VHF/UHF xmit causing Subwoofer to rumble

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eHam.net Forum : RFI : VHF/UHF xmit causing Subwoofer to rumble

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VHF/UHF xmit causing Subwoofer to rumble	Reply
by N2TLK on April 5, 2004	Mail this to a friend!
I have a 350 watt amplified audio subwoofer that has an rf remote control(for frequency cutoff and volume). The problem is when I transmit from my VHF/UHF HT and/or mobile rig in the same room as the subwoofer, the subwoofer makes a horrible sound and rumbles. I would like to keep the subwoofer on when I transmit, but I don't want to blow the speaker. Any ideas on how to stop this problem?

RE: VHF/UHF xmit causing Subwoofer to rumble	Reply
by KC2MMI on April 6, 2004	Mail this to a friend!
Rob, I would expect that you are either getting interference into the radio control circuits, or picking up RFI on the speaker/input wires. The latter is probably more common and cured by putting RF chokes on the input and speaker wires. It could be getting into the subwoofer from either side, and speaker wires are infamous good antennas. Try disconnecting the input and speaker wires from the box, see if it still rumbles. If it does...then you know it isn't getting RFI from the cables, it is getting it directly. You might want to contact the manufacturer to find out about RF shielding the active components, they may have done this before. If not...you might have to try shielding the electronics in it and exposing only the "antenna" portion. Or finding out what frequency it works on, and then filtering the antenna in it to block the VHF/UHF out of it. (A TV antenna interference filter might work. shot in the dark.)

RE: VHF/UHF xmit causing Subwoofer to rumble	Reply
by KA0GKT on April 27, 2004	Mail this to a friend!
Check the Subwoofer's operating frequency(ies) by searching the FCC's FCC-ID database. It is possible that the speaker operates somewhere near or on one of the amateur radio bands. If there isn't an RF link or control for the speaker, chances are the subwoofer's smplifier is poorly bypassed at VHF and UHF frequencies. Hopefully, there is some easy access to the amplifier, however, since much consumer electronics equipment isn't designed for easy repair, this may not be an easy thing. If you can get to the amplifier, get some ferrites and start placing beads over all of the input and output leads. If the ferrites don't do the trick, then start bypassing the inputs, outputs, power supply leads, etc with disc ceramic capacitors. It is unfortunate that manufacturers don't build RF immunity into their products, however the additional $5-10 good technical design would add to the cost of a product often moves a product to a higher price class, so I don't expect to see good engineering practices to come to consumer electronics any time soon. 73 de KA0GKT/7

RE: VHF/UHF xmit causing Subwoofer to rumble	Reply
by KF7CG on April 28, 2004	Mail this to a friend!
Be extremely careful of applying an capacitors across the output or input of the subwoofer. Changing the complex impedances at these terminals can destabilize the amplifier feedback network and destroy the unit. This is something to discuss with the manufacturer of the unit. The use of ferrites will change the input inductance of the subwoofer and play games with the frequency response of the unit and any crossovers involved. With low impedance high gain solid state devices thing get interesting in a hurry.

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