needed to drive the amplifier to a reference output of 1 watt, and the MM phono sensitivity was 0.4 mV. The MC input sensitivity was 0.06 mV. The A-weighted noise level (relative to 1 watt) was — 70 dB for aux, —77 dB for phono (MM), and —69 dB for phono (MC). The only measurement we made of the CX noise-re-duction system was a spectrum analysis of the noise output of the amplifier using the phono input with the CX switch on and off. The reduction was about 1 5 dB over much of the audio range, decreasing to about 10 dB at 10,000 Hz and 5 dB at 20,000 Hz.

The phono (MM) overload input level was between 185 and 200 mV over the 20-to 20,000-Hz range. The phono (MM) preamplifier input termination was 47,000 ohms in parallel with a rather high capacitance of 275 picofarads (pF). Many cartridges will operate perfectly well with a total load capacitance of 400 pF or more (which would result from using this amplifier with turntables having average output-cable capacitance), but some might not. The MC input termination of 330 ohms should be compatible with most moving-coil cartridges.

The tone controls had the usual Baxandall response shape (a sliding turnover frequency in the bass and hinged curves in the treble), although the control range was unusually large (the maximum bass boost, for example, was 20 dB). We also noted that most of the control action took place in the last half of the control rotation. The loudness contours provided a moderate boost of both low and high frequencies at control settings of —20 dB and below. The RIAA phono equalization was flat within ±0.5 dB from 60 to 20,000 Hz, and it was up 1.3 dB in the 20- to 40-Hz range.

The FM tuner section had a usable sensitivity of 11.6 dBf (2.1 microvolts, or μV) in mono, and the stereo sensitivity was set by its switching threshold of 19 dBf (5 μV). The 50-dB quieting sensitivity was 15.6 dBf (3.3 in mono and 35 dBf (30 μV) in stereo. The tuner distortion at 65- dBf (1,000 μV) was 0.14 per cent in mono and 0.22 per cent in stereo, and the respective signal-to-noise ratios at that level were 76 and 64.5 dB (the former is an excellent figure, the latter a little below average). The FM IHF IM distortion (14- and 15-kHz modulating signals) was —65 dB at 1,000 Hz and —49 dB at 13 and 16 kHz in mono, — 52 and —49 dB in stereo. While these readings are not particularly low compared with those of some tuners and receivers we have tested, they are quite negligible from the standpoint of audibility.

The stereo FM frequency response was flat within 0.5 dB overall from 30 to 15,000 Hz. The channel separation was about 40 dB at low audio frequencies and 37 dB at 1,000 Hz, decreasing smoothly to —30 dB at 15,000 Hz. The FM capture ratio was an extremely good 0.79 dB at 65 dBf input, at which level the AM rejection was a fairly good 53 dB. The image rejection of 36.5 dB was only fair. Alternate-channel and adjacent-channel selectivity readings were 61 dB and 4.1 dB, respectively, both typical of moderate-priced receivers. The muting threshold was 16.6 dBf (3.7 μV). The 19-kHz pilot-carrier leakage into the audio was a relatively high — 38 dB, but the only practical disadvantage of this would be a possible interaction with cassette recording using Dolby-C if the recorder lacked an MPX filter (which very few do). The tuner hum level was an extremely low —78 dB. The AM tuner frequency response was down 6 dB at 70 and 2,600 Hz relative to the 1,000-Hz level.

Comment

In many respects the Onkyo TX-51 is representative of a new generation of moderate-priced receivers offering performance and operating features that could not have been imagined only a few years ago. Some of these, such as digital-synthesis tuning, are now standard in most receivers above the budget class. Others, such as switchable MM and MC phono inputs and a separate recording output selector that does not require the user to forgo his preferred programs while recording or dubbing tapes from other sources, have been available on much more expensive separate preamplifiers and amplifiers for several years but were not at all typical of receivers in the $400 price class. The latest feature to appear in this product category is the CX decoder, and we can only hope that CX records will be more aggressively promoted.

In use, the TX-51 presented us with no bugs or unpleasant surprises. It sounded excellent, and it was a smooth performer in every respect. We played our collection of CX-encoded records through it and found it to be subjectively every bit as impressive as the separate CX decoders we evaluated after the introduction of the system by CBS. The setup of the receiver with a phono cartridge takes only moments with the supplied record, after which using the CX system requires only the press of a button. The noise reduction of 1 5 dB or so is definitely worthwhile, although it may not always be obvious because of the masking effect of the program, and we found no disadvantage whatever to the CX system when used with encoded records.

We were also pleased to find that the TX-51 has a very healthy current-output capability and can drive 2-ohm loads to a considerable output without difficulty. Although the receiver is not rated for use with loads lower than 8 ohms (this is true of most), it apparently depends more on its output fuses than on a severe internal current restriction to protect itself, and the result is a benefit to the user. The relatively high peak-power outputs of the TX-51—at least compared with many other moderate-priced receivers—should stand it in good stead not only with CX-encoded discs but with true digital discs as well.

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