MIC pre-amps with switch mode supply

[russm]

Any suggestions on this one would be gratefully received - it's driving me nuts.

I have a small microphone preamp for use with unbalanced electret mics. When powered from an off-the-shelf plug-top style switch mode psu I get horrendous mains hum (sort of) in the output signal. This is a psu without an earth pin, so the output 0V floats at around 90V (due to the two Y-class capacitors, which I believe are from L and N to output 0V). If I tie the output 0V to earth the preamp is clean, but that isn't an option in my application.

So the question is: Is there anyway to supress this noise. It's 50Hz, but not sinusoidal. I've tried a high pass filter in the audio path (3 pole, 88Hz cut-off) - no significant improvement. I've tried common mode chokes in the DC lead, ferrites in the V+, ferrites in the 0V, ferrites in both.

Don't know what to try next. Any ideas?

Thanks in advance.

Russ.

[Andy5112405]

Are you sure you have caps between L and 0V without 0V being earthed. That would be a very dangerous configuration.

[russm]

Andy, no I'm not sure. It's a proprietary power supply and I haven't opened it up. I read somewhere that in the absence of an earth pin they fit the two Y-class capacitors as described - something to do with EMC I think. The thing about the Y-class is they are guaranteed to not be able to fail short circuit, and have a capacitive reactance such that they limit the current to about 160uA at mains frequencies.

[Th3 uN1Qu3]

Originally Posted by russm

The thing about the Y-class is they are guaranteed to not be able to fail short circuit, and have a capacitive reactance such that they limit the current to about 160uA at mains frequencies.

Yes, that's so you don't get killed if the smps has no earth. But they also introduce a shitload of hum if that's the case. There is no other way, short of removing those caps from the supply, but i'm sure the noise will find another place to come from in that case. Why cannot the supply be earthed to the preamp?

[russm]

Thanks for your response.

Originally Posted by Th3 uN1Qu3

Yes, that's so you don't get killed if the smps has no earth. But they also introduce a shitload of hum if that's the case. There is no other way, short of removing those caps from the supply, but i'm sure the noise will find another place to come from in that case. Why cannot the supply be earthed to the preamp?

I was trying to make use of the vast number of off-the-shelf plug-top style psu's that are available on the market. Huge numbers imported from Asia. The significant majority of them simply don't use the earth pin - it's either not connected or it's plastic. The pre-amp / power amp I'm using it with is all housed in a plastic case, so nothing to earth there either.