I use my D50 as a DAC+preamp for the NuForce Optima STA-200, and Audience The One speakers in a really small bedroom. Music from PC, occasional gaming and movies on PS4. It's a fantastic DAC.
Been using the D50 for the past few months and experimented with a few things. Here's what I found:
• USB filter makes a difference. Quite significant improvement with Uptone ISO Regen (powered by iFi iPower 9V). Instruments & vocal plasement became more pin point, depth of stage is easier to discern, sounded less "FM-radio". But it losses a bit of "warmth" that was caused by distortion/veil which a friend preferred despite losing some resolution and clarity. To me, that warmth sounded like cheap radio (low res, high noise, high distortion, veiled).
BUT there's an issue: with the ISO Regen, D50 will sometimes stop outputting any signal suddenly. It just goes quiet. Sometimes, it can play for hours without any issue, and other times it can happens once every few minutes. Turning the DAC off & on again fixes it, but it's annoying. Not sure if it's Foobar, PC, ISO Regen, or D50 is the problem.
• Since all DACs & digital equipments (the chips and clocks) are sensitive to power supply, so I never bothered with common USB chargers. But then I found that STA-200 has a 5V output. I got curious and compared this output with iFi iPower 5V (yes, I have 2x iPowers: 9V & 5V). Opening up the STA-200 shows that its 5V output is a simple linear power supply, and likely of moderate quality.
At first, I wasn't sure I heard a difference. After many rounds of back and forth, iFi iPower is just a wee bit cleaner compared to STA200’s 5V output. This is likely to be much much better than common 5V USB chargers.
*Side track1 : of the many USB chargers, Apple's iPhone chargers do not leak noise into the main AC (tested with AudioPrism Noise Sniffer by a friend), use this if you don’t want to buy iPower or other power filters. iPower likely generates noise back into the AC mains, which may explain why the AC filter works (below).
*Side track 2: STA-200 shares the same circuitry as Job 225 and Goldmund Metis, except it has lesser watts, I opened it up and saw the PCB printed with the words “JOB”. A Taiwanese magazine interview with the owner of Optoma mentioned that he is friends with Goldmund founder, so Goldmund gladly provided this circuit to Optoma when Optoma acquired NuForce and wanted to launch a new amp quickly. It’s a stupendous amp, and superb matching with D50 silver. STA-200 is $500 from Audio Advisor and Amazon—60% off from $1299. Bought it because reviews on this amp and Job 225 mentioned that it has good soundstage depth, which to me means that it has really good resolution & capability in order to pull off the “depth” trick. And I needed such amp to test my prototype cables.
• Recently, I bought a used power filter—DA&T AC Filter. DA&T is a mid/high end audio company from Taiwan, but their AC Filters are really affordable.
The STA-200 hated the filter as it caused an audible hum/buzz at the speakers, even though it sounded better with the filter. So I plugged the 2× iPowers (for ISO Regen & Topping D50) into the AC Filter isntead.
Jaw dropped… The improvement was very significant.
As much improvement as adding an ISO Regen, possibly more. The clarity in vocals is amazing, musical instruments have even better pin point location, less veil, better clarity, better depth location, better instrument separation. When I removed the AC filter, it now sounded like radio or elevator music.
The above tweaks (or whatever you may call it) cost about 2× of D50, but it took it to another level. You could say I could’ve bought another DAC instead. But the power filter and USB filter will stay as the D50 is being upgraded in the future.
And the D50 is so amazing now, it sounds like a higher end DAC.
Cables used are almost all Audience cables: OHNO RCA & my DIY carbon-fibre prototype, OHNO speaker cables, Forte f3 powerChord. The system might seem expensive, but almost all gears were bought as 2nd-hand, or over many months from various deals/discounts.