An audio breakthrough

It would appear that there is a particular audiophile DAC with a cult following that gets rave reviews and costs over $2000, and is based on a non-audio DAC chip.

Why would they do that? Well, I think it is so they can run it “NOS” (not New Old Stock, but “non-oversampled”) and add their own “proprietary” filtering – plus it’s different from what the hoi polloi uses so it must be better. But, it would appear that someone has found a glitch, literally.

I am no expert, but I think that because this chip is a non-audio DAC, the output comes directly from a R-2R ladder, or similar. Small capacitive charges are transferred whenever the ladder switches operate, and sometimes the switches don’t all operate at the same speed. This means there is a glitch at the output whenever the DAC value changes, and it is worst when all the switches operate simultaneously i.e. when the most significant bit changes – around the mid range in other words (hmm…). Presumably there are other significant glitches at multiples of 1/4 full scale and 1/8 full scale too.

Low pass filtering the output can reduce the amplitude of the glitch at the expense of increasing the settling time. There are better techniques using a further piece of circuitry (a sample-and-hold) but, apparently, for the designers this was regarded as unacceptable for some reason (why?), and at audio frequencies still wouldn’t be as good as a typical $1 audio DAC in a mobile phone.

The evidence is all in the DAC chip’s data sheet:

glitch

I don’t know whether the glitch energy scales with the the VREF (i.e. the full scale signal amplitude), but this glitch is huge compared to the smallest signals that we might generate with the DAC.

An owner of this product now thinks he is hearing a certain harshness in the sound, and seems to have found that when reproducing a sine wave at -90dBFS, the output of the $2000 DAC contains significant glitches at the zero crossings. It would be interesting to know if there are detectable glitches at 1/4 and 1/8 full scale, too. This could be the phenomenon shown in the data sheet, or a by-product of whatever mechanism is being used, unsuccessfully, to suppress the glitches – they are rumoured to be using a combination of two DAC chips. Scrutiny of other reviews and measurements of the device seems to reveal distortion and noise figures that suggest something strange is going on – apparently.

An aspect of integrated circuit DACs is that because they are very small and constructed on a single chip, they have fantastic performance relative to themselves i.e. they remain monotonic and linear at all times. However, their absolute gain and offset may drift slightly with temperature. These temperature coefficients vary from chip to chip and can even be positive for one chip and negative for another (this appears to be the case for this particular DAC chip according to the data sheet). This means that any attempt to blend the outputs of two DAC chips externally using a combination of scaling, offsetting, inverting, mixing and interleaving would be most unlikely to succeed down at the lowest levels.

If these suppositions are correct, then this product is a great example of where the basic engineering of a basic product appears to have been sacrificed in the interests of just making something ‘different’ and supposedly ‘simpler’ – although as usual it ends up being more complicated.

[Last edited 04/05/16]

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5 thoughts on “An audio breakthrough

  1. Sorry for being coy about it. Basically, I’m not too concerned about which company this is, more that if this really has happened (and I have no proof of that), it is another little window onto the phenomenon of audiophilia.

    I’d like to be able to speculate about it. If the product is ‘anonymous’ then it doesn’t matter if I’m wrong.

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  2. Interesting! I read the blurb on a likely candidate’s website – some interesting ideas, although obviously the implementation might be a bit lacking as you point out. I’ve also read/listened to Rob Watts’ explanation of his approach to the various Chord products, Hugo, Mojo, Dave etc., and that involves FPGAs used for noise shaping etc. with again no “standard” DAC chips in the interest of implementing some specific features. He has some interesting claims about the importance of timing. I haven’t heard any of his recent products, but I was completely blown away by the Chord Red Reference CD player when that came out, which I heard in a number of different contexts. In respect of your earlier post on objectivity vs. subjectivity, Mr. Watts appears to address both, in that he’s a “proper” engineer who designs DAC silicon for other companies, and yet actually listens to the product and has pursued his direction on the basis of how it sounds, whilst measuring it very very well. His lecture from Munich 2016 is at https://www.facebook.com/chordelectronics/videos/665711883528836/ if you get time to check it out.

    I have no association with Chord Electronics, just interested in understanding why digital audio can sound different and how it all works, especially after reading your blog!

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    1. Hi. Thanks for the link – I just watched Rob Watts’ talk. I won’t dispute his claim that it’s the best DAC in the world, but where I might be sceptical is whether his hearing and listening tests are as good! He is, presumably, listening to it via an amplifier with distortion and noise 1000 times worse than his DAC, and passive speakers with distortion 10000 times worse and huge phase and timing errors. The acoustic noise from the DAC, power supplies and other equipment (which may actually be “modulated” by the signal) is presumably much, much higher than the DAC’s output noise..? One thing’s for sure anyway – I can’t afford three of them! As you may have seen, for me the ‘low hanging fruit’ is the passive speaker issue.

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  3. I agree with the thinking the company in question just wanted to separate there product by using a different approach to D/A conversion. Unfortunately they seem to have failed to meet the goal of being exceptional. I am a Mid Fi guy but have no gripe with the high end as long as the value per cost is there and this design has failed miserably in that respect.

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