Tube Power Amp Reviews

Usually, the designer must play it conservative when it comes to feedback equalization, erring toward softness at the top with most loudspeakers in order to avoid instability, or outright oscillation with a relatively few "difficult" loads. Providing adjustable compensation (called "Slope" on the 350) makes much more sense, because it allows you to get the most open, unrestrained HF quality that your loudspeakers will allow.

Sound Quality
So, how does the 350 sound? A good question. I mentioned that I had been using VTL 300s as my reference amps for the last year or so. During that time, I have had reason to temper my original reaction to that amplifier with the observation that it may not be quite as neutral as I had first thought. With the same Sound-Lab A-3 speakers, in two rooms that were quite different (smaller, mainly) from my Santa Fe listening room, I found the 300s to be just a shade on the hard side. At the same time, lest this be misunderstood (which it certainly will be if I don't elucidate), I must point out that this judgment is predicated on the use of no preamp at all (or Threshold's FET-10L, which is essentially the same thing), loudspeakers which are neither laid-back nor upfront, and a variety of signal sources: open-reel tape, CD, and Laservideo disc. The more or less reticent sound of most solid-state preamps and audiophile loudspeakers will nicely complement the 300s.

I expected the Manley 350s to sound at least similar to the 300s. And David M. told me that, at one combination of Feedback and Slope settings, they should indeed sound the same. They don't. In fact, I must say I didn't think it possible for two amplifiers from the same designer to sound so different!

Overall, the 350s are probably as suave-sounding as any amps I've heard. They are completely textureless, immensely liquidly transparent, and absolutely effortless in reproducing the kind of dynamic range I hear at live performances of large-scale music. (Yes, Peter Mitchell had me pegged when he said that I share his love for full-sized orchestras in real halls.) And while the VTL 300 had the best low end I had ever heard from a tubed amp, it must be said that the 350s are better in this respect, even if only by a small margin. The difference is in weight and control, which is exactly as would be expected. (All other things being equal, higher power always yields superior deep-bass performance.) The 350s have more foundation—deep-bass energy—than the 300s, yet the whole bass range is tighter and better controlled.

Beyond that, it was not very easy to characterize the sound of the 350, because its feedback controls afford a rather wide range of sounds. As you increase the feedback from Minimum to Maximum, the 350's sound becomes more controlled and more astringent. At the same time, as the low end becomes less full and more detailed, the high end becomes more crisp and open, and the overall sound becomes more laid-back. At the Minimum feedback setting, the 350's midrange sounds rather like that of a very good solid-state amplifier, although more laid-back than, say, the Boulder 500AE and more forward than the Rowland 5. The high end is quite soft and sweet, almost to the point of being dull. (This, remember, is on a speaker that does not soften high end at all.)

At the maximum feedback setting, the sound is very much like that of the best solid-state amplifiers, but with less solidity and apparent range at the extreme low end. There is somewhat more extreme top than at the Minimum feedback setting, but still much less than from an excellent transistor amp.

Although most of the feedback equalization occurs above 20kHz, the Slope control also has a noticeable effect on the sound, probably because of phase shift at frequencies below 10kHz. Each step of increase (there are three) causes increasing HF softness and a concomitant (apparent) improvement in musicality. For example, at the maximum feedback setting, Standard slope gave me a top end that was quite open and airy, but was imbued with a subtle tizziness. At Minimum feedback and Minimum slope, there was about the same amount of extreme HF extension, but my ears told me it was more natural. This was on my system, and it is almost certain that what you will hear from yours will be different. But my point is that the 350s can provide a wider variety of "sounds" (okay, "colorations") than any other amplifier I know of.

I'm sure a lot of readers out there are waiting, with bated breath, for me to tell them what the "correct" settings are for these switches. Okay, here's the straight dope: There are no "correct" settings, because in high-end audio, there are no standards for anything sonic. And with the Manley 350, there isn't even a middle of the road. The 350s are definitely laid-back—easily as much so as the typical solid-state amplifier, and much more so than any other tubed amplifier I can recall ever having listened to. They are almost what I would describe as dark, yet this has nothing to with midbass heaviness or high-end dullness; it is more a quality of richness that never quite verges on the cloying.

The combination of Feedback and Slope settings that sounds best with most good recordings will depend entirely on all the other components in your system, which is to say, this amplifier is a neurotic's nightmare (or nirvana, depending). The flexibility which these controls impart to the 350 means it can be tailored to the nth degree of perfection to a very wide variety of systems. It also means they can be used as tone controls to optimize each recording—a function which is best left to a sophisticated equalizer like the Accuphase G-18 or, God forbid, to tone controls.

Problems
I encountered only one problem during my tests. The power-supply storage caps in the 350s draw a prodigious amount of current from the AC line at turn-on, as evidenced by a marked, abrupt dimming of the room lights. (Manley estimates this at around 15 amps, which is equivalent to a staggering 1755W!) Switching both amplifiers on at once unfailingly tripped a 15-amp circuit breaker in my home. Turning them on a few seconds apart cured that, but I was still a bit dubious about the loud, angry-sounding snap from the power switch that would sometimes accompany turn-on of one or the other of the amps. After about the tenth time I fired them up, one of the AC switches froze. It wouldn't turn off. Without really thinking, I pushed it harder, and felt something let go. After that, the switch would move to On, but the amp did not turn on. Then I realized what had happened. The switch wipers had apparently fused together the last time they closed, and my effort to open them had pulled them off the internal rocker assembly. It seemed obvious to me that the 350's power switch is not up to the current demands being made on it.

But why did it take so many power-ups for one of them to fail? I think it was sheer coincidence. The 117V RMS AC supply is continually cycling between 0V and ±165V, and if you happen to close the switch when it's near zero, little current will flow at the instant of contact. Then, by the time the supply cycles up (or down) toward 165V, the switch's full current capability is in-circuit. But if contact is made at the instant of peak voltage swing, the full line voltage is dumped into the power-supply storage capacitors before the switch has made full contact, and until the capacitors are charged, they act almost like a short circuit across the line. It just took me 10 turn-ons (of each amp) to catch the line voltage at maximum, and that was when one of the switches failed.

I phoned David, who told me that, while they had in fact had "a few" failures from around 10,000 switches, they had all been mechanical failures (usually due to a heavy blow against the switch rocker) rather than electrical failures due to overload. He said the switches are rated at 40 amps, equivalent to 4680W, so they damn well shouldn't break down at 15 amps, or even at 30. Anyway, David got on the phone to VTL's Boulder dealer, Listen Up, who picked up my bad-ass amp and got it back to me a few days later with a new switch installed.

Unfortunately, consumers can't expect quite that good service; Listen Up said they usually ship VTLs back to the factory for repair, and that the turnaround time is typically 2 weeks—about half the time most manufacturers take for factory service. So much for the early complaints we had heard about VTL's slow service.

After the amp came back, I played it safe and used the 350s' AC plugs to turn them on and off until I had finished all the listening tests required for this review. Then I went back to using the switches. Since then, they have been cycled through maybe 30 On/Offs, and there has been no recurrence of the problem. I'm beginning to think that one breakdown may in fact have been an isolated problem, rather than a design weakness. But I still flinch at the occasional snap on turn-on, which sounds as if it should be accompanied by the steel-tongued bite of the creature from Alien.

Conclusion
So, what's my conclusion about the Manley 350s? The few cavils aside, they are really superb power amps, whose strongest points include immense musicality, effortless reproduction of fortissimos, and total freedom from fatigue or irritation even after hours of attentive listening. But they are far from being what I would call neutral. There are, in fact, few loudspeakers I can think of that would be sonically complemented by these amplifiers. Their errors are almost entirely spectral, but the nature and extent of those errors place the 350 near the edge of that variability range that could be offset by the variety of existing loudspeaker/preamp combinations.

I find this very puzzling. By every accounting, these should be the most accurate-sounding of any of VTL's amplifiers. David M. has done everything I could imagine anyone doing to ensure that the 350 performs better than any other non-hybrid tubed amplifier, yet the sonic result borders on the disappointing. How, I must wonder, is it possible that a tubed amplifier whose design shows more attention to detail in supposedly important areas than most others I know of sounds so little like either a tubed amp or a solid-state one? I can't answer this. All I can say is that the 350s never allowed my system to come to life. The soul of the music was there in abundance, but there was little of the life and sparkle of it. I'm still shaking my head in bewilderment.

Would I buy the Manley 350s, assuming I could afford to? Not if I already owned VTL 300s, and probably not otherwise either.