Basically you want to do this:
The computer output is probably unbalanced, connected to the computer's ground, which has a lot of spiky noisy currents going through it (hard drive heads seeking, display refreshes, memory access bursts, etc). Due to the (very small, but finite) resistance of the ground traces, this means the computer's ground is at a noisy voltage relative to the power amp's ground. If the power amp measures the computer signal relative to its own ground, it will see that noisy ground difference superimposed on the signal. The power amp probably has a differential input (please specify what you're using for the power amp), which you can use to cancel that noise out.
You want the negative input of the power amp completely isolated from the power amp ground. It should connect directly to the output ground of the computer instead. That way the power amp is measuring the difference between the computer's output and the computer's ground, which will be noise-free. The grounds of the power amp and computer should otherwise be isolated from each other and connected together only at a star ground point near the power supply. You definitely don't want the ground currents from the computer going past a ground that's used as a reference by the preamp.
If the class D amp doesn't provide a differential input, you can make one with an op-amp in the differential amplifier configuration.
Rg will be connected to the ground of the power amp, V1 to the ground of the computer, V2 to the signal from the computer, and Vout to the input of the power amp.
Not shown on Wikipedia:
- You should have a compensation capacitor in parallel with Rf to avoid oscillation.
- You should add an identical capacitor in parallel with Rg to keep the common-mode rejection good at all frequencies.
- Use 1% or better resistors.
This is a common problem in audio amplifier connections especially where there is digital noise and switching regulators. There is no ideal way around this problem without using differential inputs.
Why do differential inputs work - the signal return back to the box on the left won't carry any power supply currents because it feeds a high input impedance differential amplifier (or a transformer). Obviously you have to wire it correctly.
Without a differential input (silicon or transformer) it's difficult to find a signal return path that won't pass digital currents.
This problem has nothing to do with dodgy earths in your home or needing to put a decoupling capacitor somewhere.
Best Answer
From your description it's probably a ground loop.
You're presumably supplying ground to your DC-DC and BT at both power in and signal out (to the amp).
I'd break the loop, probably by having the connection to the amp grounded at only one end of the cable. I wouldn't break the loop at the power side because if you were to do that then DC-DC would have to use the amp's ground as a return path, which would be less than ideal.
An audio isolation transformer, like this one from ebay, should do what you need with minimal fuss.