Paging experts in hi-fi, electronics, computers and possible demonic possession

[CatmanV2]

OK I'm stumped at this point.
As some of you know I'm a bit of a music buff and love music around the house.
What I currently have is a couple of Raspberry Pis with HiFiberry DACs attached.
https://www.hifiberry.com/shop/boards/hifiberry-dac-pro/

They are running a custom OS called Volumio
https://volumio.org/

These are all connected to my wired Gb LAN in my server cupboard powered by a multiway USB charger like this:
https://www.amazon.co.uk/gp/product/B00PK1IIJY/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

The drive into a large Rotel power amp via good quality phono cables. The Rotel is basically a 6 channel job, 3 pairs of left and right, which can be bridged or linked but essentially three separate stereo power amps in one **** off box.

Speakers are hardwired. One pair into the bedroom, one pair into the garden.

So they both work wonderfully. Individually.
So either pi connected to either input with either pair of phono cables produces perfect sound in the corresponding location.

BUT if I plug them both in at the same time, there's a terrible hiss / crackle on both pairs of speakers. This is irrespective of if either Pi is playing anything, or even if they have fully booted. The moment you apply power to the second device, the noise starts.

Ah Ha I thought. Power issues. So if I connect one pi to an external PSU everything is fine. So the Anker is not up to the job?

Not so. I have a third pi that I can connect to the Anker and power it up just fine, and no noise on the speakers.

If I have both the pis connected to the Anker and disconnect the phono plugs from one (either one, makes no odds) the noise on both pairs of speakers vanishes.

So
Both Pis on the Anker and connected to the Amp = Crackle
One Pi on the Anker and one Pi on $other PSU. Both connected to the Amp = No crackle
Both Pis on the Anker and only one connected to the Amp = No crackle.

Cables have all been checked (USB and phono, and swapped out)

I'm waiting for another microSD card to arrive tomorrow so I can flash a third Volumio instance and connect that the the third of the stereo pairs of the Rotel to see if that also makes the crackle.

What the **** is going on?

C

 

[Markc]

To quote Not The Nine o’clock News “what’s the difference between that and a gramophone? About 30 years and a plastic cover!”

 

[CatmanV2]

To quote Not The Nine o’clock News “what’s the difference between that and a gramophone? About 30 years and a plastic cover!”

I suppose you'll be telling me you don't want slim line salad dressing!

C

 

[midlifecrisis]

Earthing, just re-read this.
Do the Pis have a final stage high output impedance balance or is it low? It sounds like the audio is being dragged down by the other one causing distortion

 

[Ebenezer]

Have you checked that there's nothing wrong with the amp itself? Have you tried plugging in a Pi and another type of source eg cd player? or two alternative source, neither of them the Pi's to se if it's actually the Pi's causing the problem?
Eb
PS so this is the way to go when my venerable Squeezebox Touch gives up the ghost?
Eb

 

[CatmanV2]

Earthing, just re-read this.
Do the Pis have a final stage high output impedance balance or is it low?

The consensus has become it's either earthing and insoluble as we've messed around with it all pm, or the 0V lines from the USB charger are not tied together and thus wandering as the charger tries to adjust charging voltage. I have a 10A 5v PSU arriving tomorrow and will butcher some micro-USB leads.

No idea if they are high or low impedance and no idea how to check.

Have you checked that there's nothing wrong with the amp itself? Have you tried plugging in a Pi and another type of source eg cd player? or two alternative source, neither of them the Pi's to se if it's actually the Pi's causing the problem?
Eb
PS so this is the way to go when my venerable Squeezebox Touch gives up the ghost?
Eb

Yes, but good shout.

And yes, I suspect something similar will see you well

https://www.picoreplayer.org/

I love Volumio as it's optimised for quality (and it's not half bad). The App is OK, website control is a doddle. Takes about 10 minutes to burn the SD, run through the config and be up and running.

C

 

[Markc]

I suppose you'll be telling me you don't want slim line salad dressing!

C

Do you want woofers and tweeters?
Do you want a bag on your head?

 

[CatmanV2]

Do you want woofers and tweeters?
Do you want a bag on your head?

No I don't want stupid things like woofers and tweeters!

C

 

[davy83]

I reckon earthing. Maybe when you connect both pi's to the amp you are joining up the earths on the output, and if they are on the same supply they have common ground on the power too. thats why when the two PI's are on separate supplies its ok.
why do you not want to have separate supplies for each PI? sounds like that works?
Although ground loops tend to hum and not crackle so it sounds odd.
If its really important what you could try isolating amplifier buffers on the two PI ouputs like

RDL EZ-HK3 HUM KILLER ISOLATION TRANSFORMER Audio and video, stereo, composite, RCA (phono) I/O

57-4942 | RDL AV INTERFACES - EZ Series | The EZ-Series offers a wide range of stand-alone designs including distribution amplifiers for line level audio, headphones, composite and XGA/XGA video, audio pre-power and mixer amplifiers, AV…

www.canford.co.uk

Or something similar.

 

[CatmanV2]

why do you not want to have separate supplies for each PI? sounds like that works?

Too many plugs is all!

Or something similar.

Hmm, interesting. I'm hoping that locking the the power supplies for the Pis will resolve it

C

 

[D Walker]

I read this earlier. But I think MLC has the crux of it, Earthing was. My shout. Getting into impedance calculus Is a mare!

 

[midlifecrisis]

W

I read this earlier. But I think MLC has the crux of it, Earthing was. My shout. Getting into impedance calculus Is a mare!

Simply put, the input impedance of the amp should be the same as the output impedance of the Pis. As each input stage for the amp is (or should be) isolated from each other, you could have a dodgy input circuit. Likewise an output stage of a Pi could be faulty. The problem with Impedance is that it's not resistance despite being measured in the same unit. Resistance is a DC measurement whereas impedance depends on the frequency of the measurement and so varies.

 

[CatmanV2]

I can't see why they would be anything other than standard line level inputs like any other bit of hifi

Power supply arrives today, we'll see what that does

C

 

[CatmanV2]

Well that's annoying. New PSU made no difference at all.

Only thing I can think of now is to link the screens of the phono cables....

C

 

[MarkMas]

No I don't want stupid things like woofers and tweeters!

God! Do I have to do EVERYTHING around here?

 

[MarkMas]

...and now I'm thinking of this:

 

[midlifecrisis]

Well that's annoying. New PSU made no difference at all.

Only thing I can think of now is to link the screens of the phono cables....

C

Earth loop current possibly.

 

[zagatoes30]

I got sidetracked as soon as you mentioned speakers in the garden, I so need to do this

 

[CatmanV2]

I got sidetracked as soon as you mentioned speakers in the garden, I so need to do this

It's highly pleasant.

C

 

[CatmanV2]

Earth loop current possibly.

Any ideas on a solution? Even the HiFi Berry team are getting it wrong so far (claiming you shouldn't run multiple Pis from the same supply) Slighly my fault as I didn't tell them I have a media server running from another Pi that is also connected to the PSU (but not the amp)

Even with two of the Pis switched off, there's noise.

C