Earth hum from headphone amp.

[vext01]

Hi audio scientists,

I've been trying to eliminate a ground hum from my desktop system, audible only when I use sensitive iems. I bet someone here knows exactly what to do.

I've systematically removed as many components as possible and narrowed it to the headphone amp -- a Sabaj A10h.

With nothing connected to the line input, but the unit plugged in, and IEMs connected and in my ears, if I move about in my office chair or stand up I can often hear an earth hum. I can eliminate it by touching the 6.35 -> 3.5 jack converter connected to the audio output of the amp. This still happens even when the amp is off, but the moment I unplug the DC jack I can no longer hear it.

What I find really odd is that the hum isn't always present. I have to move around to "find it".

I suppose there is no ground loop here, since there's only one device in question. There is a computer, monitor and lamp connected to the same outlet.

What's going on here? Any ideas? Could it be EMF induction from something else, or maybe static electricity from the hydraulics in the chair? A crappy unfiltered transformer? What can I try?

Thanks

 

[vext01]

Well, well, this amp is reviewed here and talks about ground loops:

I have seen member Wolf measuring this amplifier and scoffing at lower SINAD he got as a result of mains noise and harmonics. When I first measured the A10h, I saw the same thing but as is always my procedure, I play with various grounding modes to see if it is specific to my setup or nature of the device itself. The unbalanced Audio Precision outputs and inputs are "floating" meaning they are not ground referenced. This is normally good in that it reduces the chances of ground loops but sometimes, it works the other way around, causing such interference. Such was the case here. As soon as I connected the AP ground to the RCA shield, it substantially reduces mains interference resulting in the above measurement.

Does anyone know what the "AP ground" is in this context? Perhaps the "Audio Precision outputs"? Sounds like there's a fix!

EDIT: tried connecting the outputs to the inputs with a short RCA cable. This should connect the RCA shields together. Obviously if it worked, not a long term solution, but alas, it does not eliminate the hum.

EDIT EDIT: I think Audio Precision is the analyzer he's using. Perhaps I just need to connect the RCA shields to an earth. Will try this tomorrow.

 

[vext01]

Neither grounding the 3.5mm shield nor the RCA shields eliminates the hum. I used a single core wire to the nearby radiator. No luck.

 

[MaxwellsEq]

With nothing connected to the line input, but the unit plugged in, and IEMs connected and in my ears, if I move about in my office chair or stand up I can often hear an earth hum

You aren't hearing an "earth hum". With only the IEMS connected, there can be no earth or ground loop. If what you hearing sounds like a very low organ note which never changes frequency, then you are probably hearing mains.

maybe static electricity from the hydraulics in the chair?

Static electricity sound like a crack or splat. It doesn't sound like a continuous hum.

What I find really odd is that the hum isn't always present. I have to move around to "find it".

If it's not constant, it's probably not a poor PSU generating noise into the amplifier.

It sounds like you and the IEM are acting like an antenna, picking up mains induced energy. You may find that it goes away if you earth yourself.

It's also possible that noise is being picked up by the inputs (unplugging everything is not a guarantee that noise isn't entering via those routes.

It sounds a bit like your amplifier needs a better earth. That's what the quote from the review explains.

 

[vext01]

The antenna theory seems plausible then.

Is there an easy/cheap fix? I might just make do until I can upgrade.

 

[staticV3]

I suppose there is no ground loop here, since there's only one device in question. There is a computer, monitor and lamp connected to the same outlet.

What's going on here? Any ideas?

Sounds a lot like leakage current. Your body itself completes the ground loop: https://www.audiosciencereview.com/...these-stupid-things-behind.59279/post-2165947

I'd try grounding the A10h chassis to a mains outlet.

 

[vext01]

thanks for your responses.

I just tried attaching a spade to one of the case screws and grounding it to the radiator (in the uk all rads are ground).

Unfortunately this didn't help, but what I found really did help was attaching me (via antistatic bracelet) to the shielding of either the RCA ports or the 6.35mm output. It's not really a solution, as I don't want to have to use a bracelet to listen with iems.

I suppose this supports the theory that I'm acting as an antenna.

I'm hesitant to go playing inside electrical sockets, so I've not tried that.

Unless you folks have other ideas, looks like the end of the line.

 

[vext01]

I solved it by accident!

I was adding some cheap PC speakers to my setup (Creative Pebble V3) and connecting them to the pass-through output of the headphone amp eliminates the hum!

The speakers are powered by USB from my PC, so I guess the computer is adequate grounding.

Not bad for a £30 fix.