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Posts: 68
Joined: October 12, 2009
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Power filtering is only needed if there's something to filter. Some places on earth have insignificant noise on power lines. most do not.
the key thing to remember is that the single ended topology is one of the most prone to bad power supply noise in terms of sound degradation there is. i can avoid a lot of this. rf filtering right at the position of the CCS's used can avoid much rfi and not have to go to line filtering. it has high PSRR to high bandwidth if accomplished well. PP is inherently more noise cancelling with high PSRR built into the design.
but also, you don't need a lot of noise filtering in SE designs if you use low BW passing power transformers and use them with all center taps. like full wave rectification with center-tapped ground, 6VAC filament supplies with center-tapped ground, and on every case. if not, then add a wide BW 60-0-60VAC isolation power transformer to the system. hopefully by bifilar windings. a lot of the noise will drop.
then since no filter is perfect, especially for high frequencies, then you need more help in terms of line filtering. you can go hog wild on "X" capacitors (hot to neutral) but limited on "Y" capacitors (hot or neutral to ground). but remember the highest quality caps at microwave frequencies only happen with microwave circuits, and some power cords 6' long (wavelength = approx. 6' = about 1.8 m, and f at that wavelength = about 83 MHz or higher by some percentage for 1/2 wavelength filtering possible.) So if there's a design for microwave structure filtering in the cord, it can deal with frequencies up there fine, too. It takes a long microwave structure to get effectively to those higher frequencies as lumped elements aren't so effective anymore.
So it all depends. What amount is susceptible, what amount of noise is there, what amount hurts that particular topology, and what amount is the real actual full filtering from all attempts.
Remember that AC power lines also have a characteristic impedance and should be looked at like a radio transmission line. The best things to break it up is new fresh dedicated line with no external interference there except what passes a power pole transformer change of line impedance. those are very effective at killing the passage of noise.
The moral is clear to me. Battery power as in the early days wasn't such a bad thing, except the batteries cost so much for an AM receiver. Then AC power is a real bitch to try to emulate that battery and is really not possible. You just do as much as you can in practical terms. and if you can, a dedicated line jumps you way ahead of the game. i had one of those at one time in my life.
-kurt
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