Audio hum and best solution

General Discussion
#1

I am using an RPi3b with an IQaudIO DAC+ on board with mpd and upmpdcli as player. Source from lan connection with audio sourced from my NAS served by minimserver. Control point is Android Samsung S6 with BubbleDS.

This has been my standard setup for a while and all is well but in one situation I have an audio hum. I have only noticed this is when I am playing through KEF MUO which is connected by audio plug, (bluetooth not being used,) and with the RPi power plug and the KEF MUO both powered from plug adaptors. The hum is not perceptible when playing but when playing is paused there is a faint hum which increases after playing has been paused for about 10 seconds.

I was about to try unplugging each of the power supplies in turn but this has not been a great success because when I picked up the MUO to hold closer to my ear the playing stopped abruptly. It seems the RPi or the IQaudIO DAC+ is no longer working. I had been thinking earth loops but none of the components are earthed and that includes me! Will do some checking and return but if anybody has similar noise hum issues please share experience of solutions.

Budge

#3

Generally speaking with audio it’s best to have one device earthed and make sure all the others are earthed through that and don’t have earths on their mains connections.

But with switched-mode power supplies, anything can happen, and usually does. A faint hum which increases after a bit sounds to me like a PSU adjusting to a lower load as power-saving kicks in on a device.

#4

Hi Graham,
Many thanks and yes I agree. My problem is that none of the devices have an earth connection. The MUO is built on an aluminium extrusion but that may or may not have an internal electrical connection. The power supplies are all two lead (phase and neutral) with no earth and the only suspect may be my lan. I shall check this out but earthing a lan is a nightmare as the managed switches are all over the place and some in different buildings so I shall have to put this on back burner.

As I mentioned in my post, the system stopped playing while I was starting the process of noise elimination. Fortunately all the components, once disconnected from each other, are working, so it appears nothing has been damaged. Will press on and your suggestion seems plausible. With two switching mode psu and at least one power regulator within the Pi/HAT combination there is plenty of scope for interaction and noise. Will jsy have to put up with noise for now. Perhaps with an oscilloscope, signal generator and some spare time this would be a good audio project to add to the many in my to do list.

Thanks again Graham,
Regards,
Budge.

#5

By “plug adaptors” do you mean USB power adapters? If so, have you tried to power them both from a powered USB Hub or other means such that they only have one common power source?

#6

Good suggestion. I shall have to find a suitable adaptor and power source which has sufficient output. Will report back but will be next week, when hopefully snow has cleared.
Many thanks,
Budge

#7

Are we certain this isn’t simply an interference problem, as opposed to a ground loop?

#8

Good question. Not sure but I have never noticed it with similar Pi+HAT combination in other arrangement. Many variables, so need to work on elimination by tests and this will take a time as I am only doing it between real work! Will post findings when I can.
Thanks for the thought and I think interference would be a more correct assumption rather than earth loop as I do not have any clear multiple earth paths.