It is not a hum. It is a hiss, white noise indeed, but quite faint. I just played with this again, now that this is a weekend. I must say that the noise is not a figment of my imagination. Will read more and experiment.
Do you have a USB powerbank. If you power the RPi3 with a powerbank (as a temporary test, not permanent solution) does the hiss disappear? You just need a USB type A to microUSB cable.
Just trying to workout if this is mains power (PSU) related.
The DAC – Is this the $80 Grace Design SDAC that Massdrop sells? At that price, I have to wonder how much input filtering they can afford to put in it.
[Later] The design of that DAC looks pretty solid, but it does draw power from the RPi over the USB input – mine doesn’t. So an isolator plus separate powered USB hub, might be a good idea, to avoid any electrical noise that might be coming over from the RPi.
Do you mean a battery? I might have one around. But don’t I need 5v to power the RPi?
Thank you very much. I’ll read the links. The Grace Design SDAC is quite something for the money. I have had it on my main system and it holds its own there.
I think you and Sean are on the money in thinking that the noise is electrical. I get the idea of powering the Pi with a power bank. I do have one, but need to see whether the output is adequate. But what is an isolator?
Yes USB powerbank. People use them to top up phones / tablets battery on the go etc.
Being USB powerbanks, they output 5Vdc which is the same as the RPi needs. As I mentioned above, you’d just need a USB Type A to microUSB cable to power the RPi. If you don’t have one don’t worry.
I just thought it would be a quick and easy test, to see if it’s a PSU related issue causing the hiss. If the hiss disappears when the RPi is running on 5Vdc battery, then it’s likely mains / PSU related. Don’t charge it at the wall at the same time otherwise you defeat entire the purpose of running on battery and being disconnected from mains power.
If you don’t have one like below, don’t worry about it too much. Just thought I’d ask as it can be a quick and easy check. As I initially mentioned, for a temporary test, not a permanent solution.
Take a look at those two links I sent. In one of them, the author uses a Corning fiber USB cable to electrically isolate the Raspberry Pi from the DAC, which may be what you need. Computers can be electrically noisy, but apparently the XMOS USB input chipset, which your DAC uses (along with lots and lots of other DACs, including mine) does a pretty fine job of filtering out any noise on the signal lines of the USB cable.
But your DAC also uses the power lines of the USB cable for power, which is coming from the Pi, and may have noise transients on it. I don’t know if those are also handled by the XMOS chipset. If not, and if they are present, they may be causing the hiss. By not using the power lines, with something like that Corning isolator, you’d eliminate that possibility.
Another thing that might be causing it is your transmission lines (the USB cable, and the cable between your DAC and your amp). If those are draped over, or are too close to, an electrically noisy piece of equipment, you may have to move them or shield them.
Really, the simplest thing to do is to buy something like the Elac Alchemy DDP-2, which integrates all these things together so you don’t have to. That way, if you hear a noise, you can just send it back. But if you’re going to go the component route, you’ve got to understand each piece individually, because you’re the systems integrator!
Many thanks for this idea. I’ll try this next week. I am also going to act on the other idea – of some form of isolation too.
Many thanks. I think these are very good ideas – but quite expensive given the context. What I am playing with now is but a secondary setup – in the bedroom. But based on your’s and Sean’s suggestions, I did some more digging around and found a thing made by Audioquest called the Jitterbug. Now, I do have their Dragonfly Red, which I think is quite amazing. So, I am hoping that it will address some of the issues with the noisy Pi. I’ll post the results when it arrives. Thanks again for your terrific ideas.
Sounds like a good thing to try. I’ll be interested to know if cures the problem.
The Jitterbug came today. Definitely a huge improvement. Clearly, you and Sean are on the right track — the USB out on the RPi is noisy. And there are fixes. Thanks for pointing me in the right direction.