Noise leaking from PSU to mic amp

[Daniel Kelly (AKA Jack)]

Hi,

I have just built a PCB which holds a switched power supply and a microphone
amplifier. The power supply runs a camera whilst the audio amp is powered
directly from the 3.7v lithium polymer battery. The problem is that the amp
picks up a lot of noise from the powersupply. So much noise that the amp is
unusable at the moment. If I turn off the power supply then the amp runs
perfectly. Please could I ask your advice as to how best to limit this
noise? I've had a few ideas myself:

1) The PCB is doublesided and the back of the PCB is a ground plane. This
ground plane is continuous across the entire PCB so the powersupply and the
amp both share the same ground plane. If I separate the ground plane by
cutting the copper with a knife at the boundary between the two circuits,
will this limit some of the noise that's leaking from the power supply to
the amp?

2) Use better shielding on the PSU's inducer.

3) Build a separate PCB for the amp (something I don't really want to do
because I'm quite short of time).

Things I've tried already (but with no success):

1) Running separate power leads from the PSU and amp to the battery

2) Putting coupling capacitors all over the place.

3) Shorting the length of the microphone cable to the absolute minimum.

4) Shielding the entire PCB with grounded silver foil

5) Shielding only one circuit with grounded silver foil.

Thanks,
Jack