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Old 12th July 2018, 02:33 AM   #1871
Markw4 is offline Markw4  United States
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Quote:
Originally Posted by abraxalito View Post
Mark have you looked at the internal circuits of your DAC3 to notice the large NP0 cap right next to the DAC chip?
I have posted hi-res pictures of the inside of it in this very thread. And, no, I didn't notice caps like that. If the outputs are both held at virtual ground or thereabouts by the opamps, not much voltage there to push much current through caps. But obviously, there has to be at least some tiny voltage there since the opamps have finite gain, especially at higher frequencies. However, the C0G caps in the I/V stages and the filtering in the differential summing circuit are for attenuating that noise as much as possible. The caps in the I/V stages at least reduce the I/V output swing as much as possible at HF, but basically it looks like the opamps have be able tolerate a bit of it, and that could well be part of the reason ESS only recommends certain opamps for that use. However much it is, it appears to be smaller than I can see on my scope.

I should also mention that I see signs of heating in the opamps that I presume are due to the presence of HF DAC output noise. But, measurements and looking at FFTs over at Richard Marsh's made it look like it was not really a problem. There were some funny spurs at -130dB that largely went away when at Richard's suggestion I held my finger on the DAC chip. It could be that caps on the outputs might have a similar effect, I guess. Of course, we might not have seen the spurs if I had implemented the filtering ESS recommended.

Where I know there are still real problems have to do with clipping, jitter, and interpolation/reconstruction filtering. We can do some things to improve some of those issues as is, but the last one is the one I have done the least work on. What I would like to do is get the best handle possible on the problems we do know about, and then see how we are doing after that.

If there were still some puzzle at that point I might go poking about in the DAC-3 but I don't see much reason to suspect a problem at this point.

Last edited by Markw4; 12th July 2018 at 02:43 AM.
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Old 12th July 2018, 02:46 AM   #1872
abraxalito is offline abraxalito  United Kingdom
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Quote:
Originally Posted by Markw4 View Post
I have posted hi-res pictures of the inside of it in this very thread. And, no, I didn't notice caps like that.
Scott Wurcer's applications info for the AD797 suggests a 2nF cap prior to the opamp I/V. I'd not say that 2nF would count as 'rather large' though as C0G goes up to 100nF comfortably.
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Old 12th July 2018, 03:35 AM   #1873
Markw4 is offline Markw4  United States
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I guess someone should point out to Ian that ES9038Q2M used on these DACs does not have the same output current as ES9038PRO. Not even close.
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Old 12th July 2018, 04:09 AM   #1874
Markw4 is offline Markw4  United States
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Originally Posted by abraxalito View Post
Scott Wurcer's applications info for the AD797 suggests a 2nF cap prior to the opamp I/V. I'd not say that 2nF would count as 'rather large' though as C0G goes up to 100nF comfortably.
Given that Scott Wurcer and the guys at ESS are old friends, and that Scott maybe not too long ago did some tests on an ESS DAC for them and found some problems for them to work on, don't know if that question ever came up.

It could be that putting a modest C0G cap there would be a good idea.
It would be great if someone would like to try it and post the findings here. I am finding myself tiring of this project though, other than what I am still curious about that is. No reason I should do all the work anyway if other people are interested too.

However, part of the reason I am tiring may be that it seems like recently not much reporting from people about making progress and being happy about improving SQ. So, maybe interest is dwindling out there. Actually, it seems like people have been curious to see what turns up here as helping with SQ, but most people seem to be looking for a more cheap and easy solution to come along. Making this board sound good while possibly interesting to read about is a lot of work, likely too much work for most readers. And I didn't get involved mainly to become a technical writer, my aim has been to help people who want a great DAC on a minimal budget. That effort has come a long way, and a fully modded board sounds quite good as things stand now.

Tell you what though, maybe I will have some more of interest to report in a few days. Have to wait and see.
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Old 12th July 2018, 04:49 AM   #1875
abraxalito is offline abraxalito  United Kingdom
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I do have a cheaper solution - dunno whether its easier - so perhaps its time to open my own thread on my DAC design.... Certainly it won't suit all comers as its only 16bits.

'Work' is only tiring when it feels like work, there are no 'shoulds' here. Do it if it feels like fun, stop if not.
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Old 12th July 2018, 10:11 AM   #1876
freezebox is offline freezebox  Germany
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Hi there. I am still interested in further improvements on this 9038q2m board. Still waiting for my schmitt triggers to reduce raise time of my new NDK clock..
The 2nF C0G cap should be tied to offset voltage (AVCC/2) though and can be soldered to the 2 input pins of opamp directly, right? Would be interesting to see if opamp temp falls with this measure. Could this also help on ESS recommended AVCC reg in parralel to the 10 uF input filter cap?
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Old 12th July 2018, 10:24 AM   #1877
cdsgames is offline cdsgames  Canada
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Do you know how much phase noise those schmitt triggers will introduce ?
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Old 12th July 2018, 10:53 AM   #1878
freezebox is offline freezebox  Germany
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Hello cdsgames, I have ordered this ones: SN74LVC1G14DBVR. In SPEC I cannot find anything about Phase noise..
http://www.ti.com/lit/ds/symlink/sn74lvc1g14.pdf

Based on your recommendation to put a buffer between clock and DAC and based on Markw4 proposal to do this with such schmitt Trigger I ordered this part. Raising time seems to be lower than that of NDK clock, so I thought I give it a try... If you have a better proposal (I am sure you have..;-)), pls. let me know.
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Old 12th July 2018, 11:42 AM   #1879
Markw4 is offline Markw4  United States
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Originally Posted by freezebox View Post
The 2nF C0G cap ... can be soldered to the 2 input pins of opamp directly, right?
The cap would go across the input pins of each I/V opamp, as you say. Do you by chance have any way to measure the results? A sound card with a 24-bit A/D could probably work.

Also, I would not add a C0G cap from AVCC to ground. It could cause the AVCC voltage regulator opamp to oscillate. At the very least, one would probably want to watch what happens with the AVCC opamp with a good scope if trying something like that. Even if no oscillation one would still want to verify it did more good than harm downstream. Another reason to have something on hand to measure the audio output with.

Last edited by Markw4; 12th July 2018 at 11:51 AM.
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Old 12th July 2018, 12:06 PM   #1880
freezebox is offline freezebox  Germany
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No time to try it within next 10 days.. I have measured my DAC with my M-Audio Audiophile USB SC already. I think the THD of the DAC is below that of the SC..
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