Saturday, July 8, 2017

Updated DAC Measurements

I didn't measure the right channel of the Master 7 because of high DC offset, which started around 20mV and kept rising during the tests.  I may have broken the Master 7 in earlier testing by power cycling too rapidly.

Measuring the Master 7 left channel in full balanced connections, I found 0.009% THD.  It required some tricky connections do this measurement with a mono balanced signal.  I ran the left balanced output of the DAC into a balanced input of the Emotiva XSP-1 preamp, then switch the subwoofer outputs to full range, and take the R and L subwoofer outputs.  Testing the Emotiva Stealth DC-1 this way showed 0.0005% distortion, a bit higher than the 0.0003% it measures directly, showing the residual using this setup to be below 0.0005%, plenty good enough for measuring the Master 7 at 0.009%.

But in single ended mode, the same left channel of the Master 7 is measuring a horrible 0.183% distortion.  So I think something may indeed be broken in the left channel, as well as the right.

Master 7 Measurements

Meanwhile the Emotive Stealth DC-1 measured very well, with THD at 0.0003% in direct stereo balanced connections, and 0.0004%-0.0005% when measured through the Emotiva preamp, and 0.0005% in a mono'd measurement through the Emotiva like I did for the Master 7.  In both cases, these are right at my measurement residuals, the Emotiva dac having distortion as low or lower than my Juli@ sound card itself.

Emotiva DC-1 Balanced Measurements

The distortion test spectrum is very clean for the Emotiva DC-1 even going through the Emotive Preamp (so you're seeing the distortion of both devices compounding, but still not much).

Emotiva DC-1 through Emotiva XSP-1 Preamp

So, if low THD is important, and I think it is, why don't I use the Emotiva DC-1, which is as low as I can measure?  I tried that for an hour, but there does seem to be less there there, as compared with the 1704 based DAC 19 (let alone the Master 7).  But there's also a much greater blackness, a softer presentation that can still have shocking, perhaps even more shocking, highlights, flashing at you like on a bright sunny day.  I'm listening to it now and will decide, it's not an easy call subjectively--you think you've nailed it then you haven't.  The loser may take up super tweeter duties, where THD may be less important.

So, my subjective preference still leans toward the Master 7 compared with the DC-1, when I get the Master 7 fixed.  The Master 7 was fabulous sounding, I felt that right away and for weeks, and perhaps it even measured better in the beginning before my re-starting or something damaged it.  I hope it can be fixed to work as good as the specifications, which are only a hair behind the DC-1.

My subjective feeling is that the R2R dac makes you feel the cool breeze of reality all the time, it's right there ahead of you.  With the Master 7 the breeze isn't noticeable anymore unless you concentrate on it, but the thereness is immediate and obvious.

Sigma Delta dacs remove this by somehow equalizing the air pressure, you're in airlock, but slightly removed from reality.  But then at any given moment the doors of the airlock crack open and you've got your blast of reality in one big blast.

I generally feel that 1-bit sigma deltas such as in the Sony SCD-9000ES do this also, but the blasts of ultimate reality are finer, and therefore more musical.

Running through the higher input impedance of the Emotiva Preamp (50k) did not seem to help the Master 7 single ended measurement, in fact it went up from 0.135% to 0.183%...though that is very dependent on conditions such as the peak level, I had seen 0.18% during the earlier set of measurements but it mysteriously improved to 0.135%.*  The balanced distortion (which I was unable to measure before) reduced a huge amount (to 0.009%) because most of the single ended distortion is 2nd and other even harmonics.  I had predicted it would decrease a large amount, and I hoped for better than 0.01% which is I think a minimal requirement for digital converters, and I got the minimum technical performance I wanted even asis.  But I believe both should be much better, even considering the lack of feedback used in the circuitry, because of its sophisticated design, and of course the spec is a lot better.

(*Ahh, perhaps I didn't allow enough warmup to get to the better values the second time around, and perhaps even further warmup would that for a future set of measurements.)

One of the design features, which does make it somewhat more difficult to achieve low distortion, is zero feedback.  In principle, I like zero feedback, but for voltage ouput devices* I wouldn't trade distortion above, say, 0.0005% for it.  And in fact the Master 7 specification was exactly that.

(*The situation is very different for power amplifiers because they provide current to a reactive load.  Zero feedback has special advantages...but also so does super high feedback done correctly, it's only middling feedback that can cause great harm.  Anyway, there's a reason for having more stringent distortion requirements for the line devices than the power devices.  To be fair, only the total distortion ultimately matters, but earlier distortion compounds with and increases later distortion.  So you are much better off keeping distortion low early, and it's usually more economic and practical also, and there may be particular advantages for higher distortion power amplifier designs--though all other things being equal the lower distortion the better always.)

Zero feedback would be more essential for those hoping to get the full 'benefit' of NOS operation.  Only with zero feedback do you get the best time performance to limit of the highest frequency aliases, fwiw.

The highest slewing of transients, etc.

Now high distortion is not unheard of among high end dacs.  It's pretty typical of devices made by certain companies and designers...the same who favor high distortion SET power.  Audio Note (UK) for example.

Stereophile published a review of one of the latest Ear Dacs designed by the famous Tim de Pavarachini who also at least recently follows the "ignore distortion, instead do this..." guidelines, and Atkinson published the distortion spectrum, which featured an impressive sequence of even and odd harmonics diminishing slowly as is typical for non-feedback triode designs.  The 2nd harmonic was around -58dB and the 3rd not much lower, etc.  The second harmonic alone would correspond to a level of 0.15 or something, actually about what I'm seeing as the full THD of the Master 7 (in single ended mode and possibly broken).  So in other words, the harmonic distortion was quite a bit higher, and John Atkinson used the correct impedance and balanced outputs if available with very nice Audio Precision measuring equipment.

Well the subjective review was somewhat unfavorable also, with the reviewer complaining about grit, etc.

But Akinson didn't criticize the distortion levels much.  Instead he focussed on some very arcane aspects of the jitter rejection performance, claiming that was responsible for the negative comments.

Nobody can actually prove this (to critics) one way or another, or even if the Ear dac is audibly better or worse than any other without doing some very hard work, which we can be pretty sure nobody will ever do (dbx and all that).  Then for sure, no one will prove what is the cause of that difference, in the analog amplitude or digital timing aspects.

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