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.
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?
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?
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
Here a a couple of interference links:
Identifying Sources of Radio Frequency Interference Around the Home
INTERFERENCE HANDBOOK
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
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
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.
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.
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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.
Jim Brown also has several papers for HAM's on eliminating interference.
Audio Systems Group, Inc. Publications
Audio Systems Group, Inc. Publications
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.
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'.
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'.
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
As I said, getting rid of VHF can be different from dealing with SW HF. At VHF, 47nF can look like an inductor.
Overwhelmed.
DF96, can you say more?
Over the past days I've done some tidying of cables and it seems to be my interconnects that are major contributors. After tidying the interference is much quieter. Unfortunately I did not do this systematically so I don't know what made the difference.
Today's interesting experiment was with a BrianGT LM3875 gainclone. With the inputs shorted there is no sound out of the speakers. Dead quiet.
I then add a pair of diy interconnects. 36" long, Dayton microphone cable. It has a braided shield and a twisted pair. I connect the shield to ground at the source end. Instead of connecting this to a source, I add a shorting plug as the source.
I power up the amp and I can move the interconnects around like rabbit ears on an old TV. I can then tune in or out the 94.9 FM interference. The interference is loudest when the interconnects are at 45 degrees but very quiet when the interconnects are held vertically and very close together.
I have some Blue Jeans Cable LC1 cable I tried next. These are 4' long and the interference is louder still. Correspondence with the manufacturer pointed out that LC1 doesn't have a foil shield. The recommend 1694A for this.
I tried creating an aluminum foil shield for my Dayton interconnect but there was no difference. I made sure the foil contacted the shell of the RCA plug.
Is my issue is RFI? I ask because my google searches suggest that the classic RFI issue is pops and clicks. I've found precious little about FM interference I'm experiencing.
DF96, can you say more?
Over the past days I've done some tidying of cables and it seems to be my interconnects that are major contributors. After tidying the interference is much quieter. Unfortunately I did not do this systematically so I don't know what made the difference.
Today's interesting experiment was with a BrianGT LM3875 gainclone. With the inputs shorted there is no sound out of the speakers. Dead quiet.
I then add a pair of diy interconnects. 36" long, Dayton microphone cable. It has a braided shield and a twisted pair. I connect the shield to ground at the source end. Instead of connecting this to a source, I add a shorting plug as the source.
I power up the amp and I can move the interconnects around like rabbit ears on an old TV. I can then tune in or out the 94.9 FM interference. The interference is loudest when the interconnects are at 45 degrees but very quiet when the interconnects are held vertically and very close together.
I have some Blue Jeans Cable LC1 cable I tried next. These are 4' long and the interference is louder still. Correspondence with the manufacturer pointed out that LC1 doesn't have a foil shield. The recommend 1694A for this.
I tried creating an aluminum foil shield for my Dayton interconnect but there was no difference. I made sure the foil contacted the shell of the RCA plug.
Is my issue is RFI? I ask because my google searches suggest that the classic RFI issue is pops and clicks. I've found precious little about FM interference I'm experiencing.
36" is quite close to a quarter-wave at VHF FM frequencies. You have a good VHF antenna, pretending to be an interconnect. Simply changing the length of the cable by a few inches may make a big difference. To convert FM to AM you need the right length; you seem to have hit it by accident.
Cut 6" off and it's actually does worse in my rabbit ear test. However if I terminate it at my preamp it seems about the same as the others. To add insult to injury, when my grounded grid preamp's outputs unmute, I get more noise sounding like another radio station added to the mix.
Your amp is obviously quite sensitive to RF. It shouldn't be. You need to add filters. For example, a low value resistor (100-200ohms) in series with the input and a low value capacitor (47-100pF) to ground. The resistor can be replaced by a ferrite bead threaded onto the input wire inner. If you keep RF out, then it won't matter what length your external connections are.
A few months ago there was a thread in which someone had RFI problems with expensive interconnects. He then used the free cables which came with the equipment (Sony?) and the problems disappeared.
A few months ago there was a thread in which someone had RFI problems with expensive interconnects. He then used the free cables which came with the equipment (Sony?) and the problems disappeared.
Plugged in my old Adcom GFA-545II. It's not nearly as sensitive but I still hear the interference.
I've tried several cable different cables and variations with little effect, none of them especially high-end. The noise is always there.
I'll focus on the power amp I actually want to use. Which side of the resistor does the cap reside?
I've tried several cable different cables and variations with little effect, none of them especially high-end. The noise is always there.
I'll focus on the power amp I actually want to use. Which side of the resistor does the cap reside?
The other thing you may want to try here is to connect a samll value capacitor - say 1nf between the input socket GND and the chassis as close to the input socket as possible. Just try it on one input initially - if it works, then do the rest.
I just finished trying the RC circuit with a 100 ohm resistor and 47pF cap with no effect. The same result as previously trying just the 47pF at my preamp output and at my amp input.
100r & 47pF is only 4700ps of filtering. (=4.7ns = 0.0047us))
The F-3dB of that RC is ~34MHz
At 68MHz the attenuated RF is only about 6dB down (half voltage).
No wonder you didn't hear a change. You would have to measure fairly precisely to convince yourself that there was a change.
The F-3dB of that RC is ~34MHz
At 68MHz the attenuated RF is only about 6dB down (half voltage).
No wonder you didn't hear a change. You would have to measure fairly precisely to convince yourself that there was a change.
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Sorry, my mistake. That is what comes from guessing values, and not stopping to do the arithmetic. The values I gave would stop UHF TV and mobile phones from interfering. Try larger values, but don't go too large.
How low can I set my F-3dB? 1MHz? That would be 2nF?
What's the risk of going too large? If I used .08uF my corner frequency would be 20kHz which would probably be too low. Is there something else I'm trying to avoid?
What's the risk of going too large? If I used .08uF my corner frequency would be 20kHz which would probably be too low. Is there something else I'm trying to avoid?
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