RFI woes

[ultrachrome]

I've been dealing with RFI since moving into my house, perched on a hill with a view of a trio of transmission towers on an adjacent hill, 4700 feet away. I haven't been able to figure out for sure who broadcasts on them but there's definitely TV. However, the RFI I hear through the speaker is NPR on 94.9 FM.

Every piece of equipment I have, purchased or assembled has the issue except for my Panasonic XR-45 digital receiver.

Currently I'm using an ST-70 with Curcio CCDA driver and cap board connected to a scratch built grounded grid with a Squeezebox Touch and PS3 as sources.

I have about .6 VDC on my power line.

• With the amp inputs shorted, I very faintly hear NPR from a foot or two away.
• With a set of shielded interconnects shorted on one preamp end, NPR is louder, loud enough to clearly hear from my seating position.
• With a set of unshielded interconnects, NPR is even louder.

I added 47pF ceramic bypass caps to the amp input with seemingly no effect.

If I unplug the amp from the wall, I can still hear NPR as the caps discharge so it doesn't seem to be RFI on the power line causing the problem.

What to try next?

[Speedskater]

My ST-70 would pick-up lots of RFI but not understandable FM radio.
Here a a couple of interference links:

Identifying Sources of Radio Frequency Interference Around the Home

INTERFERENCE HANDBOOK

[jcx]

in truely extreme RFI environemnts speaker cable can be a coupling mechanism

common mode lossy ferrite may help on input, output, pwr cord

finding the transmitter freq helps in selecting the grade, ferrite form factor

they should have an engineer responsive to claims of interference and willing to help out to avoid probelms with the FCC

cheap RCA may have bad construction - video RCA or pro, Mogami or similar, should have heavy shield, 360 termination

DC on the mains is a problem in the building, principly with SCR/Triac controlled loads

[ultrachrome]

Speedskater, thanks for the links. I guess I need to be searching different terms. I don't seem to be experiencing the classic RFI issue.

jcx, interesting idea contacting the station. I have some diy interconnects using parts-express shielded microphone cable and their "super RCA" connectors. I terminated the shield at the source. I also have some Blue Jeans Cable LC-1 cables that have a double shield and crimped on Canare jacks. Both sound the same although quieter than unshielded.

Mildly frustrating afternoon. Picked up some clamp on chokes from Radio Shack to play with but when I got home, the interference was almost inaudible. It does seem to come and go a bit.

[DF96]

VHF can be hard to eliminate as it only needs a short piece of wire to pick up. Clamp-on chokes may be better for lower frequency RF. Small ferrite beads can work for VHF.

[Speedskater]

Jim Brown also has several papers for HAM's on eliminating interference.

Audio Systems Group, Inc. Publications

[Soundminded]

I lived on the infield of WTFM six blocks away for many years. I know how frustrating RFI can be. I got it everywhere, even on channel 6 on my TV set. The usual suspects audiophiles look for, power cords and speaker wire are generally not the cause, a useless waste of effort and money. You've already ruled out the power cord. If you aren't sure about the speaker wire just connect a .047 mfd capacitor across your speaker, if that's the cause it should disappear. More likely the RFI is being directly induced into the amplifier or preamplifier stage or the interconnect wires. Shielding the interconnect wires is cheap and easy, do what the major manufacturers do. Wrap aluminum foil around the outsided of the shielded wire. Strip a length of wire, any gage and wrap it around the foil. This is called the drain wire. Wrap that around the foil so it is in electrical contact with it and ground it to the amplifier signal ground. If this doesn't work and you've ruled out all other possiblities then the RF is being directly induced. Bad news you will have to construct a farady cage around the amplifier. This consists of a metal enclosure and must be grounded. Be sure to allow enough vent holes to not overheat the amplifier (vent holes should be on both the top and bottom to allow convection cooling. Aluminum works well and is cheaper than copper. Don't place the amplifier on a metal table, it can act as an antenna. One last possibility I can think of is any connection between a stranded and solid wire. I had one case where RFI caused nuissance alarms in a fire detection system for this reason, the junction of the two somehow acted as an RF detector diode, the RF locally generated by lab equipment. A small capacitor connected across the input of the zone alarms did the trick. Good luck, this is usually not an easy problem to fix.

[DF96]

If you are hearing audio from an FM station, then at least one of the following is true:
1. the transmitter has significant AM too - you could complain to the station engineer.
2. somewhere in your system there is a VHF resonator near the carrier frequency which is converting the FM to AM so you then get 'slope detection'.

[jcx]

Originally Posted by Soundminded

... If you aren't sure about the speaker wire just connect a .047 mfd capacitor across your speaker, if that's the cause it should disappear....

dangerous advice - many amps don't like pure C across the output - this should be a "Zobel" - series RC, with the R in the tens of Ohms


would work better at the amp end, with (the right combination of) ferrite on the speaker cable next to the amp pushing the RF Z of the cable into the kOhm

twisted pair rather than the usual flat wiring to the speaker could reduce diff mode pickup

[DF96]

As I said, getting rid of VHF can be different from dealing with SW HF. At VHF, 47nF can look like an inductor.