Wallwart isn't very quiet
#1
I just recorded 9 hours of audio overnight using a USB soundcard only to find it was mostly power supply trash.  This didn't happen on batteries.
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I'm scratching my head thinking my microphone preamp had started oscillating, then I realized it was hum.  But not normal 60 or 120 hz hum, this is from a switchmode power supply.  I suppose you could argue that the USB sound card should have better filtering since it's trying to do analog stuff in a digital world, but it just assumes the USB power is clean.  Is there an obligation to provide clean power?  The preamps are battery powered still, it's getting picked up in the sound card.  Which is a $10 cheapie from Aliexpress.  I do have a better one I could try.

OK, with my StarTech USB sound card some of the high frequency stuff is filtered out, it sounds more like 120 hz hum, which of course it isn't.  It's like there's some capacitance there but only enough to take out the high frequencies.

I'm recording with sox and having it normalize the level so the hum becomes most of the signal I recorded.

Just guessing, if you settled for running the soundcard internals on about 4.5 volts instead of 5.0, you could use a PNP pass transistor in linear mode with some active filtering to clean that up. You pull the base of a PNP down to make it conduct so it has less overhead than an NPN. An old LM723 chip has quite good active filtering if you use a few stages of pass transistors to provide current gain. I don't have a spare one of those barrel plugs though.


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#2
But I've worked out a routine of recording at night on battery until the battery runs out. Then next morning I transfer the files off, shut it down, let it charge until evening. Recording on battery gives very clean sound, plus my preamps are battery powered. Even powering the USB sound card from the PBP's battery I can still record for 8 hours usually.
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