unknown machine

True. However, the impedance of a capacitor decreases with freqeucny, so if it's low enough at a given freqeucy, it'll be even better at higher frequencies. The lowest freqeucny ripple you are going to get from any normal PSU is at the mains freqeuncy. A linear PSU may well have ripple at twice the mains freqeucny, an SMPSU at the switching frequency or multiples thereof. So having a low enough impedance at mains frequency should be enough. I would be _very_ suprised if any SMPU was deisgned to switch at a frequency lower than mains. One advantage of SMPSus is that, since they run at higherr freqeuencies, you can use smaller transformers and smoothing capacitors. Adding complexxity (by making it a switcher) and then having to use a larger transformer and smoothing capacitors than if it was a simple mains transformer-based supply (possibly with swithcing-type voltage regualtors to impove the efficiency over a purely linear supply) would make no sense. [There are, of course, choppers that run at lest then mains freqeuncy to generate lower-frequency _AC_ outputs from the supply, perhaps to run an indcution motor at a lower speed, or for telephone ringing, But that's not applicable here] IIIRC, power line losses increas with frequency, so this would be unusual to say the least. At one time various areas of England had their own power stations, not linked by the national grid. And they were most certainly nor all 50Hz. I think the highest was 100Hz, though, the last place to have that was Lynton/Lynmouth in North Devon. I would think making an adjustable (active) filter would eb simpler here. But if you are serious enough about electroncis to want to do soemthing like that, you are also serious enough to want to get a 'scope I think. -tony