questions about grounding and noise

[zhai]

Heys, I've "completed" 3 projects. When I started I was pretty much a beginner, so I have som design flaws that I'm now trying to rectify.

DAC: www.dddac.de , dddac1543 MK2

preamp: www.twistedpearaudio.com , kookaburra + avel lindberg trafo + darwin source selector

power amp: UcD180 stereobox, 2x UcD180, 2x UcD supply ST, 2x trafo 160VA

pictures: http://www.templar.dk/files/pics.rar

Grounding:

DAC: 220v ground pin is connected to 0v on the power supply. Not to the case.

preamp: 220v ground pin is connected only to the case.

power amp: 220v ground pin is connected to "GND" on both power supplies. And not to the case.

All boxes have ground pins in the connectors, and so do all the cables all the way to the wall socket.

primary problem:

When I connected the DAC to my old preamp, it worked fine, no noise at all. When I connected my cd player's analogue out to the preamp, it worked fine.

But when I connected the DAC to the new preamp, it gave horrible noise. I then let the 3 projects stand still for some months, tried different power cables for the preamp, and suddenly the horrible noise that was there before all the time, had gone. I also tried using the original power cable again, with no difference. I have no idea why.

So now, when I turn the volume knob on the preamp, there's noise that stops when I stop turning. When I turn the knob to high volumes, there's a "falloff" of noise, continuing after the noise that happens while I do the actual turning of the knob.

I'm pretty confused by this, but I think it has something to do with grounding. A guess is that the 220v ground pin should be connected to both 0v in the circuit and the case? Is that correct?

Secondary problem:

When my power amp is the only thing that is turned on, I get some low noise in the speakers. I'd really like to get dead silence when I turn it on, but my design includes signal cables being too close to the trafos. Is there any way to rectify this without doing major changes to the power amp?