This sub-woofer RFi kit is specifically designed to suppress audio interference triggered by Radio Frequency Interference problems from AM, FM, CB and Ham radio stations AND to minimize the interference to nearby electronics caused by a sub-woofer without magnetic shielding (Velodyne, Hartley, Hsu, etc). The ferrite filters are selected to suppress RFI from 1 to 300 MHz. The filters are easy to install, don’t require modification of the protected equipment and work in almost all cases.
What is included in the kit?
The RFI-HTS-SW kit contains a a ring filter for the AC power cord to the sub-woofer and 4 snap-on filters for the speaker wires (either for audio pass through to main speakers or direct line out to the sub-woofer).
Tech Note: This kit is INCLUDED in the full home theater RFI Kit (part #RFI-HTS) and you do not have to purchase a separate RFI kit for the sub-woofer.
AM broadcast transmits on a low frequency with large antennas. Consequently it takes a large antenna to receive the signals. Typical “antennas” are AC house wiring, phone lines, long satellite or cable TV cables, DSL/CAT5 lines running between rooms, etc. The device wires that connect to these “antennas” need to be choked with as many turns through the ferrite as possible. Some installation examples are shown below on this page.
For AC/ DC power cords and coax cables the F240-77 (2.4″ OD, 1.4″ ID) “Donut” toroid is the most popular. Also available are smaller toroid donuts and Mix 77 beads. Shown below is a graph with 8, 10, and 12 turns through the toroid – obviously more turns has higher choking impedance. Mix 77 toroid products are HERE and Mix 77 small beads (for small wires) are HERE
F240-77 Turns Comparison from .1 – 8.1 Mhz
Shown below is a graph of frequency response verses choking magnitude for a F240-77 in the AM Broadcast band for a 12 turn coil around the toroid. (Same info as above but different frequency range).
F240-77 Choking Impedance in AM Broadcast Band
Typical Installations for Donut/Ring Toroids
 AC Cable on F-240 Toroid – 6 turns |
 Ethernet Cable on F-240 Toroid – 7 turns |
 Audio or Mic cable on F240- 12 turns |
Extra Mix 75 Snap On Beads HERE – VERY effective for AM Broadcast RFI