My home, where my studio is, can be prone to some pretty bad RF interference. In fact I can totally recognize KGNU AM radio station playing bluegrass, or reggae when it perks up.
It doesn't happen with everything, certain amps with a lot of cable and effects on them, and through my tube mics/ powersupply.
Sometimes using a Furman power strip that's supposed to fix RF helps, but sometimes it doesn't work. It sounds like a job for an electrician, but what are we looking for. Is it wiring? Lights? The setup on the breaker box?
Tips for RF Interference
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losthighway
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losthighway
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The Scum
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Radio signals are being picked up by something, and then finding their way into the audio circuitry. There are two main causes:
1 - Bad grounds in the building AC. The induced noise probably wants to run off to ground, but can't find it's way.
2 - Something not quite right in the gear design.
As to #1, some electricians have a sophisticated meter than can tell when grounds have been done incorrectly. It's far more sensitive than that thing with the 3 LEDs from the hardware store, which can't tell if you've tied ground and neutral somewhere.
You're not intentionally cheating grounds, are you? No broken off prongs, or 3:2 lifts?
As to #2, there are several possible causes.
First, gear should be designed with RF filters on the inputs and outputs. It keeps RF from entering at all...sometimes as simple as ferrite beads and/or a couple of tiny caps.
Second, gear should be designed such the shield on the cable is tied to the chassis right as it comes into the box. The RF is induced in the shield, into the chassis, and then away to ground. Some gear is designed such that the shield runs around inside the chassis, maybe a few inches from the connector to a PCB, then to ground. A metal box is usually a pretty good way to stop RF from coming inside...the problem with running the shield around inside the box is it compromises the box, and serves as a new antenna. This is often called the "pin 1 problem."
Third, some gear is just more sensitive to RF. Tube mics have a number of high impedance nodes, and high impedance means they're more like to act as antennas.
Is the problem more pronounced at night? Some AM stations bump up their transmission power after sunset. Researching their FCC license will tell you if they do this.
What's the heritage of your tube mic power supplies? If they're really old, hand-cobbled, or from a questionable vendor, they may exhibit pin 1 problems.
Jensen transformers have some good whiteparers about noise troubleshooting. Look for the general troubleshooting paper, as well as the "Pin 1 Problem" paper.
A good first step is to be able to reproduce the problem with the minimum amount of gear - remove all of the pieces that you don't need to worry about.
1 - Bad grounds in the building AC. The induced noise probably wants to run off to ground, but can't find it's way.
2 - Something not quite right in the gear design.
As to #1, some electricians have a sophisticated meter than can tell when grounds have been done incorrectly. It's far more sensitive than that thing with the 3 LEDs from the hardware store, which can't tell if you've tied ground and neutral somewhere.
You're not intentionally cheating grounds, are you? No broken off prongs, or 3:2 lifts?
As to #2, there are several possible causes.
First, gear should be designed with RF filters on the inputs and outputs. It keeps RF from entering at all...sometimes as simple as ferrite beads and/or a couple of tiny caps.
Second, gear should be designed such the shield on the cable is tied to the chassis right as it comes into the box. The RF is induced in the shield, into the chassis, and then away to ground. Some gear is designed such that the shield runs around inside the chassis, maybe a few inches from the connector to a PCB, then to ground. A metal box is usually a pretty good way to stop RF from coming inside...the problem with running the shield around inside the box is it compromises the box, and serves as a new antenna. This is often called the "pin 1 problem."
Third, some gear is just more sensitive to RF. Tube mics have a number of high impedance nodes, and high impedance means they're more like to act as antennas.
Is the problem more pronounced at night? Some AM stations bump up their transmission power after sunset. Researching their FCC license will tell you if they do this.
What's the heritage of your tube mic power supplies? If they're really old, hand-cobbled, or from a questionable vendor, they may exhibit pin 1 problems.
Jensen transformers have some good whiteparers about noise troubleshooting. Look for the general troubleshooting paper, as well as the "Pin 1 Problem" paper.
A good first step is to be able to reproduce the problem with the minimum amount of gear - remove all of the pieces that you don't need to worry about.
"What fer?"
"Cat fur, to make kitten britches."
"Cat fur, to make kitten britches."
I had a similar problem on a guitar cable of all things and it was the shield. A person who is much smarter in the ways of the dark art of RF told me that all of the shields are, in effect, an antenna and will pick up the signals in the air. The fact that it is transmitting through the output leads me to believe that either
1. The ground that drains the shield isn't functioning as designed
2. One or more of your cables are the problem (one loose strand of a ground touching the wrong pin or something)
3. You are sufficiently close to the station that the problem is endemic.
I have a contact who is a broadcast engineer at work. I'll tap him for input this week. The dude is seriously smart in the ways of electronics and radio. I'll report back when I have more.
1. The ground that drains the shield isn't functioning as designed
2. One or more of your cables are the problem (one loose strand of a ground touching the wrong pin or something)
3. You are sufficiently close to the station that the problem is endemic.
I have a contact who is a broadcast engineer at work. I'll tap him for input this week. The dude is seriously smart in the ways of electronics and radio. I'll report back when I have more.
I make a living as an electrician, not recording in the basement.
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MoreSpaceEcho
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OK, so I spoke with a radio broadcast engineer and his first response was shielding. His suggestion was to solder a conductor from the ground pin of the XLR to the shell of the connector. He also said that occasionally he will run a ground from the chassis of the power supply to the ground of an electrical outlet.
He said solder the ground pin to the shell on the end of the cable going into the tube supply. Hope this helps.
He said solder the ground pin to the shell on the end of the cable going into the tube supply. Hope this helps.
I make a living as an electrician, not recording in the basement.
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losthighway
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More info on Ferrites, including source
I've had a heck of a time with RFI at my studio as well. I'm near several big radio towers. In my case, the main problems are with FM.
My biggest success in filtering out the RFI has been with ferrites, usually on my audio cables.
The ones at Radio Shack may work, but it's a gamble because they usually don't indicate the type of material used in the ferrite. There are many, and the material determines the cutoff frequency/shape of the filter they become.
Material 43 has worked best for me for FM RFI. This page has a list of the materials and their effective ranges.
http://www.fair-rite.com/newfair/materials.htm
If you have questions, try giving Fair Rite a call. They were helpful to me.
Once you've picked a material, take a look at the catalog and order some different sizes. The effectiveness is usually correlated with size (and price).
I like the Round Cable Snap-its for most applications, since you can quickly snap them on different cables/locations to test.
Looping the cable through multiple times increases the strength as well.
This is a helpful chart of the radio spectrum:
http://www.ntia.doc.gov/files/ntia/publ ... lochrt.pdf
My biggest success in filtering out the RFI has been with ferrites, usually on my audio cables.
The ones at Radio Shack may work, but it's a gamble because they usually don't indicate the type of material used in the ferrite. There are many, and the material determines the cutoff frequency/shape of the filter they become.
Material 43 has worked best for me for FM RFI. This page has a list of the materials and their effective ranges.
http://www.fair-rite.com/newfair/materials.htm
If you have questions, try giving Fair Rite a call. They were helpful to me.
Once you've picked a material, take a look at the catalog and order some different sizes. The effectiveness is usually correlated with size (and price).
I like the Round Cable Snap-its for most applications, since you can quickly snap them on different cables/locations to test.
Looping the cable through multiple times increases the strength as well.
This is a helpful chart of the radio spectrum:
http://www.ntia.doc.gov/files/ntia/publ ... lochrt.pdf
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Gregg Juke
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