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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: RC network(zobel) Radio Frequency speaker filter question?(rather long) posted by Keith Lockwood on May 9, 2005 at 12:23:41:
The speaker impedance will be large at radio frequencies because of the inductance of the wiring and drivers (in most cases), so don't worry about it.The Walker HDL uses a 0.01 microfarad capacitor in series with a 10-ohm resistor. A friend loaned me some to try and I found one was open-circuit. He gave permission to take it apart. The capacitor is in a metal case with the labeling information removed. The resistor is a nude Vishay film device, with very fragile leads. One of these had broken off, so my friend gave permission to dismantle the HDL.
A neighbor works for Agilent and helped me measure the self-resonance of the Walker mystery capacitor with an HP network analyzer. It is 21 MHz. A selection of 0.01 microfarad capacitors we tried had self-resonance frequencies in the range of 15 to 21 MHz. I'd expect a larger capacitor to have a lower self-resonance frequency.
The capacitor becomes inductive to frequencies above the self-resonance, so these networks made with 0.01 microfarad capacitors will not dampen RF at higher frequencies. Most audio cables ring in the UHF band, and these networks would be ineffective there. We found that a 390 pF silver-mica capacitor had a self-resonance frequency of 58 MHz. This is starting to become useful, so an alternative design to use in high-RF areas is to connect several sizes of silver-mica capacitors in parallel, and then in series with a metal film or metal oxide resistor.
I did a bunch of listening tests with different capacitors and resistors, including Mylar, polystyrene, and silver-mica. The silver-mica caps sound best to me. The Holco (older style, without steel caps, or use the PRP replacements) resistors from Michael Percy sound best to me. My final recipe is to place 390 pF silver-mica caps in parallel with 0.01 microfarad silver-mica caps, and then the combination in series with 20-ohm Holco resistors. Alpha-Core silver spades provide good connection over spade speaker cable spades. These networks are at both ends of all my speaker cables. I may add even smaller caps in the future. You can find good prices on silver-mica caps at Hosfelt Electronics.
The resistor value is not critical. I chose 20-ohms to match the impedance of my speaker cables because I asked their builder what it was. I think 10-ohms is a good compromise if you don't know.
This tweak is useful but not the limit of what you can do for RF pickup by speaker cables. I've got Power Wraps on my cables, and found that both tweaks improved the sound in complementary ways. Treating local RF sources is also useful: cable or satellite TV converters, and TiVO boxes, are strong sources that are under your control.
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Follow Ups
- My results. - Al Sekela 05/9/0515:35:21 05/9/05 (1)
- Re: My results. - pkell44 14:48:14 05/10/05 (0)
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