Noise, Resolution & Benchmark

The most notable aspect of Benchmark Media Systems' DAC3 HGC ($2195), which I favorably reviewed in the November 2017 Stereophile (footnote 1), is its low noise floor. John Atkinson's measurements corroborated Benchmark's claim that the DAC3 is capable of "at least" 21-bit performance. While significantly less than the theoretical potential of a 24-bit data format, 21 bits is still the state of the art, and corresponds to a dynamic range—the ratio of the highest achievable digital-domain volume to the DAC's internal noise—of 128dB. That's well above the dynamic range that most power amplifiers can achieve. A good-measuring high-end solid-state amplifier is likely to have a dynamic range—the highest attainable ratio of signal to noise—of about 100dB ref. its maximum power.

Benchmark created an amplifier that more than matched the DAC3's dynamic range: the AHB2 ($2995). Bridged to mono, the AHB2 has a claimed A-weighted dynamic range of 135dB (footnote 2). Kalman Rubinson reviewed the AHB2 in the November 2015 issue; it now sits in Class A of Stereophile's "Recommended Components."

With the DAC3 HGC and the AHB2, Benchmark seems to have achieved the lowest-noise, lowest-distortion system of source and amplification on the planet. The signals this combo sends to loudspeakers should be cleaner—truer to the source—than any other audio components can achieve. Wanting to know what that means musically, I asked Benchmark to send me a pair of AHB2 amplifiers to complement the DAC3 HGC already in my possession.

Setup
This article is unusual in that my goal was not to take the measure of a particular component but to determine whether Benchmark's entire low-noise system could resolve musical detail that a more typical audiophile system, with noise and distortion levels that are low but not as low, could not. More broadly, I wanted to determine whether the two systems sounded meaningfully different—especially in ways I could correlate with the presence or absence of distortion and noise.

I set up two systems independent of each other except for the data source (my NUC-based server, which runs Roon ROCK) and loudspeakers (my longtime reference DeVore Fidelity Nines).

I configured the Benchmark system for optimal technical performance, with the DAC3's internal output pads disabled for the greatest possible voltage. In this configuration, the DAC3's maximum output is an extraordinarily high 18.3V—too high for any other amp-and-speaker combination I'm aware of, but appropriate for the AHB2s at their lowest gain setting: a very low 9.2dB. Benchmark does things this way because the combination of high source output voltage and modest amplifier gain is optimal for minimizing noise and distortion. The AHB2's two other gain settings, of 17 and 23dB, can be used in more mainstream systems. I used the Benchmark amps' bridged outputs because, in contrast to most other amplifier designs, the AHB2s get even quieter, by 3dB, when the amplifiers are bridged.

The speaker cables, Benchmark's own, employ SpeakON connectors on the amplifier end and, on the other, banana plugs from River Cable with a nifty screw-down locking mechanism—a great cure for loose-fitting banana plugs. I used the power cords Benchmark provided; they're nicer than most stock cords. Balanced interconnects were high-quality Canare star-quad microphone cables (L-4E6S) with Neutrik connectors. This is a professional-audio system, so why not use pro cables?

In addition to the Roon ROCK and DeVore speakers, the comparison system consisted of PS Audio DirectStream DAC and BHK Signature preamp and, variously, PS Audio BHK 300 and Pass Laboratories XA60.8 monoblocks. All connections among these fully balanced components were made with Clarus's top-of-the-line Crimson interconnects and Clarus speaker cables. The ROCK was hooked up to the two DACs with USB links from AudioQuest. Switching between systems was quick and easy: disconnect one pair of speaker cables, connect the other pair, and switch zones in Roon.

But first, levels had to be matched. Starting with my main system, I adjusted the volume so that the musical peaks of the recordings I planned to use were loud but tolerable. Then I played the pink-noise track from Stereophile's first Test CD (Stereophile STPH002-2), measuring the level with a sound-pressure-level (SPL) meter. I hooked up the Benchmark system and adjusted the DAC3's volume control so that the white-noise SPL reading was within 0.1dB of the other system's level. After setting the levels, I left the volume controls alone. The maximum system SPL was 92dB (footnote 3).

The Art of Noise
If you follow John Atkinson's measurements sidebars, you know that just because a digital audio format is 24-bit doesn't mean you get 24 bits of audiophile goodness in your listening room. The analog side can't match what digital is doing—AC line noise is especially pernicious. JA's measurements of the DAC3 HGC corroborated Benchmark's claim that it's capable of about 128dB of dynamic range, equivalent to about 21 bits. It probably isn't possible to do much better than that.

But what about the rest of the system? I've already mentioned power amps: If noise and distortion limit the system's signal/noise ratio (SNR) to about 90dB—that's pretty good—then even under otherwise optimal conditions, noise and distortion in the amplifier will be higher in level than, and so potentially mask, the quietest 38dB of the DAC3 HGC's output (footnote 4).

This is the problem the AHB2 is intended to solve with its claimed dynamic range of 135dB—that's the ratio of the noise (inputs shorted) relative to the full output voltage. A more relevant figure is 0.0003%, the AHB2's total harmonic distortion plus noise at full power, relative to the maximum output voltage. That corresponds to an SNR of about 110dB. That number should be better bridged, and JA's measurements show that just below full power, the distortion is lower by at least a factor of 10, so it should be possible to achieve significantly better SNR by listening just below maximum volume. Which means that 120dB is in sight.

Assuming, then, that there are no other noisy components in the signal path—no preamp—the AHB2 delivers to your speakers more of the resolution and dynamic range of the DAC3 (or other high-performing DAC) than will most other amplifiers.

What happens after that depends on your speakers and your room. Loudspeakers are distortion factories, especially in the bass. Rooms, too, have a dynamic range. The top of the range is set by the highest volume you can tolerate—or, since music is supposed to be pleasurable, the highest volume you can enjoy. The bottom of the range is set by . . . what, room noise? Yes, but precisely how that works is not entirely clear (no pun intended).

By convention, in-room noise is referenced to an SPL of 20µPa (micro-Pascals), which corresponds to 0dB, the quietest sound the average human ear can hear—though that's sort of average, ignoring how the ear's sensitivity depends on the frequency: in the highs and lows, the volume necessary for audibility is much higher than 0dB, reaching 73dB at 20Hz; in the presence region it's significantly lower, plumbing –7dB at 3kHz.

In my room, on a good day, with windows closed and appliances turned off, I measure an SPL of a few dB above 30dB. That's typical for a "quiet rural area," according to one widely distributed chart, so it's pretty damn good for the middle of Manhattan. On a bad day, with service trucks idling along my stretch of Riverside Drive with pumps or generators running, or with heavy rush-hour traffic on the road itself, the noise level climbs above 40dB, which the same source says is the "lowest limit of urban ambient sound." The noise in my listening room can hit 50dB at times, usually weekends, and late nights in summer when the building's boiler fan isn't running. That's equivalent to a "quiet suburb, conversation at home," or "large electrical transformers at 100 feet."

So there's electronic noise inside components and there's room noise. How they relate depends on your speakers' sensitivity and the volume setting (footnote 5), but in general room noise is louder. To risk stating the obvious, the widest in-room dynamic range is usually achieved at high volumes. But 110dB SPL is the pain threshold for the average human; to go above that is to risk permanently damaging your hearing. An SPL of 110dB is, I think, a reasonable upper limit to the high end of your room's dynamic range. In real rooms, music's quiet passages, and quiet sounds within louder passages, will always lie below the room's noise floor.



Footnote 1: Benchmark Media Systems, Inc., 203 E. Hampton Place, Suite 2, Syracuse, NY 13206-1633. Tel: (800) 262-4675, (315) 437-6300. Fax: (315) 437-8119. Web: www.benchmarkmedia.com.

Footnote 2: Bascom H. King, designer of PS Audio's BHK monoblocks, is quoted on Benchmark's website: "I measured less distortion and noise in the Benchmark AHB2 than in any other of the many power amps I've measured over the years." Not only does the Benchmark amplifier have very low distortion; that distortion gets even lower as the power increases, to quite high levels.

Footnote 3: Room noise, then, was higher in level than all but the loudest 60dB of music.

Footnote 4: I'm ignoring the effects of noise-shaped dither, which can improve effective dynamic range beyond 6.02dB per bit.

Footnote 5: This isn't simple, though. When you turn the volume up or down, you're altering the electronics' SNL—the dynamic range in practice.

COMMENTS
rt66indierock's picture

With my first office system I had no noise at idle but substitute the Klipsch Hersey's from my home system and I could hear noise at idle.

My office is pretty quiet. Currently 36 to 39 dB with a "river" running underneath it. Without the river at night 24 to 28 dB.

Last year an acquaintance brought his Brenchmark DAC and Amp in with Joseph Audio Pulsars. We noticed and then measured a small difference in the dynamic range of his system and mine. So good job.

John, I'll take "Down in Hollywood" exactly as the 3M machine recorded it. See you Friday.

JimAustin's picture

I'm very curious about your river. I've thought about it and can't figure it out--unless you live on the Pacific coast and over a sea cave. Or something.

rt66indierock's picture

The Phoenix Mountains Preserve is less than two miles away and when it rains like it has the last 72 hours (3.43 inches) a large amount of water flows down the mountains to the flood control canal creating a river.

This river runs under my office into to the flood control canal next to it. I was curious how much extra noise it created so I measured it. Actually not much extra noise.

Nothing left of the river today but it is a regular part of our monsoon season.

Bogolu Haranath's picture

"A River runs Through it" .............. Mark Isham :-) ...........

ok's picture

..but I wonder whether the same or similar listening remarks (since we’re not talking highly differential observations here) wouldn’t have also been possible by using various random configurations for whatever reasons unrelated to real bit depth or THD+N. In my opinion any kind of electronic THD under 0,1% or so is musically meaningless at best (especially when achieved by means of excessive negative feedback which is related to other forms of distortion) mainly due to speaker's erratic frequency response and harmonic distortion bottleneck; pretty much like trying to bypass nearsightedness by increasing screen contrast. However higher figures of electronic THD could arguably accumulate to or even compete certain speaker distortion patterns with rather unpredictable results.

JimAustin's picture

..but I wonder whether the same or similar listening remarks (since we’re not talking highly differential observations here) wouldn’t have also been possible by using various random configurations for whatever reasons unrelated to real bit depth or THD+N.

Yes, that is possible. It would be exceedingly time-consuming to rule out that possibility.

In my opinion any kind of electronic THD under 0,1% or so is musically meaningless at best (especially when achieved by means of excessive negative feedback which is related to other forms of distortion) mainly due to speaker's erratic frequency response and harmonic distortion bottleneck; pretty much like trying to bypass nearsightedness by increasing screen contrast. However higher figures of electronic THD could arguably accumulate to or even compete certain speaker distortion patterns with rather unpredictable results.

I was, frankly, surprised to hear a difference. And while it was obvious once identified, it's not something you'd notice in casual listening--regardless of whether or not it correlates with THD+N. This is very close to what I set out to test--though, as my answer to your first question acknowledges, the test is more suggestive than definitive. Maybe the obvious conclusion from this experiment is that the kind of numbers Benchmark has achieved are most useful in the studio--and especially at the conversion stage, where there may be several conversions. In a studio, a power amplifier is used for monitoring only--so, a single pass. The AHD-2 is a great amplifier, but unless the differences I heard are indeed attributable to distortion and noise, I don't know why anyone, even a studio, would need an amplifier that clean.

JimAustin's picture

I thought I should add, having just rewritten what I wrote a few weeks ago: I'm convinced that noise is not responsible for the differences I heard. I'm convinced that, as I wrote, if you can't hear noise, noise is not a problem; after all, you can hear through noise. I'm far less convinced about distortion. Distortion, correlated with the music, can obscure detail. At what level, I'm not sure.

ok's picture

..I've also heard this amp; it sounded truly remarkable – and I don't mean merely on the detail department.

Ortofan's picture
ok's picture

of Bruno’s views on a handful of matters and as far as I know he’s not a proponent of feedback excess in non-class D amplifiers. In this very paper (p. 14-15) he clearly states that his use of excessive NF in conventional amps is purely experimental and mostly unstable, while “normal” amounts of global feedback admittedly render their sound somewhat unpleasant (well, not that I'd personaly reject any amp merely on feedback grounds). By the way his class D amps and active speakers tend to sound arguably better than some 90% of the current hi-end circulation.

Bogolu Haranath's picture

Manufacturers of conventional SS amps like Ayre and Pass use almost no NF ........... Two examples ........

dalethorn's picture

When it comes to power amps, I think it's foolish to skimp on anything. If it doesn'r weigh at least 300 lbs and raise the room temperature noticeably, it's too weak.

Ortofan's picture

... negative feedback (as it applies to electronic circuits), perhaps you can quantify the amount of negative feedback that you deem to be "excessive" and describe how you make that determination?

ok's picture

should be considered within context, not per se. Compromises usually need to be made between low THD+N, wide bandwidth, high damping factor pros and gain drain, higher order distortion pattern, brick wall clipping etc cons. Class D amplifiers may need more than 30 db of global feedback to effectively operate, while some class A/B ss or tube designs are doing all right with just 3 db bordering to none. There are also power amplifiers (CH Precision comes to mind) that allow the end user to swap between global and local feedback depending on speaker impedance and personal taste. Your question, if not teasing, otherwise reminds me of the ancient greek “sorites paradox” which seeks to determine the exact point where accumulated grains of sand actually become a heap.

Bogolu Haranath's picture

Some tube amp manufacturers provide user adjustable feedback controls .......... Cary Audio is one example ............

Ortofan's picture

... amplifier. How do you determine whether or not a given amp employs what you refer to as an "excessive" amount of negative feedback?

ok's picture

..THD and damping factor; if it works, I don't mind.

Ortofan's picture

... do you want an amp to have?

ok's picture

..above 20 something is actually marketing overkill. When cable/crossover/voice coil is taken into account, there is no such thing as a DF of 1000. I've heard zero-feedback amp with a measured DF of 18 that handled bass with Darth Vader force grip; same figure in a feedback-based amplifier would arguably suggest an embarrassing piece of junkware.

Ortofan's picture

... that audible variations in frequency response won't be introduced when such an amp is used with a speaker having a non-linear impedance?

ok's picture

and even measurable to a certain degree, though it actually depends on many a factor (output devices, power supply, bias current, signal path etc) amongst which nominal output impedance is probably the least significant one. In general I find speaker measurements (please someone add distortion figures!) far more straightforward and informative than the electronics counterpart, the actual meaning of which remains in many a case rather obscure. Or, as Putzeys puts it (referring to MQA): “Meanwhile I'm happy to do speakers. You wouldn't believe how much impact speakers have on replay fidelity”.

Bogolu Haranath's picture

Putzeys designed Kii Audio Three loudspeakers using his Class-D internal amplifiers ........ The speakers are listed under Class-A Stereophile ............

Bogolu Haranath's picture

Nelson Pass talks about amplifier design in an article published by Stereophile .......... That article can be found at Stereophile References ..........

Ortofan's picture

... are you then willing to accept as a by-product of using an amplifier with a relatively lower damping factor (or higher output impedance)?

ok's picture

since I already own a decade old, modestly powered, zero feedback integrated combined with some 88db/m, mostly 4 to 3 ohms, almost flat 35Hz-20KHz response (all according to third party actual measurements) 3-way floorstanders and I can attest that the results are nothing less than extraordinary from whisper to shout for any casual city dweller. Cables are of crucial importance but nothing really extravagant. Never mind names and prices – not to be found at Recommended Components either.

Ortofan's picture

... the fabled mystery amp and speakers.

Their performance is always described as "extraordinary" when the make and model of the equipment somehow can't be revealed.

ok's picture

..but it was just an interview, not an advertisement as your average posts.

Ortofan's picture

... the Socratic method won't succeed.

ok's picture

..the aforementioned speakers are favorably presented in JVS’s latest RMAF report driven by some tube integrated. Measurements are easily accessible through the net I suppose ;-)

jeffhenning's picture

So the amp with the greatest linearity, least noise and least distortion might not be the most resolving, truest amplifier?

Sorry, but that seems nonsensical. Are you sure your speakers are really up to the task? Sorry again, I don't think they are.

I'll closely paraphrase Benchmark's John Siau in what he wrote me 3 years ago (my memory is not photographic): "When you first hear an amp like the AHB2 if you are used to amps with more noise and distortion, the AHB2 may sound wrong. Given some time, that fades. In the end, the superior amp will sound always better."

JimAustin's picture

Can't help thinking you didn't read the article, or not carefully.

Bogolu Haranath's picture

HR wrote an interesting article about loudspeakers, which was recently published on the AudioStream website ....... Interesting article and interesting comments :-) ..........

Solarophile's picture

So what did HR say that was of any relevance here? As usual, I found his thoughts rather vague in that article.

Bogolu Haranath's picture

May be the loudspeakers are the weak link? :-) .............

Solarophile's picture

Speakers and headphones are always the weakest link compared to any decent DAC or amplifier whenever we start comparing things like distortion.

Robin Landseadel's picture

No LP playback will ever have s/n ratios like these. Kinda hard to think of LP reproduction being "high resolution" in this context.

Bogolu Haranath's picture

The SNR of vinyl LP is approx. 50-55 db ........... which is approx. 8-10 bits .........

Bogolu Haranath's picture

The SNR of analog reel to reel tape is approx. 72 db, which is approx. 12 bits ...........

Bogolu Haranath's picture

The SNR of analog cassette tape is approx. 63 db, which is approx. 10 bits .........

Bogolu Haranath's picture

The SNR of DAT is approximately the same as CD .............

Bogolu Haranath's picture

Nagra makes a digital reel to reel audio tape machine (the company says is) capable of recording in 24/96 resolution ..........

Bogolu Haranath's picture

JA recorded "Duet" album with a Nagra-D digital tape recorder at 24/88.2 resolution using "Decca Tree" 3 microphone configuration ..........

dalethorn's picture

I'm still waiting on a definitive list of "Decca Tree" recordings, with all of the facts in one list. I'm so busy arresting immigrants these days that I don't have time to build my own list from scratch.

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