Solid State Power Amp Reviews

"Grand Integra" is the name Onkyo has given to its line of perfectionist-oriented audio products, and the M-508 is the cheaper of Onkyo's two Grand Integra power amplifiers. (The flagship model is the $4200 M-510, reviewed by Larry Greenhill in Vol.8 No.8, featuring "high current capability," and rated at 300W continuous per channel.)

The rather bulky, but surprisingly lightweight (for its size and power) M-508 is a true dual-mono design, consisting essentially of two completely separate amplifiers on a single chassis. That it is not designed for outstandingly high current capability is evident both from the modest size of its power transformer and the gauge of its AC cord. The M-508, however, does feature Onkyo's "Real-Phase" power supply, where a series transformer between the rectifier bridge and the reservoir capacitors couples the positive charging currents to the negative, improving the rejection of common-mode power-supply noise. Incidentally, the 0.003% distortion rating at "rated output" is questionable. "Rated output" is supposed to be that point where an amp starts to go into overload; 0.003% is obviously nowhere near overload. Either the THD figure is too low, or the amp overloads above 200W.

The M-508 is an impressive-looking piece of hardware, with good, clean metalwork finished in flat black on all sides, and two unusually large (6!9 wide), black front-panel meters. Their calibrations and indicator needles are edge-lit in what might be described as rain-forest green, making for quite a sexy appearance. The meters are described as "high-speed" types, and are optimistically calibrated up to 500W and +4dB—the latter relative to a 200W 0dB level.

The amp has two sets of inputs, marked Direct and Variable, the latter feeding through a pair of small input-level knobs on the front. Four pushbuttons select Normal or +10dB meter sensitivity, or turn the meters on and off. In the +10 position, meter sensitivity is increased 10 times, so you can still get meaningful readings at reduced volume levels. Another switch selects the Direct or Variable inputs, and can be used to select either of two sets of permanent input connections. This means that, for instance, you could leave your CD player plugged into the power amp, and select it with the push of a button, using either its own or the M-508's level controls for volume adjustment.

There are also two sets of loudspeaker connections, and two more front-panel buttons select either or both sets of speakers. One depressed speaker button does not, however, automatically pop out if you press the other, which means it is easy to shut off both speakers at once. As there is no headphone output on the amp, I cannot see the advantage of being able to kill both sets of speakers, but I can see it causing some consternation when a user accidentally punches the one button that is already on, and is suddenly faced with a Dead System. (Even the meters are deactivated when both speakers are Off.)

Although the power-supply rails are internally fused, there is no loudspeaker overload fusing; fast-acting, long-holding relays in the speaker output lines are claimed by Onkyo to provide all the speaker protection necessary. They're probably right with regard to woofer overload, but I do not believe any relay is capable of responding fast enough to protect a typical tweeter from a hard clipping overload (footnote 1). A tweeter with plenty of thermal inertia (as from ferrofluid damping) would probably be OK, but something like a ribbon could vaporize before the relay opened up. The saving grace here is that the amplifier has enough power and headroom (claimed to be 265W for up to 200ms) that HF overload due to clipping—the usual cause of tweeter destruction—is unlikely.

Sound Quality
I left the M-508 on for four days before listening to it, so I won't be relating with relish this time my usual horror story of how it sounded right out of the box. This may be one of the most economical 200W amplifiers to operate. In its quiescent (no-signal) state, it runs so cool that the top of the unit barely feels warm to the touch. Even at sustained signal levels peaking out at between 200 and 300W (according to its meters), the amplifier was never more than moderately warm, which should, if nothing else, make for long, trouble-free performance.

Sonically? Well, since Onkyo is a Japanese company, I am well aware of my responsibility as a purist snob to report that the M-508 is "a good buy for the money, but is really quite compromised, sonically." After all, it's no secret that the Japanese don't know what real high-end sounds like, preferring instead a harsh, strident top. (It's cultural, of course; their music's that way.) The proper way to review such an amplifier is patronizing condescension, like the way my Publisher and Managing Editor treat me. ("It's just a Japanese amplifier, you know, but I think you'll be amused by its pretensions!" "He does surprisingly well, considering his age.") But I fear I must shirk my duty and undermine my own credibility, for truth demands that I report that the Onkyo M-508 is a very good amplifier.

It did not have the legendary Japanese top end. In fact, its extreme top sounded more like that of an excellent tube amplifier: rather soft and sweet, with a remarkable degree of musical euphony. The only thing about the high end that one might fault the M-508 on is a subtle shortage of openness and delicacy—a result, I suspect, of some audible-band phase shift due to the amp's 20kHz bandwidth limiting. Most Stereophile readers know how I feel about high end, though; I prefer to listen to a much more closed-in high end than this, than one that is even the slightest bit edgy or tizzy. Note, however, that this HF assessment of the M-508 was as heard through Sound Lab A-3 speakers, which have all the high-end openness and delicacy I have come to demand in reproduced sound but usually hear only from electrostatics. With most dynamic tweeters (the JBL and Altec titanium domes notably excepted), the M-508 is just as likely to sound dull at the top as will most tube amplifiers.

The middle range from the Onkyo amp was nearly perfectly neutral—neither close up nor distant. It was slightly more up-front than that of my reference Threshold SA-1s, and more than slightly less so than most tube amplifiers, falling about halfway in this regard between the SA-1s and the Audio Research D-250 Servo II. Bass was in excellent balance with the rest of the audio range and had good detail, but lacked that last bit of visceral wallop, instant start-stop control, and palpable solidity of some of the very-high-current designs (like the Krells and the big Threshold SA-series amps). I have not tried the Onkyo on any of the large full-range ribbon speakers like the Apogees, but suspect it might not be the wisest choice for use with them.

Soundstaging was unusually good, with ambience (and, occasionally, instruments) occupying space beyond the speakers, but front/back depth was a little compressed in comparison with the SA-1s, and noticeably compressed compared with the top tube amps (which I sometimes suspect of exaggerating this characteristic). In one area of soundstaging, though, the M-508 was equal to the best amplifiers I have heard, regardless of price. I refer to imaging specificity, where the Onkyo produced as tight, definite, and stable an image as any.

Although I have no way of monitoring peak power levels, I did get the impression that the M-508 has quite respectable short-term headroom and very graceful overload characteristics. There were times when the front-panel meters were kicking up to the 400W mark, yet the sound remained remarkably clean. (And SPL readings indicated about 4–6dB higher levels than I am usually able to get, cleanly, from my 160W Thresholds.) The clean bass quality suggested that what I reported some issues back as diaphragm-bottoming from the Sound Lab A-3s may in fact have been due to amplifier overload, as there was no such low-end stress from the Onkyo at spls up to 104dB!

Inner detailing of the M-508 in complex signals was good, but by no means outstanding, tending toward a moderate amount of congestion and loss of specificity during forte and louder passages. This, interestingly, seemed more related to signal complexity alone than to playback level, as it was not mitigated by reducing the listening volume.

Conclusion
How, then, would this rank with the competition? While I acknowledge the M-508's imperfections, I must add that I found the amplifier more enjoyable to listen to, overall, than any circa-$1000 amplifier I can remember from the past couple of years. In terms of overall performance, I would put it on a par with the McIntosh 7270 (reviewed in Vol.10 No.2), but at a little more than twice the Onkyo's price, I'm not sure the McIntosh is really "competition." Of the other similarly priced competitors tested recently, I would say the $895 NAD 2600 comes closest in performance to the Onkyo. I think I would opt for the latter, however, despite its slightly higher price, mainly because of its super-silky high end and alive-sounding midrange.

At $1150, the Onkyo M-508 is not a worldbeater, but it's a very good amplifier for the price. I wonder, though: if Onkyo got rid of the frills—the meters, the extra speaker outputs, the input level controls and input switch—could they sell this for $800? If they did, would there be any reason to consider buying anything else? Hmmm.

---

Footnote 1: When the time comes that audio signals are digital right up to the loudspeakers, it will be a snap to delay the signal in the power amp without further degrading the sound, so that potentially destructive HF overloads can be suppressed before they even reach the amplifier outputs.