Amplifier hum

[FlyingHacker]

Connect a ground wire from every involved instrument to a single grounding point in a star configuration. All the grounds should meet at a single grounding point, not separated by a conductor of any length.

Alternately you could try an isolator between the stereo and your computer.

[timelessbeing]

I connected a naked speaker between the grounds on the two household circuits, and I got the same buzz. That's not supposed to happen right?

[timelessbeing]

Well sure enough ...
When I plugged the stereo into the same outlet as the monitor ... complete silence. The hum/buzz completely disappears.
It's not ideal, but it will get me going until for now. In the meantime, I'll have to look into where that mains earth noise is coming from.

I think I'll test the old transformer again. It had no earth reference, but the neutral line goes back earth. If that works I'll go back to the old design.

[FlyingHacker]

Glad it is working.

[CJay]

Nasty, I've sat in an office of around 150 people and argued with the management that they should turn off and unplug all their PCs because there was obviously an electrical fault, it'd caused several PCs and monitors to go bang rather spectacularly. By the time they agreed with me they'd lost around 40 PCs and a similar number of monitors, a very expensive Avaya IP phone system, the UPS for the servers and phone system and managed to trash a disk array that, fortunately, was fully backed up but took a day and a half to restore (it was *big*).

All caused by a fault which sounds similar to yours, a poor neutral connection which had started to arc in a conduit.

[timelessbeing]

Wow that does sound nasty. My problem is trivial by comparison. The current was barely enough to light up an LED.

[CJay]

It was, with the exception of the Avaya, not actually expensive in hardware terms, a couple of thousand pounds to rectify, the major cost was downtime, a day until the building management got a generator in place and then two days to get their storage array up, running and restored.

It did have the nice side effect for us of selling them a full disaster recovery plan with additional testing days.   

My point was that when you have a fault like this, it's probably best to find it before it becomes a real problem, could be as simple as a loose connection in a socket somewhere. .

[timelessbeing]

I tried the transformer again, and it still causes interference. It's just a bad design. So back to the DC power supply again, and it sounds great as long as I keep it on the same circuit as audio source.  I thought I'd post some pics in case somebody wanted to make a similar mod ...

Lenovo 20V 4.5A 90W power supply from eBay: $ 8 US (shipped from NJ)
DC jack from eBay: $1 US

[timelessbeing]

There was a weird looking hole where the fm antenna came out (just a thin piece of wire). Since I never use FM, I enlarged the hole and mounted the jack into it. Two stiff bits of wire from some clip were used prevent the jack from getting pushed in. The slotted right into some grooves on the sides of the jack.

Then I just routed some wires from the jack over to the power supply board.