Re: HiFi Power cords
Actually switch-mode PSUs run a lot faster than 20kHz these days. The main reason is that the higher the frequency, the more power can be transferred through a smaller mass of (ferrite) transformer core, and then smoothed back to DC using smaller capacitors.The upper limit is approximately where the extra power lost in the power transistors while they are changing state starts to exceed any economic benefit of making the power supply less massive.
A long time ago I repaired what must have been pretty much the first ever switched-mode power supply (a 20A bench power supply using OC42 - Germanium! - power transistors ) It switched, very audibly, at a few hundred Hz.
Incidentally, the output of a typical switched mode power supply is very poor for analogue audio use. Digital circuitry such as a computer doesn't care about tens of millivolts of ripple on its power, and just because the oscillator runs at a MHz doesn't mean it can't be (and is) modulated at lower frequencies by (for example) AC line noise.
Biting the hand that feeds IT
