I have a noise in an Aviom personal monitoring system that I believe is coming from a non-stock SMPS which is powering the 8 port A-net hub. The sound is like a very high frequency whine, like 4K or higher.
I want to build a filter on the output of the SMPS and was thinking I could simply parallel a large capacitor on the output.
Am I on the right track here? Should I add an inductor as well as a capacitor? Are there certain values for each I should stay above/below to avoid a complete waste of time?
Thanks for any advice.
One thing to be aware of is that "rolled" electrolytic capacitors have a certain amount of internal inductance, primarily due to the geometry of foil "plates. So larger capacitors behave like they have an inductor in series with a capacitor. That's no good for high frequencies, and is why switch-mode power supplies use "folded" foil plates instead of "rolled" plate. No, I'm not making this up.
A workaround for low-tech power supplies is to add a small disc capacitor (.01 mF or so) in parallel with the big soda-can sized (many thousands of micro-Farads) filter capacitors. The big caps filter the low frequency ripple, while the tiny disc capacitors filter the high-frequency "whine".
That suggests that the noise you're hearing would most likely be filtered out with a disc capacitor of some sort. The exact value isn't critical, as long as it's rated for high-frequency use (flat - not rolled). Even a really big disc cap will likely be high voltage (that's OK) as well as fairly low capacitance (.1 microfarad would be pretty common).
Others can chime in on this, but that's what my creaky brain remembers.
