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If you’re reasonably handy, you can probably build your own digital-to-analog converter. It won’t cost much, and if you’re careful, and knowledgeable enough to understand and follow some rather technical instructions, or if you have patience enough to follow advice from a few different online discussion forums—and the judgment to distinguish the good advice from the bad—then the DAC you make may end up sounding very good.
So it’s no surprise that you can buy very good Chinese-made DACs that measure very well, very cheaply. Those Chinese DACs are probably designed by first-rate engineers, and while extracting maximum technical performance from a good DAC chip requires care and attention, it isn’t rocket science (footnote 1).
What, then, is the point in paying tens of thousands of dollars for a D/A converter?
It’s a reasonable question, one that every DAC shopper must answer for themselves. Is extremely low measured jitter, noise, and distortion all that matters in a DAC? Is it sufficient assurance that it will sound “perfect,” as good as a DAC can sound? Or is it possible to take this basic technology further, despite what the measurements show? It’s easy enough to find people who are quite happy with their $1k DAC and smugly confident that they’re getting the best possible sound. But in perfectionist audio (and certainly in this magazine), it’s axiomatic that progress is always possible, that you can always do better, and that measurements—at least the easy and obvious measurements, such as S/N ratio, distortion level and profile, and Miller-Dunn J-Test jitter—don’t tell the whole story. And if you listen with trained ears through topnotch audio systems well set up, it’s frankly hard to miss the improvement in sound achieved by expensive DACs produced by companies committed to achieving the best possible digital sound.
And if you disagree? Then you just saved yourself a ton of money.
The CH Precision C1.2 D/A Controller
I’m sitting back in my lightly chewed IKEA chair, listening to Benjamin Grosvenor’s performance of the Liszt B-minor sonata, S.178, recorded in Queen Elizabeth Hall at London’s South Bank Centre. It’s from Grosvenor’s album Liszt, and it’s streaming from Tidal (24/96 MQA, Decca). I’m listening on a system most would consider very good; it certainly isn’t cheap. It includes the Wilson Alexx V loudspeakers, two Burmester 218 amplifiers (each bridged for mono, in for review, footnote 2), the Pass Labs XP-32 preamplifier, and not-quite top-level cabling by Nordost and AudioQuest.
The source of this music is the new CH Precision C1.2 D/A Controller ($43,000 as equipped), aided at the moment by a complete CH Precision digital front-end: the X1 power supply ($20,500), the T1 clock ($24,500), and the D1.5 transport ($49,500 but not currently in use). I’ve set the volume to what I’d expect to hear if I were sitting in the first few rows of the concert hall—and indeed, the sounds I’m hearing could be emerging from a Steinway on the stage of a good concert hall.
Well, to be completely honest: not quite. This is a very good performance and well-recorded, but, while the highs I’m hearing have an appropriate, crystalline “ping,” the lower-midrange keystrokes seem ever so slightly dulled; a touch of transient bite is missing. There’s also some congestion on the loudest passages, a sense that the piano’s case is filling up with sound and distorting a little, some thing I’ve noticed in live performances but not this much. Despite these minor flaws, this system is delivering a spectacular experience. The piano has real grunt—more than makes it to my listening seat at most of the piano performances I attend (footnote 3)—and lots of high-end sparkle. Decay, of notes high and low, is natural and even.
But what of those flaws I heard? Should we blame them on the CH Precision digital front-end? No. It’s clear that the fault lies in the way the piano is miked, which trades transient clarity for low-end impact.
How to build a CH Precision DAC
If your goal is to make a DAC that’s better than one you can make with a very good DAC chip, the way to do it is to start with a concept. You need a theory for how to proceed, or, as baseball commentators like to say about hitting, you need a thoughtful, fundamentally sound approach. It helps, of course, if your theory is correct, and if it’s just plain wrong you’re in trouble. But for reasons I think will soon become apparent, your theory need not be precisely on the money. CH Precision’s approach—shared generously with me by Florian Cossy and Thierry Heeb—is based on the notion that timing is everything. Getting the frequency part right is easy enough. What’s hard is getting things right in the time domain.
Both Cossy and Heeb are engineers. Heeb is the digital guy. In addition to being the “H” in “CH,” he’s a senior researcher at the University of Applied Sciences and Arts of Southern Switzerland, specializing in DSP for audio. Cossy—the “C” in “CH”—is the company’s CEO; his engineering expertise is on the analog side (footnote 4).
The first step toward understanding why timing matters in a D/A converter—or why it makes sense to assume it matters beyond mere 1s and 0s—is to recognize, as Heeb told me months ago in a Zoom conversation, that in audio, a digital signal is best thought of as analog. “Even if the signals or the electrical signals are supposed to be digital, basically just two levels, a zero and a one, as soon as you get into an electronic board, they are actually analog signals, current or voltage flowing through components. That is especially true, for instance, for clock signals. If you just consider clock signals as being a shift between two values between zero and one, you don’t really get what clock is. The most important point in clocking is in the time domain”—well, duh—”with finite resolution. Basically, it boils down to an analog signal again.”
I’ll just throw this in: In the physical world, music happens in the time domain. True, we do hear frequency—as pitch, and combinations of frequencies at chords, or as vocal or instrumental timbre—but, strictly speaking, those musical signals exist only as a function of time: In your ear canals, there is only one level of pressure at an instant of time, and it changes.
The frequency domain is, strictly speaking, a mathematical abstraction.
There are two things (at least) that lead to time-domain errors: timing randomness—also known as jitter (footnote 5)—and an intrinsic lack of precision in D/A conversion, which Heeb (and others) call time smearing. Time smearing is the same concept that MQA is intended to address—they too call it time smearing—and, indeed, CH Precision’s approach to dealing with that phenomenon seems quite similar to MQA’s approach. In a comment published in my review of the CH Precision D1.5 transport/player, Heeb said, “Time smearing is basically if you put a single pulse through the system, if you have a filter with a very long impulse response, that single sample will extend over a large number of samples.” The goal, then, is to shorten the impulse response so that the musical content of an input sample extends over as little time—over as few samples—as possible. How is that achieved? With an approach to conversion that’s quite different from the approach outlined by the foundational document of digital audio, Shannon’s theorem.
Shannon’s theorem says that if certain conditions are met, the output of an A/D–D/A sequence can exactly match the input, mathematically. But that’s not true in the real world, under any real-world circumstances, because the conditions are unphysical. They do not exist. For example, the basic mathematical function Shannon employed for sampling and reconstruction—the sinc(x) function—goes from minus infinity to plus infinity in time, which in the real world never happens. (“There is no energy in the signal before the instant where the musician starts playing,” Heeb wrote to me by email.) Anyway, CH Precision would not want to use a sampling/reconstruction “kernel” that’s infinite in duration, because, well, that’s a lot of time smear (footnote 6). “We prefer to use splines, which have a much more compact support (footnote 7), which makes it so that when the sample goes in, what comes out has, in our case, [no more than] 100Ês of pre-ringing and post-ringing,” Heeb said. A particular spline can be used to represent music locally; a long series of overlapping splines can represent a whole song or symphony.
In my review of the D1.5, I found it to be a transport of obvious quality. I also found it to be, with its two monophonic D/A converter cards, an excellent player of CDs, SACDs, and MQA CDs (footnote 8). Good as it was, though, those DAC cards are limited implementations of the CH Precision conversion approach. The C1.2 is an outright assault.
The C1.2 upsamples everything (except, according to the Roon Signal Path display, MQA data, which makes sense) to 16 times the base rate: 44.1kHz data and its multiples are upsampled to 705.6kHz; 48kHz data and its multiples are upsampled to 768kHz. This, though, is not your mother’s upsampling. In performing this upsampling, the C1.2 does something that was common in the early digital era but that’s surprisingly rare these days (so maybe it is your mother’s upsampling): It keeps all the original data points, interpolating new samples between them. Other approaches, most notably asynchronous sample-rate conversion, obliterate the original stream completely (except the very first sample) and replace it with a completely new datastream. The time series described by the new stream may be very close to the old stream; nevertheless, this strikes me as an interesting point, philosophically and perhaps sonically: How can you claim the original spectrum is perfectly recreated (it’s not) when all the data have different values?
The “base”-model C1.2 doesn’t include a USB input, but you can get one ($3000). CH Precision’s USB input card is a bit different from others. While it does reclock incoming data—that’s the advantage of an asynchronous, isochronous USB interface, in principle—it does not resample. Even via the USB input, the original samples are preserved.
At the end of this chain of conversion technologies is something surprising: a DAC chip. Not just any DAC chip, but one that was an important step forward for digital audio when introduced—in 1998. It is Burr-Brown’s PCM1704 R-2R ladder DAC chip, four per channel. Why do it this way instead of laying out an actual R-2R ladder with resistors, as several much cheaper, excellent-sounding Chinese imports do? I asked that. “The fact that it is a monolithic chip makes it both consistent and wonderfully accurate to work with, something that a discrete ladder cannot achieve even with the highest precision resistors,” Cossy answered. He also wrote, “Even though it is an ‘old’ chip, it more than meets current requirements.”
You wouldn’t expect CH Precision to use a boring old volume control (footnote 9), would you? Well, they don’t. Instead, the C1.2 utilizes a hybrid analog/digital control, which combines three large analog steps (via relays) with smaller digital domain steps.
The C1.2 from the outside in
Everything I’ve written up to now was true of the earlier C1 DAC (except maybe the part about the volume control; I’m not sure about that). So, what’s new in the C1.2? What has changed?
First, though, an aside on naming. Why name two products released so close together so differently? The D1.5 came out months before the C1.2. Why not call them both “1.2” or “1.5”?
Footnote 1: Yet, a look at some of JA1’s measurements reveals that commercial implementations of common DAC chips often fall short of a chip’s potential.
Footnote 2: According to Stereophile policy, reviews must be performed in a well-known room, mainly on well-known equipment, so I have already listened extensively—for several weeks—on my reference Pass Laboratories XA60.8 amplifiers. See my review here.
Footnote 3: Although not, I’m thinking, at Manhattan’s newly rebuilt Geffen Hall. I’ve attended two shows there now. Though a little bit dry, that hall has serious grunt.
Footnote 4: Also, of course, “CH” stands, in Latin, for “Confoederatio Helvetica,” or Swiss Confederation—for Switzerland.
Footnote 5: I’ve been hearing for years, from digital designers, that jitter can affect sound at far lower levels than previously thought—and that the effects of jitter are manifold: It’s not just the edginess heard, for example, on the jitter tracks on Stereophile Test CD 2 that affect imaging precision, subjective tonal balance, and other aspects of musical presentation.
Footnote 6: Modern sampling theory long ago abandoned the idealized notion of perfect reconstruction. An example of this is the use of a reconstruction kernel (a spline function, say) that differs from the one used for sampling (perhaps a sinc(x) function). “The key question is, how do the sampling and reconstruction kernels combine?” Heeb wrote in answer to another question. “In other words, what is the result of a reconstruction kernel applied to a sampling kernel on a unit pulse? If the result is close enough to identity (in a given frequency band and a given time space), then different kernels can be used with no apparent drawback.” So, wise designers long ago stopped being slaves to Shannon’s theorem, favoring instead an approach that attempts to minimize error and to shift error to where it does the least harm. This, I believe, is why there’s more than one legitimate approach to D/A conversion—and why it remains an unsolved problem. There are various legitimate approaches—valid assumptions as to where the inevitable error does the least sonic harm.
Footnote 7: “Compact support” is a mathematical term that means that, outside a certain finite range, the value of the function is zero.
Footnote 8: Although as an MQA-CD player, I had nothing to compare it to. It was the only MQA-CD player I’ve ever auditioned.
Footnote 9: The volume control can be difficult to locate among the C1.2’s many menu options. It’s hidden in the “Factory” menu, presumably because it’s such a fundamental choice: whether to use the C1.2 just as a DAC or also as a preamplifier.
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CH Precision Sàrl
ZI Le Trési 6D
1028 Preéverenges
Switzerland
(41) (0)21-701-9040
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