Yamaha M-50 Power Amplifier Review

The Yamaha M-50 power amplifier features the company’s ZDR (zero distortion rule) amplifier circuit and the “signal-tracking" X power supply that is intended to provide higher overall operating efficiency. Also in the interest of low distortion, Yamaha employs a “linear-transfer bias circuit” in the output stages to virtually eliminate crossover distortion. Although no circuit details are given, the block diagram of the amplifier suggests that the ZDR feature is a form of amplified feedback of the extracted distortion signal.

The performance specifications of the Yamaha M-50 include a 120-watt output into 8-ohm loads from 20 to 20,000 Hz with no more than 0.002 per cent total harmonic distortion. With 4-ohm loads, the only specification is a clipping output of 200 watts per channel, and there is no rating for any other load impedance. The M-50 is finished entirely in black. The front panel contains a large square illuminated pushbutton power switch, and a protection light comes on if the amplifier’s protective circuits operate for any reason (they delay the connection of the speakers for a few seconds when the amplifier is turned on to prevent turn-on transients from reaching the speakers).

At the right side of the panel there are separate level controls for the two sets of speaker outputs, either (but not both) of which can be selected by a button near the level controls. Another button shuts off the speakers entirely. Although the two controls (marked A and B) resemble the individual channel-level controls sometimes used on power amplifiers, each of them affects both channels equally for the selected speaker output.

A large rectangular window in the center of the panel contains a dual peak-power display. In it are two vertical columns of short horizontal bars that light in orange to follow the instantaneous output level of their respective channels. Each column is calibrated in power output (based on 8-ohm loads) from 0.03 to 150 watts, with a row of corresponding decibel calibrations in the center of the display. A small button to the left of the window shuts off the display. On the rear apron of the M-50 there are insulated speaker connectors, the input phono jacks, and a single unswitched a.c. outlet. The speaker connectors, which resemble conventional binding posts, have holes that accept the stripped ends of the wires; a quarter-turn of the connector clamps the wire firmly in place and makes a solid electrical connection.

Laboratory Measurements

The 1-hour preconditioning of the Yamaha M-50 at an output of 40 watts into 8 ohms, followed by five minutes of full power, resulted in a rather hot top cover above the transistor heat sinks. The power output at clipping (1,000 Hz into 8 ohms) was 153 watts per channel for a clipping-headroom rating of 1.06 dB. With 4-ohm loads, the clipping power was 240 watts, well above the 200-watt rating. We could not make any measurements with 2-ohm loads since the protective circuit shut off the amplifier outputs before the waveform clipped. The dynamic-power output was measured with the 20-millisecond tone bursts of the standard IHF test, with the outputs clipping at 156 and 260 watts into 8- and 4-ohm loads, respectively. The 8-ohm dynamic-headroom rating was about average at 1.14 dB. When we tried to drive 2-ohm loads with the pulsed signal, we found that the waveform distorted (rounded off symmetrically) long before the protective system operated. The maximum reasonably undistorted output into a 2-ohm load was roughly 20 watts per channel.

Nevertheless, Yamaha’s claims to ultra-low distortion were essentially confirmed with 8-ohm operation. At 1,000 Hz, the distortion was between 0.0003 and 0.001 per cent from 1 watt to at least 120 watts output (with 4 ohms the results were quite similar, the 0.001 per cent point being about 140 watts per channel). At rated power or less, the 8-ohm distortion was between 0.0003 and 0.001 per cent from 20 to 3,000 Hz, rising to a low maximum of about 0.003 per cent in the 5,000- to 20,000-Hz range. The high-frequency linearity was also measured with the two-tone IHF intermodulation-distortion test signal consisting of equal amplitudes of 18- and 19-kHz signals whose peak amplitude was equal to that of a 120-watt sine-wave output. The second-order (difference-tone) distortion at 1,000 Hz was an excellent —90 dB, and the third-order (17-kHz) component was —64 dB, both referred to 120 watts.

The sensitivity of the Yamaha M-50 for a reference output of 1 watt was 0.105 volt, and the A-weighted noise in the output was less than our 100-microvolt measurement limit (better than 90 dB below 1 watt). The amplifier’s slew factor was greater than 25, and its rise time was 3 microseconds. It was stable with reactive simulated loudspeaker loads, and its IHF reactive-load rating was 1.18 dB.

Comment

In service as part of a home music system, the Yamaha M-50 worked perfectly, with absolutely no sound of its own. Even turn-on and turnoff operations were completely silent. It delivered the type of performance we would expect from such a deluxe audio component. Recalling its limited output with low load impedances, we paralleled several pairs of speakers in an effort to hear distortion when the amplifier was driven hard. In this we were only partly successful. Distortion was audible only when we drove the M-50 very hard, producing sound levels far beyond what any rational person would find enjoyable in a home environment. While the distortion disappeared when we switched to a much more powerful amplifier that was able to drive low-impedance loads without difficulty, this is hardly a fair comparison with the M-50.

Yamaha’s awareness of the current-output limitations of the M-50 is demonstrated by their using an output-switching system that makes it impossible to drive two pairs of speakers simultaneously regardless of their impedance ratings. The apparently unruffled behavior of the amplifier when we drove a group of speakers whose combined nominal impedance must have been less than 2 ohms can probably be explained by the fact that actual speaker impedances vary widely with frequency, and no two are likely to be alike in this respect. It is therefore probable that the amplifier never “saw” an impedance as low as 2 ohms; at most frequencies it may have been closer to 4 or 5 ohms. Combined with the frequency distribution of most music, it is not hard to understand why the apparent deficiency on the test bench might have been of little practical importance. However, there are speakers whose actual impedance comes perilously close to 2 ohms at some frequencies, so caution is advisable when using them with this amplifier.

One surprising discovery about the M-50 was that it is not a particularly cool-running amplifier despite its “X” power supply. Even operating with no signal, its top became distinctly warm—not actually hot, but certainly not as cool as even some conventional amplifiers of the same or higher power ratings. It is also not especially small or light for an amplifier of its power rating, although it must be admitted that there are heavier and larger “120-watt” amplifiers on the market.

The M-50 is a good amplifier, with extremely low distortion and no vices other than those already mentioned. It is not inexpensive, but it appears to be well built and adequately protected.

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