Lots of people have been complaining about ground loops and noise from PSU’s on their computer getting into their audio equipment. Also, if you listen on a radio scanner or a HF (shortwave) receiver, this interference can also cause a nuisance.

I recently gutted a old PC PSU for components, and also extracted the mains input socket and its filtering components to illustrate why it happens.

First of all here is a very simplified explanation of how a PC PSU (and many other PSU’s for all sorts of equipment) works.

The 230V (or 100-120V if you are in another country) AC mains electricity at 50 or 60 Hz is converted to DC with a rectifier, and smoothed with a big capacitor.

Then another circuit converts it back into AC but at a much higher frequency (way above 20 000 Hz so humans cannot hear the noise) and it is sent through a transfomer, and converted back into the DC output that the gadget such as a computer or printer requires (there is also some other complicated circuits to make sure the output voltage stays correct). Why is this done? A transformer can be made way smaller at a higher frequency. If the PSU was designed the old way with a 50 / 60 Hz transformer (a “linear” power supply), then our equipment would need a PSU as big as itself and many more times as heavy!

However, there is a cost here – converting the mains into a high frequency creates interference signals. They are multiples of 50/60 Hz but go all the way into the audio bands and also the radio frequencies, and they get transmitted both into the power rails of the equipment the PSU is feeding, and also back down the mains cables. Also, the computer itself has circuits what generate these audio and radio frequencies as a unwanted byproduct, and at various points on the motherboard these unwanted signals are sent to earth (ground) via capacitors.

Here is the input socket of the PSU..

and the circuit diagram (apologies, I haven’t got any decent schematic drawing software on this PC, and have to admit I’m that good at using it anyway, so did it the old skool way :wink:)

NB: at present I can’t identify the exact values C1 and C2 as I can’t read the bloody markings on them with the naked eye (tiny grey on blue text FFS :rant:) and have mislaid the magnifier – but this doesn’t affect the explanation of this circuit.

The diagram does assume you are in the UK or a similar country where the live and neutral on a mains socket cannot get reversed. In other European countries they can, so either L or N on that diagram could be at top or bottom.

However this doesn’t affect the working of the mains filter or SMPSU. It is also the law across the world that computers and other gadgets must not chuck out excessive interference, and that they are electrically safe. So mains earth is connected to the metal frame of the computer case, and often also to the 0V of the power supply DC output, and is often also used as as ground point for audio, to avoid any fault at the PSU making metal parts you can touch alive to 230 or 120V AC (which can kill people).

What this circuit does is use capacitors which have a low impedance to the interference frequencies to send them all back down the mains earth connection. This is why you must always have earth/ground connection on equipment with a 3-pole power inlet (like the IEC shown above or the “cloverleaf” socket on some laptop PSUs) – otherwise through C1 and C2 some mains electricity will leak through and the metal case of the PC is then alive to half mains voltage – which particularly in a 230V country is unpleasant and dangerous! Also at the DC end of the PSU, similar circuits send other unwanted interference to the 0V rail.

Unfortunately this causes the other problems – if you are using the 0V rail as a ground reference point for audio, all the crap is also there, and gets into your sound equipment. Worse still, the earth wiring in your house (and there should be a lot of it, otherwise your house wiring is dangerous!) radiate it and so radio equipment can pick it up too.

So for both audio and radio equipment, we must often use transformers to get rid of the unwanted interference. For audio the “ground loop isolator” transformers work well

http://www.partyvibe.com/forums/sound-engineering/48352-ground-loop-illustration-cheeseweasel-deserves.html

and for RF a ferrite core and wire wound around it to make a “balun”, and using a different source (not the electric mains earth) for “RF Earth” (such as copper pipe or metal stake driven into the ground and a wire connected to it) can work well. I’ve heard some old sparks claim that pissing upon the earth stake makes it conduct better – don’t do this if you are transmitting or there could be any fault/leakage current. that will be painful!