The 8th page of
http://www.apqi.org/file/attachment/2008724/91729.pdf shows an interesting concept, the "Narrow Bandwidth Transformer." The secondary winding (or the primary winding) is extended with an extra winding (the fp winding), with equal number of turns, connected in reverse phase to the existing secondary winding through a capacitor Cfp. At low frequencies, the impedance of the capacitor Cfp is high, the capacitor acts as an open switch, only one secondary winding functions and thus the 50/60 Hz is free to cross the transformer. At higher frequencies (above 1 kHz), the capacitor begins to act as a closed switch. Both secondary windings are now at 180 degrees phase difference. Therefore the voltage induced in the two windings cancel one another and no transfer of high frequency signals occurs. The transformer acts as an effective low pass filter with adjustable bandwidth. The paper claims that this approach gives better noise attenuation than a more conventional balanced power solution, and it occurs to me that it might have an advantage in the safety department as well. (When properly implemented, balanced power is plenty safe, but this sort of narrow bandwidth transformer seems like it is more foolproof.)
Wouldn't it be fairly easy to DIY a Narrow Bandwidth Transformer? There are plenty of bifilar wound transformers around, such as 120Vx2/120Vx2 isolation transformers. Alternately, even without dual windings on one side, perhaps two *identical* transformers could be wired together for similar effect.
Obviously there would be at least one inefficiency in taking the approach I'm suggesting here -- for example, the original current rating of the transformer would need to be derated by half. (Since transformers can be seriously overloaded for brief periods of time with no harm, perhaps a transformer manufacturer could find a way to design a commercial version of such a transformer with less than a 50% loss of current capacity relative to the core size.) But the inefficiency I just mentioned seems like a small price to pay if benefits similar to those described in the paper can be obtained.
To me this seems like such an obvious candidate for DIY knock-off that I expected to see something come up in a Google search on "DIY Narrow Bandwidth Transformer" but I couldn't find anything. I can't believe I'm the first DIY fan to think of knocking off this idea, so the fact that my search turned up nothing could mean I'm overlooking some really major obstacle to accomplishing it. But then again, maybe not. So . . . what do y'all think?
-- Chris
