Hi Torben,
No provocation perceived, but very valid questions.
Apologies in advance, this will be a little “wordy”
A quick explanation of the test method might help you to fully understand. In assessing the “noise” coming via the streamer/Ethernet connection, simply playing nothing to the DAC can make some DACs mute their outputs.
In order to prevent this, you need to keep the DAC “awake” by sending it a signal. We don’t want to hear the signal, so we make it “silent”. dBFS is a way of quantifying a signal’s amplitude relative to full scale output. So the maximum signal level that can come from your DAC is 0dBFS, for a balanced connection, this is typically 4 Vrms. It is generally agreed that -120dB is below the threshold of human hearing (-120dB is 1 millionth as loud as 0dB). In fact, in reality it’s way below. It would be like listening out for the sound of a pin dropping to the floor whilst standing trackside at a Formula 1 race!
-120dBFS relative to 4 Vrms is only 4 microvolts, which is a very low signal level indeed. In fact it’s about as low as the inherent noise level from even the best performing hi-fi components.
Benchmark Media’s kit is a little out of the ordinary. It’s origins are in Pro Audio, where much higher signal levels are common. The DAC3 HGC has a calibrated 0dBFS gain (it’s how mine is set to feed the HPA4) of +24dBu which is 12.3 Vrms output, over 3 times a “normal” DAC.
The HPA4 maximum output level is +28dBu which is 20 Vrms. This means that at maximum volume, it can amplify a -120dBFS track to 20 microvolts signal level. An extra ~14dB of gain. But it also does so at vanishingly low noise levels.
When I looked at the specs again, it’s pretty obvious why I could (potentially) hear the 1 kHz tone. Between the DAC3 HGC and the HPA4, I have 135dB of gain at my disposal.
However, if the claims around ethernet noise were to be true, I wouldn’t be able to hear the test tone because it would be lost in the “noise” from my Ethernet connection and the “noise” from my RPi’s PoE SMPS.
The reality is that I could hear the 1 kHz tone, which means that claims of noise from ethernet polluting the audio are unfounded. It also means that the RPi’s SMPS has no influence on audible noise either.
I hope you’ve been able to follow so far.
As to the PSAudio link,
Really? I’ve just proved that it can’t (provided it’s properly implemented, of course)
Which brings me on to the next point. “Audiophile” ethernet cables are almost invariably shielded. They claim Cat7 and Cat8. Few would test out to the rigorous specification targets needed to achieve 10GbE (Cat7) or 25GbE/40GbE (Cat8) transmission rates. Cat7 was never a proper EIA/TIA/IEEE standard and has been superceded by Cat8. Cat8 has no place other than in data centres (this is one of @ElTel 's ares of expertise)
Shielding provides a ground link between components which if you used Cat6 (as you should), wouldn’t exist.
So your fancy snake-oil “Cat7” or “Cat8” has introduced a ground path which shouldn’t be there. One way to break that ground path is via optical isolation “optical fibre”.
Your fancy ethernet cable has introduced a problem you now need to solve… An in the true audiophile fashion, you have to spend even more money.
Optical fibre solutions are solutions to problems which shouldn’t exist and that prey on FUD.
One of the best ways to make money in audio is to create a problem, then sell a solution. It doesn’t even have to be a real problem. Just a suggestion of an issue affecting sound quality that can be “solved” by buying your product.
The reason high-end equipment manufacturers are now including optical capability is, I believe, purely a marketing strategy. It gives them something to set them apart from the competition.
The Sonore optical module is nothing more than a fibre media converter. However it’s an “audiophile” FMC, so they can charge you more for it…