That's a good question...
So many times I've had the suggestion made to me that any DC power supply that sticks out the right voltages, has sufficient current capability etc. is all one needs to power audio circuits.
Improving PSRR (Power Supply Rejection Ratio) is often quoted as a better way of dealing with noise problems. To an extent that's true, but I've never met a single circuit whose PSRR is sufficiently high to be immune to the massive musical improvements a really good PSU brings.
I've measured lots of PSU's over the past year, and the one thing I can say, with absolute confidence is NONE are pure DC. All have AC content, be it circuit / reference noise, mains harmonic content or a host of other load-related noise content.
Those headline figures we so often read in the data sheets are the best figures the manufacturer can show, usually at very low frequencies often at DC. As one moves up the audio band you find that in may cases those wonderful figures have suddenly degraded to the point where they really do matter, a lot.
Couple this with the fact the the human ear seems more sensitive to low levels of high frequency distortions than LF ones, and you have a real mechanism for sonic degradation.
Let's hand over to Walt Jung, from his article in EDN Magazine.
So what can we do about it?
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