Solid State Power Amp Reviews

Jim Austin wrote about the BHK Signature 300 in August 2017 (Vol.40 No.8):

When I came up for air after reviewing PS Audio's BHK Signature preamplifier for our June 2017 issue, there was still a pair of BHK Signature 300 monoblock power amplifiers ($14,998/pair) sitting on the floor of my listening room. (PS Audio had provided them to use while reviewing their preamp.) Also sitting there, next to the monoblocks, was a pair of DeVore Fidelity's Orangutan O/96 loudspeakers, their lovely wood baffles gleaming. (They're in for a Follow-Up.)

It was an intriguing match: The BHK Signature 300 is a beast of an amplifier. Each one weighs 83 lbs and can put out about 1000W. As for the apes, John Atkinson called the O/96 "one of the easiest speakers for an amplifier to drive that I have encountered."

Big brute of an amplifier; sensitive, easy-to-drive speakers—what's not to like?

What do we really know about matching?
It's usual to pair highly efficient speakers with low-powered amplifiers, and save powerful amplifiers for especially demanding speaker loads—or, anyway, for the kinds of speakers we expect to present demanding loads: big, complicated, lots of drivers. But does that really make sense?

This is a Follow-Up review of the BHK Signature 300 monoblocks, but I'm going to spill more ink on technical issues than I usually do. Although Stereophile is devoted to the subjective reviewing of audio components, technology is the foundation on which rests perfectionist sound reproduction—and, hence, our subjective judgment. Technical specifications and measurements can tell us what is safe to try, what's likely to be necessary, and what to avoid. They can also serve as checks on our perceptions: If measurements seem to contradict what we're hearing, maybe we should listen again. You can't remove subjectivity (or, more to the point, the subject) from a subjective review—you wouldn't want to—but you can impose upon the review a little quality control.

Many people are surprised—I was—to learn that the matter of amplifier power vs load efficiency was a serious, productive area of research at least through the end of the 20th century (footnote 1). The detailed answers are complicated. You often need more than you think: as much as two-and-a-half times more current than the speaker's load impedance—considering magnitude and phase angle—would predict. That's one more reason to try a powerful amplifier with a loudspeaker you wouldn't expect would need that much power.

Big Doesn't Mean Difficult
When Michael Fremer reviewed the BHK Signature 300 monoblocks in the February 2016 issue, he used his big, expensive Wilson Audio Specialties Alexandria XLF loudspeakers (seven drivers, 655 lbs, $210,000/pair). He also listened through the Marten Coltrane IIIs (five drivers, 209 lbs, $100,000/pair), which he had in for review. This is the kind of system you expect to see muscled by big, powerful monoblocks.

The PS Audios worked well for Mikey. "I would be hard-pressed to identify anything wrong in the BHK 300's sound," he wrote. He "thoroughly" enjoyed them, "without reservation—especially their engaging transparency, transient delicacy, harmonic and textural richness, and convincing timbral accuracy." They weren't quite as good, he judged, as his striking, sweet-sounding darTZeel NHB-458 monoblocks—but then, while the BHK 300s aren't cheap, Mikey's reference amps cost nine times as much: $135,000/pair.

I aimed to try the BHK monoblocks in a different kind of system—but just how different was it? John Atkinson's measurements show that, despite its impressive size and complexity, the Alexandria XLF is not a difficult load. Indeed, its B-weighted voltage sensitivity, as measured by JA, is 92.6dB/2.83V/m—higher than the putatively ultrasensitive DeVore Orangutan O/96, also as measured by JA! (footnote 2)

Sensitivity, though, is just one aspect of amplifier-speaker matching. There are two others: the amplifier's output impedance (which influences the frequency response), and making sure the amplifier can provide enough current. Both of these things depend on the speaker's impedance—both its magnitude and its phase. JA found that the big Wilson XLF is not a current-guzzler. The XLF "will not be a difficult load for the partnering amplifier to drive," he concluded. The Orangutan O/96 is easier—far easier—but there's not as much difference as you might think. So let's listen.

It's all about the bass . . . isn't it?
I compared the PS Audio monoblocks with the kind of amplifier you'd normally see paired with a speaker like the Orangutan O/96: my Leben CS600, an all-tube, point-to-point–wired integrated amplifier that puts out something like 30Wpc. Although it's an integrated, the CS600 has preamp inputs, so it can be used as a standalone power amp (with, however, the volume control still in circuit). The CS600 ($6495) is retro in styling and technology, but it has a modern sound: While John Marks was listening to it for his June 2010 column, an audiophile visitor, on learning that the CS600 had tubes, exclaimed, "Wow, this doesn't have the typical 'tube sound.'"

If there's one area where a big, powerful amplifier with a solid-state output stage (hence low output impedance) ought to outperform a lower-power tube amp like the Leben, it's in the bass—it takes current to move big bass drivers. Then there's the whole "damping factor" thing—the idea that a dynamic driver's movement through a magnetic field generates current that must be dissipated if that driver is to be well controlled. Assuming it's a real thing, damping factor depends on an amplifier's output impedance—almost always significantly higher in tube amps. Both things would seem to favor a powerful solid-state amp like the BHK over a less powerful tube amp like the Leben (footnote 3).

The Orangutan O/96 is an especially interesting candidate for such a test (footnote 4) because its 10" driver produces some of the most comfortable, strain-free—in a word, natural—bass I've heard. It doesn't plumb the absolute depths, but its low frequencies are abundant, and I'm not sure I've ever heard a double bass reproduced quite so convincingly: like hearing it up close through a wide-open window. And yet the presentation isn't microscopic, not etched in glass at high resolution. Would the O/96's bass sound tighter with a high-current, low-output-impedance amplifier like the BHK 300? And if it did, would that be a good thing?

I set up my system with PSA's BHK Signature preamp, which I still had on hand, alternately feeding the two amplifier choices. I used the Leben CS600's preamp inputs, turned the volume up most of the way (to about 5 o'clock), and controlled the volume from the preamp. I switched back and forth between the CS600 and the BHK Signature monoblocks, using unbalanced cables for the former and balanced for the latter—feeding each in the best way it permits.

Before I tell you what I heard, consider what I was comparing and what's likely to matter. JA found that the BHK 300 outputs 300W into 8 ohms, which means that its power reserves will probably be a bit smaller into the 10-ohm O/96—that's the nature of solid-state amps—but it's still much more powerful than the Leben CS600. I haven't seen an output-impedance spec for the CS600, but JA measured the impedance of a similar Leben amplifier—the smaller, less powerful CS300—at 3–3.5 ohms at low frequencies, falling at higher frequencies by an amount that depended on the setting of the output-impedance switch on the back. At 3 ohms, the output impedance of the CS600 would be about 15 times that of the BHK 300 monoblocks. If you buy into the damping-factor thing, the monoblocks should do a far better job of controlling the Orangutan's 10" woofers.

To test the bass, I used "Otiot," from Masada's Zayin, aka 7, featuring John Zorn on tenor saxophone, Dave Douglas on trumpet, and Greg Cohen on bass (CD, DIW DIW-915). Cohen explores the double bass's natural range in its entirety; if your room has significant resonant modes above about 41Hz (the lowest note on a four-string bass), you'll hear them (footnote 5). Similarly, if an amplifier runs out of steam—has a current shortage—on a particular note, you should hear that, too.

In the event, I heard little difference. At first I thought a couple of Cohen's lowest notes lost some definition with the Leben relative to the BHK monoblocks, but then I went back, listened to the monoblocks again, and heard the same thing: This was room acoustics, not speaker impedance. It may have been on the recording, but it probably was a low-level resonant mode in my room (footnote 6).

In retrospect, this may not have been the best test. Unfortunately for reviewers, tracks on which it's easiest to hear a problem are often the tracks where the problem is least likely to occur in the first place. The biggest challenges for amplifiers are loud, complicated passages, with lots of frequencies at once. White noise can torture an amp, but you probably don't spend any more time listening to white noise at ear-piercing volumes than I do. I may be going out of my way to cause trouble, but I should at least stick to music—music I like, in fact.

So I pulled out Ralph Vaughan Williams's Symphony 7, Sinfonia Antartica, with Kees Bakels conducting the Bournemouth Symphony Orchestra (rip from CD, Naxos 8.550737, footnote 7). Starting at about the 8:30 into the third movement, the pipe organ comes to the fore with sustained low notes and, a little later, swirling high notes—and then the brass. This is the kind of music I would expect to seriously test an amplifier . . . but into this easy speaker load?

The BHK monoblocks maintained their composure throughout this difficult passage. Perhaps more surprising is that the Leben did, too. In one complex passage near the nine-minute mark in the third movement, at the highest volume level I felt comfortable subjecting my neighbors to (Leben volume knob at about 5 o'clock, BHK preamp at 55), the high, swirling organ notes and the orchestra lost a little clarity—but even then, the organ's pedal notes were adrenaline inducing.

When reviewing components, I always do far more listening than I specifically comment on. Over the course of this comparison, I heard some significant differences. The BHK 300s produced a bit more energy in the mid-to-upper bass than the CS600; counterintuitively, the low bass wasn't very different. The Leben, meanwhile, had more treble energy than the PSAs, especially in the upper treble, which sometimes led to a clearer sense of the performance venue. These differences weren't especially subtle—they made a qualitative difference in the character of the sound—but neither did one amp have a clear advantage over the other. A good illustration was À la Santé, from Shostakovich's Symphony 14, in the recording by Simon Rattle and the Berlin Philharmonic (CD, EMI Classics 3 58077 2; MQA/Tidal HiFi 68735772). This is dark, bottom-heavy music, and through the BHK it sounded darker and weightier; the Leben emphasized rhythm and texture a bit more.

"If it's not baroque, don't fix it."—Cogsworth, the Prince's butler in Beauty and the Beast
Of the two amplifiers in this comparison, the BHK Signature 300 monoblocks were probably the most technically accurate in the sense of hewing more closely to the signals pressed on or encoded in the recording media. The differences in tonal balance I heard were probably the results of the interaction of the Leben's output impedance with the Orangutan O/96's load impedance: The DeVore speaker may not require much current, but the Leben's impedance still interacts with the speaker and affects the balance of frequencies reproduced. But this is little more than speculation, and anyway, it's just a way of saying that each of these amps has its own character, with the PSA monoblocks probably having higher fidelity, in the original sense of the phrase. Overall, I slightly preferred the BHKs' weight to the CS600's airier, spacier sound, but it depended on what music I was playing—and, perhaps, the day of the week or what I had for dinner. These are two great amplifiers, and I'd be happy with either. Or both.

This was an interesting and fun test with a clear, simple outcome. First, in defiance of common norms, I heard no disadvantage from pairing a big amp with easy-to-drive speakers; the sound was wonderful with big orchestral music or intimate chamber jazz—the BHK 300s may be powerful, but they're no brutes. But neither do these easy-to-drive speakers require, or even much benefit from, so much power. I did hear a bit of congestion in one especially complex passage—by all means, take that into account. But remember, too, that it passed in a heartbeat; I probably wouldn't have noticed it if I wasn't listening analytically.

From both objective and subjective perspectives, then, PS Audio's BHK Signature 300 is another interesting option to consider with highly efficient speakers, another thing to try if you're so inclined.—Jim Austin

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Footnote 1: The last really important paper in this area that I'm aware of is Eric Benjamin's Power Amplifiers for Loudspeaker Loads," JAES, Vol.42 No.9, September 1994, pp.670–683.

Footnote 2: While the sensitivity of the Orangutan O/96 is specified at 96dB/W/m, Stereophile's convention is to specify sensitivity in units of dB/2.83V/m; for a true 8-ohm speaker, the different units give exactly the same answer. But the O/96 is a 10-ohm speaker, so its power efficiency is higher than its voltage efficiency. See John DeVore's response to Art Dudley's review of the O/96 in the December 2012 issue.

Footnote 3: JA found the BHK's output impedance to be a little high for a solid-state output stage, but it's still much lower than those of most tube amps.

Footnote 4: Sorry, I know, animal testing is wrong, especially on our brother apes.

Footnote 5: To me, what's really alluring about this track is the way Dave Douglas's muted trumpet and John Zorn's sax combine on a sustained note to create the eerie illusion of a human voice—then they move apart and the illusion fades. I doubt this was intentional, and when I realized what I was hearing, the illusion disappeared. After that, I had to struggle to hear it that way again.

Footnote 6: I could have checked this out with headphones. I didn't.

Footnote 7: If you're an amplifier masochist, the thing to do is to send it a loud low note at a frequency where your speakers are reactive—lots of capacitive and inductive impedance—along with some energetic highs near the speaker's minimum impedance. What better than a symphony featuring a full orchestra, a pipe organ, a soprano, and a women's chorus—not to mention a vibraphone, plenty of low percussion, and a wind machine?