Manley Labs SE/PP 300B Monoblock power amplifier

This should have been a recipe for disaster.

It's no secret. David Manley has not been big on single-ended amplifiers. Not enough muscle. You're better off with push-pull. Still, with a growing market for single-ended stuff, I'm sure Manley saw a need to do something. Of all tube designers, Manley has always been among the most prolific.

"Want a single-ended amp, mate? No problem. I'll build one up for you." (This is the kind of thing Manley would say, but didn't in this case. I made it up.)

That's what troubled me initially. Manley doesn't seem committed to single-ended amps. And the ads for the SE/PP 300B Monoblocks ($3500/pair) in Stereophile didn't inspire much confidence: "Single-ended or push-pull? Have it both ways in one amplifier."

Which way should it be, David? Single-ended or push-pull? I asked him, but I couldn't get a definitive answer. "Try it both ways, mate, and see what you like. That's the whole point.

"We believe," said David, in the two sheets of paper that pass for an SE/PP 300B instruction manual, "that we are the first manufacturer to ever offer a tube amplifier which can be switched between Single Ended and Push Pull modes of operation. This required a very clever output transformer design (among other things!) which is wound right here on premises at Manley Labs."

I always knew you were a clever chap, David. Now...get this. You can change it from single-ended to push-pull with a flick of the switch while the amps are playing. Want detail and delicacy and the ultimate (or nearly) in harmonic refinement—for a string quartet, maybe? Use single-ended. Want more balls for rock or country music or Mahler's Sixth? No need to switch amps. No need even to turn these amplifiers off. Just pull the switch. It's the kind of thing that shouldn't work, but does (footnote 1).

But wait—Manley's not finished trying to please everybody.

You can also dial in varying amounts of feedback. Yes, nasty, politically incorrect negative feedback—almost as politically incorrect as those exotic wood finishes Rankin will supply, so long as you're willing to pay him a $1000 penalty.

Feedback? In a single-ended amplifier?

That's heresy, man!

"I like the amp with no feedback," a friend told me, just after he'd received his pair of SE/PP 300Bs. "I also prefer it in the single-ended mode. With push-pull, the soundstage comes forward, gets wider, but you lose depth and palpability."

Dialing-in a modest amount of feedback can improve the bass performance—it did for me with the Thiel CS.5 speakers. But dial-in too much feedback and you can hear the onset of distortion quite clearly. The amp loses its clear, clean purity.

Adding feedback also decreases the output impedance, but not by much, since Manley designed these amps to have very low source impedance anyway. I found with the Thiels and the Infinity Composition Prelude P-FRs that 1 or 2dB of feedback sounded best—tightened up the bass—but any more than 2dB audibly detracted from the clarity.

The amplifiers use two 300Bs per side—no-name Chinese tubes selected for Manley in the Orient. ("I don't believe in designer tubes," declared Manley. "These are just as good, and they are better than run-of-the mill Chinoise." Manley speaks French, too.) The tubes are wired in parallel—according to single-ended triode purists, a no-no: you want just a single 300B output tube per side. Rectification is solid-state, with four full-bridge systems per amp: one for B+, one for negative bias, and two for each of the two 300Bs' filaments.

Manley says he "eschews/hates/despises" (his words) vacuum-tube rectification for the B+ for several reasons—but mainly because vacuum tubes cannot drive a "stiff" B-rail power supply and are a source of "loose, drooping bass" in other single-ended amps. Manley calls this the "bikini effect;';'—that is, "a flabby bottom with a drooping top." (Don't get Manley started; he won't stop.)

The gain/driver stage consists of a 12AX7T and a 7044, which Manley calls a "very foxy tube...that swings huge volts into 300Bs." By the way, the amps are quite small—they measure 12" wide by 10" deep. Finished in metallic gray, the amps are plain-looking but attractive. They look...well, manly.

So how do these amps sound?

Depends on how you use them. Even in single-ended mode with no feedback—the purist mode, if you will—the amps lack some of the Jadis SE300Bs' incredible detail and refinement. The Wavelength Cardinals, too, have a purity of sound and a beauty of timbre that the Manleys, even at their best, can't quite match.

But—here's the kicker—the Manleys come very, very close. They almost have as much delicatessen and detail as the Jadises, and they have balls in the bass, too—even in single-ended mode, with no feedback.

And, of course, there's the price. The Manley amps are much more keenly priced than the Cardinals or the Jadis amps. Why, they're only $97.22 per watt per channel in single-ended mode, and just $48.61 per watt per channel in push-pull. A bargain.

Bass? Not quite so rich and full as the Wavelength Cardinals, but perhaps better controlled. Certainly not wimpy. There's even enough bass, subjectively, with the lean-sounding (but overall quite excellent) Infinity Compositions Prelude P-FRs.

Midrange? The Manleys are capable of much of the harmonic beauty I hear with the Wavelength Cardinals. Treble? Not so extended as the Jadises, but not rolled-off, either. That goes for either mode—single-ended or push-pull.

With the Manleys, you get the special magic of a 300B-based amp in an amp that can drive "real-world" speakers. For instance, the Manley SE/PP 300B monoblocks had no trouble at all driving the Thiel CS.5s to realistic levels, even in single-ended mode. Indeed, this was an excellent combination.

True, the Jadis SE300Bs elicited a more refined, more exquisitely detailed sound, with much more sense of air; but the Manleys delivered more punch—even without resorting to push-pull (footnote 2).

Sound too compressed? Soundstage too narrow? Want a more balls-out sound? Wish you had a few more watts to open up that castle in Capri? No problem, mates. Just pull the switch and go into push-pull mode.

With push-pull you'll get not just a more powerful sound, but a bigger sound. But you also get a dimensionally flatter sound. I consistently found myself switching the amp back into single-ended mode (footnote 3).

As for paralleling two (or more) 300Bs per side: it's true, you seem to lose some delicacy, some detail, some purity—some je ne sais quoi. But you do gain power. That's true, too, of the Jadis SE300Bs, which sound much more powerful than amps using a single 300B per side, and far more powerful than their "official" 10W rating. With the Manleys, you not only get real-world power, you also get it at a real-world price.

Unless your present speakers require gobs and gobs of power, or your room is very large, these amps will likely work. So you won't be looking to trade in your present speakers and go looking for others with 92dB, 94dB, 98dB sensitivity.

There you have it. Trying to offer a single-ended and a push-pull amp all rolled into one, with variable feedback too, should have resulted in failure. When you try to please everyone, you usually end up pleasing no one. But that didn't happen with the Manleys. I'm going to have trouble sending these back.

Footnote 1: I actually played the first and last movements of Mahler's Sixth, with Pierre Boulez conducting the Vienna Philharmonic (DG (445 835-2), in push-pull, and the middle two movements single-ended: single-ended for the Scherzo and the Andante, push-pull for the Allegros.

Footnote 2: The Manleys are optimized for a 5 ohm load, which may be one reason they sounded so terrific with the 4 ohm Thiels.

Footnote 3: Some tube-amp designers seem to regard push-pull as the greatest advance ever. It certainly has allowed designers to provide more power more cheaply. But single-ended design—and I've heard enough single-ended stuff by now, both tube and transistor, to tell—has a character and a rightness of its own. Don't be surprised if more and more solid-state amplifier designers turn to single-ended. (Nelson Pass is already leading the way; I expect to have more to say about that soon.)

Manley Laboratories, Inc.
13880 Magnolia Avenue
Chino, CA 91710
(909) 627-4256