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Inductors have self-resonance.

Posted by Al Sekela (A) on September 19, 2006 at 21:35:46

In Reply to: Re: No, standing waves are in the UHF band. posted by Ugly on September 18, 2006 at 18:54:40:

Even air-core inductors have parasitic capacitance that reacts with the inductance to cause a primary resonance. A parallel L-C circuit has very high impedance at its resonance, then looks like a capacitor above that frequency.

Whether such an inductor would provide useful filtering of RF noise frequencies depends on its design (L, C, R) and the noise spectrum. Consider that such a device is installed. The remaining sections of cable are then free to resonate, since the inductor will present an impedance mismatch to the cable sections. The inductor would be confronted with the high level resonance tones. Even if it blocked them effectively, they could still induce noise into nearby cables.

The Power Wraps work by dissipating the energy in the standing waves. They do this by forcing the magnetic components of the standing waves to generate circulating currents in a resistive medium. Resistance causes damping, while reactance simply reflects the energy somewhere else where it can do mischief. Making the cables highly resistive would also prevent resonance, but would create impedance problems for the audio signal. There are some resistive interconnect cables designed for this purpose (I can't speak to whether they sound at all good), but obviously power cords and speaker cables could not be made this way.

Another approach to reducing the levels of standing waves on cables is to provide effective impedance matching at the ends. It is the impedance mismatches of audio connectors that permit audio cables to resonate. I've had good results by providing R-C networks at the ends of my speaker cables, where R is 10 to 20 ohms, and C is 0.01 microfarads. These do not affect audio frequencies, but absorb energy from standing waves. It is necessary to bypass the capacitor with smaller units to have effective filtering into the UHF, as most capacitors in the 0.01 microfarad size have self-resonant frequencies (yes, every capacitor has parasitic series inductance) in the 15 to 21 MHz range, and we want a filter that works up to at least 50 MHz. Thus, use good, non-magnetic resistors.

I have not experimented with interconnect cables this way, but there is no reason why it would not work if suitable resistors and capacitors were used.

Finally, a cheap parallel noise filter for the AC line consists of a filament transformer with an R-C filter on the secondary. R is chosen to look like 120 ohms to the AC (use the square of the turns ratio to calculate impedance transformation), and C is chosen to respect the power rating of the resistor for 60 Hz operation. Never exceed half the rated power of a resistor, so keep the power down to a quarter watt for a half-watt resistor.

This noise filter provides a rough match to the AC wiring characteristic impedance, even through the outlet and plug, and the transformer core provides spike protection and additional energy loss mechanisms. The larger the core, the better it works.

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