Bad hum

[Jurek]

No, not a smell, but a noise which emanates from my speakers when the charger for work laptop is plugged in.
If I touch any part of the laptop metal or otherwise the hum increases.
If I put my palm flat on the screen it sounds like a hive full of angry bees.
I don't actually need to touch the laptop to make the noise - I just need to bring my hand near to it.
It was never like this with the old work laptop, which was retired a couple of months ago.
If I unplug the charger lead from the laptop the noise ceases instantly.
When I plug the charger lead back in, nothing much happens.
But within 10 seconds or so, the hum is back.

I know that the words 'magnet' and 'field' will be somewhere in the answer.
I'm more interested in knowing how I can stop it (without unplugging the charging lead).
The one thing that has changed between the old lappy and the new one is that the old one was connected with an ethernet cable, the new one is on wifi.
Disconnecting the wifi connection makes no difference.

Any ideas?

[PaulF]

Have you got a different charger you could use? Even as a temporary measure.

Does it still happen if you move the charger further away from the laptop?

[Kim]

Some sort of ground-loop-style fun, likely because the laptop isn't actually connected to mains earth, but the power supply has filtering capacitors to live and neutral, causing it to float at a fraction of mains voltage.  (The capacitance is small, so you won't get enough current to cause more than a barely perceptible tingle.)

Try an audio isolation transformer between the laptop and the speakers.

[Jaded]

You have created a Theremin!

[Jurek]

Have you got a different charger you could use? Even as a temporary measure.

Unfortunately not.

Does it still happen if you move the charger further away from the laptop?

Sods law being what it is, this noise now appears to be an intermittent fault - I've just turned the lappy back on and all is silent - However, that's a good call, as the old charger was on the floor, whereas the new one is taped to the underside of the desk. I'll check it out when conditions dictate that I can.

Try an audio isolation transformer between the laptop and the speakers.

There is no physical connection between the laptop and the speakers. The speakers deliver the sound from the iMac on my desk via an audio amp. The only commonality between the laptop and the rest of the IT junk that resides here is that everything runs off the same mains double wall socket, via half a dozen daisy chained 4-way ext. leads.

You have created a Theremin!

I so wish I had 

[PaulF]

There is no physical connection between the laptop and the speakers. The speakers deliver the sound from the iMac on my desk via an audio amp. The only commonality between the laptop and the rest of the IT junk that resides here is that everything runs off the same mains double wall socket, via half a dozen daisy chained 4-way ext. leads.

On the assumption that the speakers are Bluetooth do you still get a buzz if you connect the speakers to another sound source?

Can you test with everything turned off except the laptop and speakers and those 2 connected directly to the mains?

It sounds like what guitarists call a “ground loop”

[Jurek]

There is no physical connection between the laptop and the speakers. The speakers deliver the sound from the iMac on my desk via an audio amp. The only commonality between the laptop and the rest of the IT junk that resides here is that everything runs off the same mains double wall socket, via half a dozen daisy chained 4-way ext. leads.

On the assumption that the speakers are Bluetooth do you still get a buzz if you connect the speakers to another sound source?

Can you test with everything turned off except the laptop and speakers and those 2 connected directly to the mains?

It sounds like what guitarists call a “ground loop”

Incorrect assumption.
Fat. Copper. Cable.
Proper old skool here.

[PaulF]

Sorry misread

[Wobbly John]

My diagnosis matches Kim’s, which is encouraging.

Is there somewhere before the amplifier for the speakers, that you can plug a ‘ground loop isolator’ in?

I have one if you want to try (left over from when teachers’ laptops did not play nicely with amplified speakers and projectors), but you may need adapters for the connectors. I think it is on phono plugs.

[Kim]

Yeahbut how is it a ground loop if the laptop+PSU which is clearly the source of the noise isn't in the audio path?

[Feanor]

I'm not clear exactly what hardware we are dealing with and how it is interconnected.

What speakers?
Internal ones in the laptop?
Passive ones plugged into the laptop via 3.5mm jack?
Active ones plugged into the laptop via 3.5mm jack?
Speakers fed from an external amp, connected to the laptop how?

Without knowing this, it's all guesswork.
But yes, a ground loop of some kind is likely.

[Jurek]

External speakers connected only to an iMac via an audio amp in the proximity of the work laptop.
No physical connection between the work laptop and the iMac/audio amp/speakers.
Two separate systems which happen to be located on the same desk.

[Feanor]

Wait, what?
The Laptop is not even physically connected to the system we are discussing?

You have an iMac - Amp - Speakers, with no physical connection to the laptop, just proximity?

[Jurek]

Wait, what?
The Laptop is not even physically connected to the system we are discussing?

You have an iMac - Amp - Speakers, with no physical connection to the laptop, just proximity?

Correct.

[Feanor]

OK, that changes everything.

I get that the amp is connected to the speakers with chonky copper cables.
But how is the iMac connected to the amp?

And what is the nature of the 'hum'? is is a constant-pitch 50 Hz tone of varying volume, or is it changing pitch?
Your description of angry bees makes me wonder.

[Jurek]

OK, that changes everything.

I get that the amp is connected to the speakers with chonky copper cables.
But how is the iMac connected to the amp?

And what is the nature of the 'hum'? is is a constant-pitch 50 Hz tone of varying volume, or is it changing pitch?
Your description of angry bees makes me wonder.

Thanks.
The iMac connects to the amp via its headphone output to a phono input on the amp.
The pitch of the hum doesn't change, just increases in volume depending on how much contact I have with the surface/proximity of the laptop.
ETA - 50Hz at varying volume but constant pitch would probably describe it accurately.

[aidan.f]

My money is on near field radiation from the laptop PSU and associated cables being coupled into the low level loudspeaker amplifier input.

I guess, 'cos work supplied it that it is a genuine dell/HP/..  lappy psu not a knockoff?

[Feanor]

OK, then it looks like it's not a hum loop, as there's no connection between the systems.
Your description of the effect varying as you touch things suggests that it's an electromagnetic pickup thing, and you are acting as an antenna.
There's one thing in your post that was a red flag, and I'll come to that.

So yes, the laptop PSU seems to be a bit noisy and is radiating 50Hz.
And you are it's antenna.
You could try another PSU, it could be faulty. Is it genuine?

On the other hand, it could be the sound system is being over-sensitive to 50Hz in the air.
The thing that raised a red flag was you saying you were connecting a headphone output to a phono input.

That word 'phono' can be used to mean two different things.
Some people use it to describe the type of connector: the RCA/Phono jack.
But it can also mean the name of the input on the amp.

On the back of the amp, there will be a bunch of different inputs: Phono, CD, Tuner, Tape, Aux etc etc.
They will all have the same RCA/Phono jacks as input.
But an input labelled 'Phono' is different from the others: it's super-sensitive and is designed for a vinyl pick-up cartridge. It's expecting a few millivolts.
All the others are 'line level', and are expecting a signal of up to a volt.

You should not be feeding a headphone-level output into a input labelled 'phono'.
That's going to hideously over load it, and the input may also be sensitive enough to pick up 50Hz noise because it's not properly terminated into a low-impedance cartridge coil.
Connect a headphone output to any of the other amp 'line level' inputs.

[Kim]

When I was a PSO, I molished a little adaptor board to convert line level audio to something suitable for a turntable input, because reasons.  As well as the difference in level and impedance, there's the de-emphasis to deal with.  I don't recall it working particularly well.

[PaulF]

Yeahbut how is it a ground loop if the laptop+PSU which is clearly the source of the noise isn't in the audio path?

At that point I  was assuming the speakers were connected to the laptop but as they’re connected to a different computer it’s not a ground loop.

[Jurek]

[snip]
You should not be feeding a headphone-level output into an input labelled 'phono'.
[snip/]

My mistake. It is actually connected to an AUX input.

[Feanor]

OK, so there's pickup happening somewhere.

Try unplugging the cable at the iMac end, leaving it connected at the amp end.
Does it still pick up hum?

Then try unplugging the cable at the amp end, leaving nothing plugged into the amp AUX input.
Does it still pick up hum?

Try plugging the PC and PSU into various different sockets increasingly far away.
Any change?

<ETA>Look at the mains plugs on both the iMac and the amp: do they both have metal earth pins and 3-core cables, or is either of them plastic or flat 2-core cable?
I'm considering the possibility that there *is* an earth loop, but not one involving the PC, just one between the iMac and amp.
This earth loop may remain quiet in the absence of being tickled by an external 50Hz field, but may be getting tickled by the radiated 50Hz from the laptop.
In which case, try passing the connection between the iMac and the amp through a Ground Loop Isolator (Google will throw up lots of hits using that phrase ).

But basically, beyond that, I Don't Know.

[Kim]

One thing I've discovered is that my PC's audio output seems to float after a few minutes of not playing any sounds^[1].  Presumably some power saving thing.

ETA: And apparently jibbleable by changing the value of /sys/module/snd_hda_intel/parameters/power_save, at least on Linux.

Obviously this is of limited use on a Mac, but it may be doing something similar.


_{[1] The pertinent question being whether you get the hum while the iMac is playing audio?}

[mrcharly-YHT]

When the hum is running, try touching the usb sockets or 3.5 audio jack. If you get a small shock, it might be that your mains is insufficiently earthed.

House mains should be earthed at multiple points, otherwise there can be an induced voltage on the earth; particularly if you have multiple power supply systems like those on computers.

Ensuring the socket earth wire is 'earthed' will fix this.

Sadly, a socket body in plasterboard and screwed to wood is effectively insulated from true earth.