Before we get to the specifics of this amplifier I should first comment on the current incarnation of PS Audio. The BHK 600 amplifiers are somewhat extreme, with the result that they might not be what you expect from PS Audio and they might surprise you. Those of you who’ve been around for a while, I mean decades, may think of PS Audio as a solid, high-quality manufacturer of mid-priced, high-end audio. But in the last, perhaps, ten years, Paul McGowan, who is still running the show, has engineered something of a transformation of the company into a full-fledged, fairly extreme high-end audio brand. And I think this product, the BHK M600, is particularly representative of that transformation.
Basic Description
To put some technology description around that idea, these are big mono amplifiers. Each one weighs 108 pounds and is about 17 x 12 x 14”. Nominally, we’re talking about an amplifier that’s rated at 600 Watts into eight ohms, 1200 Watts into four ohms, and 2000 watts into two ohms, per channel. That assumes that you’ve got sufficient electrical power to actually meet those output specifications. What’s interesting, if you read the details of the PS Audio website, I those numbers are intended to be quite conservative because the actual rated power on the PS site is 950 Watts per channel into eight Ohms and 1500 Watts into four ohms. The price point is $32,500 per pair in the U.S.
Why Big Power Might Help
Clearly the power is the headline grabber, but I think that’s almost a distraction if we stop there. PS Audio does a good job, and other manufacturers have similarly done a good job, of pointing out that a lot of our listening is likely done in the one-watt or two-watt range. That’s because the average power output required to do music at 78 to 80 dB (a typical average volume – you can easily see what you use with a sound level meter app on your phone) doesn’t tell you what you need in terms of power output to reproduce transients. To figure out what we need for transients, we need a benchmark. An easy one to find is that a symphony orchestra can put out 110 dB peaks for the listener. I’m not saying that symphonic music is what you listen to, I’m merely pointing out that there are good measurements of symphonies that can serve as a reference point for measurements of what we might want to be able to reproduce if we want an amp that doesn’t clip or otherwise struggle with peaks. In this imaginary scenario, our average level is 80 dB with 1 watt input and we have 30 decibel peaks. The striking thing is that you need a lot of power to reproduce those peaks. If you have relatively inefficient speakers, you’re going to need even more power; if you have relatively efficient speakers, maybe not quite so much. But the differences are astounding when you calculate what is required to actually produce transient peaks:
If you use 1 watt to generate 80 db SPL average, you need 1000 watts to produce 110 db peaks
I think what PS Audio has tried to do with this amplifier is to say “let’s come as close as we can to taking the transient limitations out of the system”. And as I’ll tell you in a little bit, that’s certainly what I heard. What’s interesting about this amplifier is that my listening notes say “always sounds relaxed”. I mean, it sounds very dynamic, don’t misunderstand. But by “relaxed” I mean that it doesn’t sound like it’s trying hard. It sounds like it’s reproducing music, and that just doesn’t seem to matter what kind of music it is, how transient or poppy or dynamic it is. This amplifier just seems to be able to reproduce those dynamic swings without compression or grunge or harshness or blur. So, I got the feeling that the PS Audio people are onto something in designing an amplifier that I initially regarded as overkill. Whether you need this much power or you could use half as much power or a third as much power, I really can’t tell you because your speakers and listening room and musical selections will matter. That said, for those who find themselves concerned that sonic error in their systems might be caused by power limitations, PS has done a very nice thing by nearly taking power output out of the mental equation.
The Designer
But that’s not all that’s interesting about this amplifier. Another interesting point here is the designer of this amplifier. This amplifier, and several of the other PS Audio amplifiers, were designed by Bascom H. King. If you’ve been around for a while, you’ll probably have heard that name. Bascom designed some Marantz equipment in the late 60s or early 70s. He was then involved with Infinity Systems where he did the Infinity Class A power amp and all the servo systems in the speaker systems. Go look up Infinity Servostatic or Infinity IRS, and you’ll see some of what were extreme state-of-the-art speakers from the 1970s. And Bascom King also worked with Jim Bongiorno at Great American Sound on some of the Ampzilla products and some of the other electronics that GAS was involved in producing. As you may remember, there was a fairly narrow window where Great American Sound was recognized as pushing the envelope quite a bit, so he was involved with some famous designs. He had set himself up in the electronics consulting business way before that became a popular way of doing things. So, he subsequently went on to other manufacturers.
Bascom was also a reviewer for Audio Magazine, where he had a chance to listen to a lot of circuits, study the circuits, and knowing how preamps and amps worked, he was able to look at the circuit designs and get a lot of knowledge comparing listening results to circuit features. In the recent time period, Bascom King was involved in the design of some of the Constellation amplifiers, which, as you will know, we have given Editor’s Choice Awards to. He passed away when he was 82 years old, and his last amplifier design is the one under review today, which is the BHK M600. PS Audio has video interviews with Bascom on how he went about the “little” brother of this amplifier, and I think you see a typical good engineer who was very careful about how he thought about circuit topology.
Bascom knew a tremendous amount about individual devices. A lot of what you’re doing when you’re doing a power amplifier or preamplifier design is looking at the available components, be they tubes or transistors, bipolars or MOSFETs or ICs. This amplifier uses both MOSFETs and tubes. This amplifier also uses tubes in the front end and power supply. The designer is looking at the device characteristics and trying to find devices that are readily available so you can actually support consistent manufacturing and that have highly desirable sonic characteristics.
I’ll give you an example of how King applied this knowledge. This is not my area of expertise, but I can tell you just enough about it to paint the picture. In a Class A/B amplifier, you have output devices that are of complementary types. With Bipolar transistors you would have PNP and NPN types. In this case, where MOSFETs are used, you would have N-channel MOSFETs and P-channel MOSFETs. King observed that the readily available MOSFET devices were from International Rectifier, and the P-channel devices that fit with this amplifier’s goals have a characteristic that makes them not ideal complements to the N-channel devices. As a result, he decided to use N-channel devices for both parts of the output stage of this amplifier. That takes a special circuit design and creates some other issues he had to compensate for. He figured out a way around the problems of using “same sex” MOSFETS so that he could optimize this particular device. Interestingly enough, he leveraged his Audio Magazine experience by recalling a circuit feature that he had discovered in a Yamaha amplifier back in the early ‘90s. Standing on the shoulders of giants is how great things happen. Not surprisingly, it really benefits your work if you have a deep and broad understanding of the devices that are available to you, and then you have a deep and broad understanding of circuit topology so that you can match the circuit topology to the available devices to compensate for their characteristics. And, of course, a third key is that you actually care about listening to the result and compare it to the sound of music.
The result is that the BHK M600 is big amplifier, very powerful, with an interesting circuit design that really tries to take each stage in the amplifier and optimize it around currently available, state-of-the-art devices. PS Audio, because often you don’t get that far from your roots, has priced this amplifier at $32,500 a pair. To some of you may sound astronomical, but actually given the power output and some of the amplifiers it competes with, this is an amplifier that’s at perhaps 50% to 70% of the price of some competing products, at least as measured by power classification.
Sound Quality
I’ve spent about six months listening to the BHK M600 with various speakers and source components (digital and vinyl). I wanted to make sure when we’re talking about something at this level that I really knew what was going on and that I could identify some of the characteristics or lack of characteristics that are on hand here.
I’m first going to refer to the dynamic envelope of this amplifier which is, in my mind, exceptional. But I want to be sure I’m being clear that you notice it by it not making errors. It doesn’t clip or it doesn’t crush or it doesn’t crunch the sound. This yields a more relaxed and natural sound that’s just super easy to fall in love with. Part of this is because you find that you stop subconsciously preparing for distortions on dynamic swings.
I do think I crept up my average listening level a bit while I was reviewing the BHK 600, but when I measured this, it was only about 2 db. The effect of a bit more average level and bigger dynamic peaks was a general sense that music was more dynamic and more open. The important change is that I could get to dynamic swings that made the music much more involving.
This shouldn’t be too surprising if we recall that when we are running 80 dB on average and we want to reproduce 110 dB peaks, we need something like 1000 watts to reproduce that transient correctly. The amplifiers that I can bring to mind from recent reviews are typically just not capable of that, so the exceptional openness and purity of tone across the dynamic envelope of the BHK 600 are maybe to be expected. So, perhaps, my big point is that this simply made the music more listenable and involving.
Now, for analytical purposes, I’ve used symphonic dynamics as a reference for my calculations. You might think that there’s a lot of pop music that’s more compressed and you would be correct. But there’s a lot of really well-recorded material as well and a lot of it is in high res. I’ll give you some examples of some of the music I listened to, though over the course of listening to this amplifier, I probably listen to 250 or 300 pieces of music.
Examples on the pop side include Sigur Ros, Wilco, Balmorhea, Radiohead, Cindy Blackman Santana, Metric, LCD Soundsystem, Alicia Keys, Steely Dan, Khruangbin. I listened to singer-songwriters like Adele and Taylor Swift and Leonard Cohen and Joni Mitchell and Jennifer Warrnes. Jazz artists like Miles Davis, Dave Brubeck, Bill Frisell and Cecil Taylor. And symphonically, since I mentioned the dynamic range of symphonies, I listened to pretty much everything from Beethoven and Brahms, through Mahler, Stravinsky, Walton, and some of the moderns like John Adams. The important point about sound quality is that opening many of these tracks up dynamically makes them more real and more impactful and more poetic.
To try to exercise the amps, I used Magnepans (very low sensitivity) with this amplifier, and right now, I’m using the Magico A5. Both of these speakers, and others, worked well and I heard consistent benefits to having the BHK 600s in the system.
Of course, besides exercising the amps, we also want to be able to reproduce musical delicacy. And for that leads directly to soundstaging. Soundstaging is a great measure of superior musical delicacy and detail. That’s because soundstaging is significantly a byproduct of having a low noise floor and the ability of the amplifier to reproduce low-level signals with really high fidelity. if you just think about it, it’s quite easy to wrap your head around how a concert hall works. The same thing is true in a club. The space that you’re in is communicated to the microphones, and therefore, ultimately to your ears, by reflected sound captured in the concert hall as those reflections occur and go around the room, whence they get lower and lower and lower in level. And as they drop off in level, you’ve got to be able to capture them accurately. And that means that the amplifier has to have a very low noise floor and has to preserve the delicacy and beauty of the low-level signals extremely well (low distortion on small signals).
I have not heard soundstaging like I heard with this amplifier on previous lesser amplifiers in the reference system. The stage with the BHK 600 is wide and deep. The performers are, when the recording is good, accurately positioned on the stage. The BHK 600 excels at “spotlighting” or putting space around the performers so that you can follow specific musical lines more easily.
Now I will say that you might get this with a Soulution or a CH Precision or a Constellation amplifier (I did not have any of these on hand during the test period). Soundstaging might be even better with these amps, since it is such a litmus test of electronic excellence. But as you will know, those amplifiers cost 2, 3, 4, 5 times what these amplifiers cost. And so I was impressed with the soundstaging of the BHK 600. Let’s just leave it at that.
There’s a third sound quality element I want to talk about with the BHK 600. There is a characteristic sound of this amplifier that I like, but I’m not sure it’s 100% accurate. There is a purity and a naturalness to the high frequencies that stands out. But if I had to swear where the accuracy level was, I would say the high frequencies are 99% accurate, but that the amplifier errs slightly, 1%, on the side of being soft. Now, my experience is that’s the side you want to err on. And since all reproducing devices have errors, engineers have to pick their political party. A decade ago, there was a movement in high-end audio to go the other way, to err on the side of edge enhancement and brightness. Many experienced reviewers and audiophiles look back on that period as a wayward time. I personally think that 1% is a very minor deviation, and one that happens to be desirable because if anything, it compensates for the less well-designed equipment in the chain, which can get really ugly when it’s errors are too sharp, transients are overloaded, or there are other kinds of nasty additive distortion. The BHK 600 does just a little bit of polishing of that stuff, which is why I say I believe it’s the side to err on. I found the softness to be a more than acceptable characteristic because it is complimentary and because it is subtractive and therefore not distracting.
Modern solid-state amplifiers don’t vary much in things like frequency response, but they do sometimes struggle to drive some speakers and in doing so can seem to have tonal balance and clarity issues. I didn’t hear any of that with the BHK 600, but I didn’t have some of the really tough loads, like big mbl speakers, on hand to try to simulate fringe cases.
Stepping back from very difficult loads, the damping factor of the BHK 600 is over 100. I felt the bass was nicely balanced (and, note, room positioning often tends to swamp the effect of damping factor), but if you want or need the class-D level of damping, that is not on offer here. Similarly, if you like the bass boost you can sometimes get from the interaction of high output impedance (low damping factor) tube amps and speakers with bass impedance peaks, that isn’t here either. Bass on the BHK 600 is just natural and well-defined.
Downsides
What are the downsides with the BHK 600s? Well, if you can afford these amplifiers, not much. First of all, they’re big and heavy. You have to have the room for these. And related to that, they put out a fair amount of heat. Yes, they are not heavily biased into Class A, but they’re big, powerful amplifiers. At idle, they generate a fairly substantial amount of heat. I would want to run them in an air-conditioned room if you live in places where it gets warm. It’s not quite as much thermal dissipation as you would have with a Class A amplifier, to provide some perspective.
The BHK 600 uses tubes in some of the circuits. PS does a very nice job of explaining why tubes are desirable for the power supply and why they work well on the input side of the amplifier. They’ve also done a nifty thing by providing a power switch on the rear, which is the main power switch for the amplifier. They recommend is you leave that main switch on generally, and then you use the power button on the front of the amplifier to turn off the tube circuitry that’s on the input side of the amp. That lowers your thermal dissipation when the amp isn’t being used and it preserves tube life. Nonetheless, there are tubes on the input circuit, and at some point, those are going to have to be replaced, maybe every two years or so. And while we’re talking about all of these thermal properties, not only are the amps big, but when you have something like this that has this much thermal dissipation, you can’t just shove it in a closet or rack and expect it work properly.
Let me mention that the packaging that the product comes in is very nice. The box has an interesting feature, which is that it has casters built into the box and a handle on top. So you can roll it like a rollerboard suitcase. These guys have paid attention to the fact that there are real humans using their products. And in addition, I’ll say I think they’re very nicely designed. It doesn’t look like a mad scientist laboratory experiment. I thought they were clean, modern, and they look extremely well put together.
That’s a brief rundown on the PS Audio BHK 600 monoblock amplifiers. I hope you can tell that I’ve really enjoyed having them in my reference system.
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