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Deep-Digital Circuit Bypassing...

Jack,

So I've run the Juli@ digital card with the additional ceramic cap bypasses in for a couple of weeks... and while it's improved, I'm still not thrilled by it. There's a bit of 'confusion' to it and a very slight sense of 'harshness' in the highs on some recordings.

Very curious about this as these are all deep-digital processing chips... the controlling CPU, the PCI/Audio controller, and some helper chips. Looking at a lot of the commercial, even high-end commercial DACs & digital playback units (SACD/DVD/CD/SD players, etc.) out there, they generally had some number of ceramic caps as PS bypasses. And the Juli@ already has all of those ceramic 'BCxxx' caps.

Darned... it makes a LOT of sense to me to use very low-inductance caps mounted in a way to keep that inductance low around these deep-digital processing chips. And the ceramics seemed to fit the bill AND be the industry standard.

So my next experiment will be to do another Juli@ digital that's identical to this one except these bypasses will use some small Oscons... I have some 33uf/16v that will work well here and are small enough to fit without a lot of hassle.

Continuing my research, I went looking for situations where people used caps other than ceramics for digital chip bypasses... and found these:

http://www.diyaudio.com/forums/digital-source/186245-dac-2496-ak4393-dac-kit-cs8416-ak4393-5532-a-5.html#post2622112 ("PPS film (again if you can get them) bypassing on the digital electrolytic's and those massive power filter caps")

http://www.diyaudio.com/forums/digital-source/142562-microsd-memory-card-transport-project-post2594576.html?highlight=pps+capacitor#post2594576 (for putting non-SMD PPS caps on the regulator outputs on their SD Card Player)

http://www.diyaudio.com/forums/digital-line-level/79452-building-ultimate-nos-dac-using-tda1541a-81.html (he also has some interesting regulators that filter both the rail and the ground... very curious about these!)

And looking closer at the AckoDAC & the RAKKDAC, I now suspect they both also use some SMD poly caps around their digital circuits, at least on the digital side of the DAC.

Of course, a number of them also use some tantalum caps there too (as does the Jul@ around the AK4114 digital input-output transceiver and the ADC & DAC & output stages on the analog card). I'm not ready to go there yet, but I don't have a problem with PPS SMD caps.

Now I'll also take the one with the ceramics, pull them off, and put on 1uf PPS caps (which I just happen to have a small supply of here).

I also found some people who swear by using BG NX caps as local bypasses, at least around DACs. I agree that's a good bet, but most people won't be able to duplicate that and I'm trying to come up with mods that can be duplicated with easily-available components, so I'm not planning to try my limited supply of those no-unobtainable caps on my Juli@ digital section.

I still owe some replies on this thread to Uncle Leon on his latest capacitor work and to fork. I hope to get to them this week.

Later!

Greg in Mississippi

P.S. Another interesting poly cap trick is discussed in this thread: http://www.diyaudio.com/forums/digital-line-level/196474-ess9018-try-new-try-more-3.html where Bunpei replaces the exotic TP shunt regulators for the Buffalo-II's AVCC with simpler & more pedestrian LT1763s (Actually a Toshiba TAR5SB which is a near equivalent) & a 100uf/800 polyprop cap from CDE on the output & likes it a LOT in comparison!


Everything matters!


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  • Deep-Digital Circuit Bypassing... - GStew 21:39:00 11/24/11 (0)

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