RF into Sub-Woofer? - elmers Forum at eHam.net

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eHam.net Forum : elmers : RF into Sub-Woofer?

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RF into Sub-Woofer?	Reply
by NV2A on November 7, 2009	Mail this to a friend!
My shack is in the room just above the living room. When I key 2 meters, either 65 W desk radio or handheld, I get a bodacious hum into my 250 watt subwoofer that rattles the windows? My handheld set at 1/2 watt still causes the problem but not as prominent. Some Radio Shack torids to the Sub Woofer input made no difference? Any suggestions would be appreciated. This 2 meter is like an intercom to my friends chasing DX in this neighborhood and I miss using it.

RE: RF into Sub-Woofer?	Reply
by K0BG on November 7, 2009	Mail this to a friend!
First of all, the ferrite materials RS sells aren't worth the effort. However... Depending on the quality of the subwoofer, the most likely input is the AC line, not the low level audio. Cheap subs, like the Infinity B and 250 lines, use integrated amps, and about a cheap of a PS as you could design. There is no effective way to "fix" them. Been there, done that! On higher line models, the low level input is the usual path. If you use the 3/4 ID, mix 31 split beads, you can get about 8 turns through the beads. This will take care of most problems. High end subs just don't have RFI problems. My home theater has two 1 kW Sunfire subs, and no RFI or any sort, on any frequency. The amp modules are totally enclosed in aluminum, and properly bypassed internally. That's what you get for your money. By the way, decent audio cables help too, but you sure don't need those high-end "hoses" from MasterBlaster. Alan, KØBG www.k0bg.com

RE: RF into Sub-Woofer?	Reply
by K6AER on November 7, 2009	Mail this to a friend!
To amplify on what Alan said (sorry I could not resist) disconnect the audio cable feeding the sub woofer and see if you are getting RF hum into the amplifier section. If not you might benefit from a toroid on the audio feed cable. I have had mixed success with Radio shack RFI bead clamps but for stereo I would use a 2 inch type 43 mix toroid and take 15 turns of the audio feed cable through the toroid just before it connected to the sub woofer. This can also be done with the AC line as well. Part of the problem in sub woofer design is if they leave out the necessary components for RF I prevention and they save 50 cents on each sub woofer. That could be $100,000 bonus for the production manager. Also the cabinets are unshielded and Hamsters getting into audio systems is a very small problem. Easier to ignore the problem. As you get older the Stereo/Entertainment system get used less. Problem eliminated.

RE: RF into Sub-Woofer?	Reply
by VA7CPC on November 7, 2009	Mail this to a friend!
One more possibility: If there's a cable from the output of the amp to the loudspeakers, RF can be induced in that cable, and then get back to the amp input via the amp's own negative-feedback loop. So, if amp and loudspeaker are separate, you'll want to try some ferrites on the loudspeaker leads, as close to the amp as possible. Charles

RE: RF into Sub-Woofer?	Reply
by N3OX on November 7, 2009	Mail this to a friend!
"My shack is in the room just above the living room." Where is the ANTENNA?

RE: RF into Sub-Woofer?	Reply
by W8JI on November 7, 2009	Mail this to a friend!
Two meters requires 15 turns of something?? It requires beads with a high permeability? Let's think about this. A two meter Marconi antenna is 19 inches long. What is the likelihood the same thinking we use for HF, where a 1/4 wave antenna is 30 feet long, would work on a two meter system where a 1/4 wave antenna is 19 inches long? What are the chances the same beads with 10-20 turns would make a good choke? The first thing I think you should do is pull the input cable but leave the sub woofer powered up and see if it still hums. If it doesn't hum, I would try some 100 pF bypass capacitors or a few 73 mix beads with a single pass through the hole right at the input jack.

RE: RF into Sub-Woofer?	Reply
by K6AER on November 7, 2009	Mail this to a friend!
Tom, Eventually the ham will be getting on HF. Why go through the exercise twice. That why I said for so many turns. Is it overkill, probably yes? Does it hurt anything? No. Most hams are uncomfortable taking apart their stereo equipment to add bypass caps. Also the RCA jacks may no be grounded. You don’t know with so much coming from overseas. If by pass caps do the trick that is great. I suspect the RF is getting into the amplifier directly regardless of the bypassing that might be attempted. Until we hear about pulling the audio feed leads reducing the hum we are guessing. Mike

RE: RF into Sub-Woofer?	Reply
by W8JI on November 8, 2009	Mail this to a friend!
Mike, If you put too many turns on an inductor the impedance eventually goes down, not up. 10 times good is not always good. No one even knows if the wiring is the issue. One would think someone would have him power the thing (if possible) and pull the audio input to see if it is affected by the wiring. With a two meter signal, which is what he is complaining about, it could be the wires inside the woofer box acting like an antenna as much as things outside. It does NOT take more than a few inches of wire to pick up significant RF on two meters. A few hundred pF of capacitance across the input, if he can get to the jack, is a substantial shunt impedance on two meters. Going through a few inches of 73 material sleeving is a substantial series resistance. He is complaining about two meters, not 80 meters. While HF techniques might work, it would be pure random luck if they did. Tom

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