I think my solution to reduce or remove hiss/noise was to use a decent 5v USB power supply, but can't exactly remember if it worked 100%. I also remember having ground loop issues, so it may have been a ground loop isolator that fixed my problems.

I wasn't using the type of high quality HATs that graphitone is using so it may all be related.

That ground loop isolator is now in use in my car because of a similar issue with a noisy 5v USB supply and a Bluetooth adapter.

The Pi, DAC and amp are all powered from the laptop type PSU I linked to. It plugs into the amp itself and powers all three through the onboard connections.

In my setup, you can power the screen from the Pi too with the ribbon cabled included with the official touchscreen, but I kept getting the not-enough-power symbol appearing, so powered the screen seperately.

Hmm. That really is interesting. I saw a version of the DAC pro pre-fitted with a socket for a power brick to connect to, but I forgot where it was and Google hasn't been my friend. I wonder why they don't make one when it's such a popular mod that HiFiBerry publish instructions online.

OK, an update in case anybody cares (or not, since I'm posting this anyway). Decided, if that's the word, to combine building a mini music-player computer with a bit of extra online storage. So I'm going to build a tiddly little PC with a couple of large capacity HDDs inside. Having looked at loads of ready-made ones and decided they don't quite do what they want for one reason or another have ordered one of these from ebay.

Interesting though to consider a project where there's a range of enthusiasts doing different things with tiny PCs but the various manufacturers don't have much interest in it. It was amazingly hard to source a mini-stx mobo*, and they cost stupid money new. And they often come without power bricks which is fine except that it's not always clear where to get a suitable one and guess what - they cost stupid money too. Those do seem to be the rules-of-thumb: lack of info, scarcity of supply and stupid money. 

*and I wanted one that had support for SATA HDD/SSD as well as as M.2. When I say stupid money, that includes postage and tax since they're almost unheard of in the UK. 

What OS and media player are you going for?  And what format are you storing the music in?

I haven't yet decided on a media player. In some ways that's not so important because (as per my original post) it's quite important that Mrs WilliamA can get her Apple library and that we can both get at Spotify where we have playlists. As for other stored music, I'll probably settle on flac as I want to dump some of our hundreds of CDs, so several media players will do. OS will probably be Windows 10 as Mrs W knows her way around that.

On the storage side, we're mainly looking at music although some of my thinking is that extra disk space could help with the shortage on my main backup PC. 
 

Yeah, smaller = order-of-magnitude more expensive. Must be a novelty/enthusiast/lack of demand thing.

That's a thing I've found with using a full pc OS for a dedicated function: a slow mission-creep, because it can.

I can understand it, because the main market as far as the manufacturers are concerned is industrial and the whole thing is a bit of a moving target. Do the buyers want a traditional mobo in miniature or something far more granular: some kind of SoC arrangement without all the extras? The STX format is already a bit niche which is why the CPU and RAM support is out of date or patchy. The board I'm getting is socket 1151 but only CPUs up to 7th gen, and DDR4 although others boards are mainly DDR3. When I started looking I had no idea how many different attempts had been made to create a standard for embedded applications such as the Intel NUC things. I suppose the most popular very small form with home builders is mini-ITX and even that is on the wane. 

I was after low(ish) processing power somewhere in the range of Celeron up to i3, but not as feeble as the Atom range which starts to struggle if anything serious is waved at it. I also wanted support for standard SATA drives. I thought it would be nice to use up some DDR4 SO-DIMM memory I have. It had to be small and preferably not use a big standard PSU and quiet so it can sit next to us without being annoying. Not much to ask.

Anybody got a spare i3? Pentium, Celeron??

I use an old HP SFF office pc with athlon something, Ubuntu 18.04 for a home music & file server (MrsD.s old pc, she ran xp on it)... probably massively bigger than what you're going for, but free. It sits under my scanner so doesn't really eat up any extra space.

 :-D Yes, I'm aiming at about 6x7x4.5 inches. So a bit smaller. That said, my back-up PC is about the size of an enormous old desktop and has noisy Corsair water cooling, but it sits in the spare room so that's OK

There's a punk to New Romantic progression. A visionary manufacturer strips out all the fat and offers a refreshing barebones machine that challenges users. Then to build business they start to offer add-ons and new iterations with greater capabilities. Before long, it's just another machine with all sorts of gingerbread and bloat lashed on.  Then somebody has the bright idea of stripping out all the fat.

I prefer Bauhaus to Roccoco progression (I actually like both styles but, as I said, I've a low threshold of boredom).

It's the circle of life.

You say dumb terminal, I say thin client...

Just tuning in to say "Damn and Blast!"

I knew I shouldn't have talked openly about my backup PC. Went to look at the Plex server this morning and it wasn't there. None of the shares were available either, so with that sickening feeling so familiar to all home PC builders I went and looked at the box. Dead as a dead thing. Optimistically tried a new kettle cable but nada. Let it rest unplugged for a minute then tapped the on/off switch which produced a half millimetre twitch from the PSU fan. Unbelievably unable to lay my hands on a sodding paperclip right now, but it does look as though 3 years of running without a reboot has killed the PSU. As soon as I can confirm that, it's time to pray that's all that failed. May be time to invest in a new case as the present arrangement has got to be the shittiest ever with a PSU replacement involving a complete dismantle of the whole thing - it's one of those boxes where the PSU is inaccessible with a pass-through power cable to the socket at the back of the case.

Bah! Even more buggering bollocks. With a paperclip the PSU fan spins like a spinny thing. Now I've got to try and remember where I put my multimeter. Oh for the days when I had a couple of spare PSUs lying around!
 

MB fuse? Blown caps?

Possibly, and I am beginning to look at the cheapo no-name motherboard now.

So I hauled the backup PC over to my main desktop and hijacked the known working power supply from that. Yes, well. OK, the PSU started up as expected, but then there was a spark somewhere in the row of caps near the CPU power connector, so I turned things off.

Reassembled the backup PC with it's own PSU and wacked the power switch just out of curiosity. Damned thing started up, went through the bios screens and into Windows, but just before the desktop it shut down dead again. Beginning to think that all the twisting and turning closed a broken circuit (or something), or some worn-out caps gave it one last go,  but it didn't last.

So, I'll go over the PSU with a multi-meter and if the pins are providing the right voltages I suppose I'm looking at getting a new P55 motherboard.

 

Not the power-supply. All outputs within tolerance.

Now to see if I can find a socket 1156 motherboard with plenty of SATA ports.