The very best performing S/PDIF receiver we found (we thoroughly tested & tweaked all popular S/PDIF receivers over and over again) was the DIR9001 with a tweaked loop filter and in combination with the ElectroTos low jitter protocol. The DIR9001 supports 96 KHz maximum.
Higher sample rates require higher clock and data rates in source, interlink and DAC circuits. Circuits are never perfect, the major issues with digital audio are stray capacitances and stray inductances in source, interlink and DAC. The combination of high frequency clock and data rates with these parasitics will cause major issues with digital audio that are still not fixed. One of the results is source dependency (every bit-perfect source still sounds different). When clearly audible source dependency occurs, digital audio is still far from optimal.
Every imaginable interference that is added to the always present digital audio related signals will make matters worse, think of (RF) ground loop noise and power supply (mains) noise. All data and frequency signals will crosstalk and create one big unpredictable interference spectrum that can reach into the GHz range (harmonics). Even Bluetooth, GSM and WIFI add to this mix. This complex and constantly varying large bandwidth spectrum will be de-modulated by P-N junctions for example (think of radio receivers). Then demodulation products can be formed from two or more inaudible frequencies that will now fall within the audio spectrum (same happens with radio, television and wireless phones, but here this is desired to get the information across).
This interference cannot be blocked by 20KHz brickwall filters because these frequencies fall within the audio spectrum itself. The result is audible degrading / change of sound even when using world's best synchronous re-clockers in a DAC.
This is the major (not the only) reason that bit-perfect sources still sound different with DACs that should technically be close to perfection. Source dependency is noticed by many audiophiles and the most hilarious fixes are used like audiophile LAN switches and filtering equipment that is more expensive than the digital audio set.
But it gets even much worse. Analogue audio was never perfect either, every amplifier (driven by an analogue audio source) sounds different. No analogue amplifier sounds or will ever sound the same and or perfect. There are too many variables to keep under control and there are fundamental problems with analogue signal processing that are not even discovered. I think of analogue audio circuits as non-linear circuits.
Now mix imperfect digital audio and imperfect analogue audio and one created one huge problem that is never going to be fixed. Think of digital and analogue as water and fire, these will never mix well.
Chances of success are a bit higher when going all analogue (tape / vinyl + analogue (pre) amplifiers) or all digital (no active analogue circuits allowed, only digital on/off switches).
So I personally arrived at the conclusion that the conventional digital audio setup with a digital audio source, DAC and (pre) amplifier will always be doomed to fail. Recently I gave up on (pre) amps combined with digital audio equipment and developed an alternative that might trigger a revolution in top performance digital audio. More on this later.
We offer the UPL96ETL and U192ETL. After some time it became very clear that the UPL96ETL USB-memory-stick based player simply performs a lot better than the U192ETL, understandable, that's why we developed it in the first place, because of the never ending issues with USB audio. We still offer the U192ETL because most audiophiles prefer USB audio streaming and take related degrading for granted.
I was asked if I could give it another try and slightly improve the U192ETL so it comes closer to our UPL96ETL reference player. The U192ETL is an XMOS-based USB receiver with asynchronous feedback and master clocks that have better phase noise specs compared to the popular Crystek CCHD-759 clocks. We use a 8-core XMOS (XU208). Such configuration can be found in many audiophile DACs and this is perhaps the best performing solution for USB audio streaming, but it still fails to match UPL96ETL, SD-card players or even audiophile CD transports.
The irony is that both, UPL96ETL and U192ETL use exactly the same master clocks, same USB bus power supply, same USB interface and same synchronous re-clocker circuits. Find the 10 differences .......
Logically speaking it -should- be possible to get very similar performance from very similar circuits. So I was determined to pinpoint this decades old issue with USB audio and fix it once and for all.
After systematically testing -all- possible causes over and over again, one issue remained, this simply had to be it. I even mentioned this problem on this thread multiple times ... I should have known. The fix was a different matter, it basically boils down to blocking noise, but how?
I tried most popular digital isolators, opto-couplers constructed with Toslink transmitters and receivers. Nothing worked, only made matters worse. The theoretical galvanically insulated approach with master clocks on the other side of this barrier (feeding clocks back to the XMOS using a digital coupler) also failed miserably. For a while it seemed that this issue cannot be fixed and we simply have to live with the clearly degraded sound (compared to the reference) of USB audio.
After many sleepless nights I came up with one other option and this one finally worked, it fixed the USB issue. Not only that, when applied to the UPL96ETL reference, even that reference improved significantly. Now both, UPL96ETL and U192ETL perform on a virtually similar level. Sure there will still be small audible differences, nothing is perfect, but when these are very difficult to hear or cannot be heard at all by direct comparison with a reference, it is more than good enough.
Our listening test setups:
Macbook -> USB (UC250 interlink) -> Fractal PowerDAC -> 90dB Open baffle speakers (no analogue signal path, all digital).
Macbook -> USB (UC250 interlink) -> Modified DA96ETF (187.5 Ohm output impedance) -> SVC24 shunt volume control -> Beyerdynamic DT990-pro studio headphones (250 Ohm).
Macbook -> USB (UC250 interlink) -> DA96ETF -> SVC24 -> servo power amps -> 90dB Open baffle speakers.
We used unfiltered mains power supply, WIFI, Bluetooth all switched-off. No smartphones anywhere near the test setup.
We used Swinsian application
Swinsian — The Advanced Mac Music Player
We tested 44.1, 48, 88.2 and 96 KHz files.
We played all different kind of genres and different recording quality. Above systems make it very easy to hear big differences in recording quality and the highly destructive impact of DSP on already mastered audio files.
After getting annoyed with the flimsy USB interlinks with poor shielding and super thin wiring I decided to construct a better option. I use individually shielded data and power wiring pairs and thick wire with low DC resistance so I get a decent power supply at the USB receiver (reduced voltage drop / reduced load induced ripple voltage) and more reliable data transfer over larger distances. Note that with USB audio streaming, data transmission errors cannot be corrected. The individually shielded wire pairs minimise unwanted crosstalk between both, USB bus power wire pairs and USB data wire pairs.
I attached a picture of cable construction