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Radio signal coming through my preamp.

Discussion in 'Preamps / Channel Strips' started by Studio 9, Nov 28, 2012.

  1. Studio 9

    Studio 9 Active Member

    What do i do to get rid of that? The landlord said that the power outlet was on its on circuit but apparently thats not the case.
  2. Davedog

    Davedog Distinguished Member

    Ahh....theres more to this problem than a dedicated circuit methinks...The only way radio frequencies can get into a circuit like a mic to preamp or a preamp to recorder is through a poorly shielded cable. Could be the mic cable...could be the feed from the pre to the recorder system be it an interface or tape or direct to the computer firewire or whatever you use. We actually have to guess since there isnt a lot of information available. It COULD be an ungrounded receptacle at the end of a run of receptacles in that circuit. See, the more wore, the better the antenna. Funny how that works. Could be a really poor job of isolation in the preamp.....again, the more wire the better the antenna.....theres a lot of wire inside an amplification device with a transformer....But then we dont know what you have.......

    Anyway....there is a field that is captured when power is applied to a circuit and if you live in an area with a close proximity to a radio repeater/transmitter, you'd better be certain of your cabling.
  3. Studio 9

    Studio 9 Active Member

    Thank you for your response. i'll have to go check all my connections and see if i can't find where it's coming from.

    I'm using a profire 2626 interface. It has never done this before i moved into my new place. the receptacle im using right now is ungrounded and i'm using standard mic cables. never had problems before.
  4. Davedog

    Davedog Distinguished Member

    Try another outlet. Run an extension cord. Hook things up until you get the radio signal again. Leave everything hooked up to the original set up and get the radio signal going again. Turn it down low so you dont pop a speaker and unhook until it stops. Test the cable that stops the signal.

    If it is coming into the house all the time then you are in an area where this can be a problem. If it only happenes at night then you are in an area that is only effected when the station rotates its signal for nighttime broadcast.

    This will require a small transformer as a supplemental power supply and protection device. A plug-in type.

    Look up the solutions with RFI problems. a lot of sites dedicated to this problem and a lot of gear available.
  5. RemyRAD

    RemyRAD Well-Known Member

    Unfortunately, there is a lot of equipment out there that is rather sensitive to RFI. Do you know if you are near any radio/TV broadcast towers?

    Here in the Washington DC area, all of the major television stations and most of the major radio stations are all in very close proximity to one another. Radiofrequency interference is a huge problem for people that live in and around this vicinity. This problem doesn't come just from crappy cables or improper connections. It penetrates everything. And sometimes there really isn't any cure, short of an expensive process of shielding the entire room and/or mounting the equipment in a mu metal shielded box. And that costs big bucks and will probably weigh you down quite a few pounds.

    We had a prominent recording studio here in the Washington DC suburbs. It was in the same shopping center area as a daylight AM religious radio station. This studio had an API console and Ampex MM 1200-24 machines. This equipment was all the best of the best and the studio for many years was plagued with this damned radio station of which, they could never get rid of the interference. So you might be facing the same problem? And there is no fix except to move. That studio moved eventually, 25 miles away, which finally solved the interference problems. And that was with a $50,000 console and a $36,000 multi-track machine. And that stuff is already, well shielded.

    One of the other items that can be used to help squelch RFI is the use of ferrite traps. These traps have to be placed around Cables, very close to the connectors. They are bulky and a hassle and they're not cheap. Especially when you need a whole bunch of them. So you should just start with a few on a couple of cables and give it a test.

    This may not have a happy ending?
    Mx. Remy Ann David
  6. kmetal

    kmetal Kyle P. Gushue Well-Known Member

    a dude i work w/ had to rebuild his high end commercial studio including a Faraday cage to get rid of interference from a high powered local radio station in Rhode Island. (92 pro fm)

    his breakdown of it was basically you need to use 'chicken coop wire' all around the walls/floor/ceiling, somewhere in between the layers of drywall, and flooring. he stressed it needs to be connected to itself, as if it were one surrounding.

    this stuff is beyond my current (no pun intended) knowledge, but i figured i'd mention it.
  7. RemyRAD

    RemyRAD Well-Known Member

    Yup, I was thinking about including the suggestion of getting chicken coop wire fencing. Yes it not only all has to be connected together, it also has to be grounded to a great big grounding post.

    Of course that kind of fix can be wholly impractical. However other alternatives can also be tried. So while you want to completely surround and enclose your surroundings with this shield, you may in fact be able to get away with something scaled down from that? You may be able to build that chicken wire fencing, mounted to the inside of the racks in which your equipment is mounted in or placed. And where the front controls are available and not necessarily behind the shield. This may be all that's necessary? But that really depends on how high the RF field is in which you are in? I ran into a similar but different scenario from this at NBC radio back in 1982. Tape recorder heads on some brand-new machines were picking up hum on the MCI JH-110's. This was getting into the playback head. The playback head had a single facial shield but nothing else around the bottom or sides of the head. Playback heads and other professional recorders frequently had the head mounted in this small mu metal box. So, MCI sent us new head cabling. But that was not the problem as I told them. I told him this had needed a full mu metal shield. They told me I was out of my mind and sent to technicians up from Fort Lauderdale to look at these machines. Of course that was not the problem. So then they sent me a couple of pieces of mu metal. But these had to be formed around the head. So I just bend them into a U, shape, and slid them around the sides and bottom of the head but not the back. That was the trick! Even the chief engineer of NBC-TV yelled at me telling me that the mu metal had to be re-annealed (a heating process). Because it won't shield anything where the metal has been bent because it affects the molecular structure of the metal. But I didn't do any of that yet it still worked perfectly, 100%. MCI had told us that these machines didn't need head shields and I just happen to prove them wrong again. Then they made a whole bunch of custom shields that they supplied to our station for me to install on all of the other machines LOL. So this is where theory collides with practice. One is the right way. One is the wrong way. And the wrong way still worked. That doesn't mean it will always work. So put your thinking cap on and try to picture how you might be able to shield the offending piece of equipment? It may only be a single piece of equipment you need to do this to? It might be all of it? RF interference is nasty to deal with.

    It's an imperfect world we live in.
    Mx. Remy Ann David
  8. Studio 9

    Studio 9 Active Member

    Wow. All of this information has been super helpful. I was playing with it last night and it seems as though the mic cable is acting as an antenna. I moved it around different positions near the mic stand and around the room and each position either changed frequencies, picked up another station all together. one area i found to be a dead zone but its about a square inch perimeter hanging in the air. lol. I think i'm gonna play with different cable routings to see if i cant disperse the signal and kill it before the mic picks it up.
  9. RemyRAD

    RemyRAD Well-Known Member

    In a similar scenario to this, other problems that have been experienced even in well-built studios is a guitar picking up and delivering an inordinate amount of hum. Frequently, because of the way guitar pickup works, it's very sensitive to stray electromagnetic fields. And even in the finest designed environments, there may be power cabling under the cement floor? And the cure is to simply have the guitarist rotate 90° or so to null out the electromagnetic hum field.

    In that scenario with the MCI JH 110 analog tape recorders that NBC had purchased that were plagued with hum, I surmised there had to be a high current power feed under this together control room. I was told there wasn't any power cables under the floor. Blueprints were pulled out to prove that. But I was able to verify that there must of been a power cable by taking a spare, trashed tape recorder head. I stuck that head at the end of a yardstick. I wired it up to a simple little amplifier. I proceeded to walk around this control room waving this stick with the head at the end of it just like you see those treasure hunters down at the beach. Yup, I was able to find that trace line, right through the cement floor. Then another blueprint was discovered that verified my find. Because those tape decks without that shield I created would only work properly if ya rotated the 90° on the floor. And that was not a practical thing to do for this particular control room as it but the machines all in the wrong direction to the engineer. And so I had to incorporate that shield that I created because I couldn't move the power cabling and we couldn't move the machines. And that's how I go about my wacko way of solving problems. It's all based on instinct and really not on education. The education is important. So is the abstract thinking. A lot of this is simply not logical. But nobody said life was fair right? Right. I mean I think life paid a pretty big trick on me? And some people here might be able to surmise what that trick was that was played on me? LOL. I'm not tellin'.

    The rain in Spain falls mainly on the plain I mean plane. Where's the TSA?
    Mx. Remy Ann David
  10. kmetal

    kmetal Kyle P. Gushue Well-Known Member

    we had some nasty nasty buzz on guitar/bass amps and it increased as i turned up the gain on the amp. the work-around was to as Remy said, make the player rotate until the buzz went away, and keep the gain pretty and find creative ways to add it later.

    this was particularly embarrassing in a professional situation, but not as embarrassing as having unnecessary/unwanted hum.

    about 8 months ago it was discovered that the power transformers were discovered to be "leaking", not exactly sure what that means, i was told that it can cause some serious problems in audio equipment, like hums and buzzes. The power transformers were replaced and we have not have any such issues since.

    it may just be a coincidence, but i'm not sure that it is.
  11. RemyRAD

    RemyRAD Well-Known Member

    It's really amazing how many guitarists get up set and their noses out of joint when ya tell them to sit down and rotate. LMAO. No really! It's happened to me. On numerous occasions. And then of course you have to excuse yourself and rephrase.

    Some have responded in vile ways. LOL.
    Mx. Remy Ann David
  12. rmburrow

    rmburrow Active Member

    Studio 9: Copper screening is available to screen a room in a high RF environment. At least two radio stations I am personally familiar with have copper screened studios since the 20 kW/5 kW AM transmitter and antenna is in close proximity (Northern VA). Hopefully, your situation won't come to that.

    First, can you hear the call letters of the radio station in your equipment. If you do, the interference is probably from a AM station and the interference mode is audio rectification.

    Can you look out your window and see any tall towers? Do you see any nearby towers closely spaced in line or in a rectangle? This may be the antenna array of a AM directional station, and the antenna pattern and/or power levels are likely to be different day and night. This station may possess an instrument called a Field Intensity Meter ("FIM") which is used by the station to monitor the directional pattern(s). This instrument can provide the signal strength (mV/m or V/m) at your location. If you hear the call letters of the radio station in your equipment, call the station and ask for the engineer. The engineer may be part time so you may have to leave a message. Tell whoever takes the message that "I am receiving interference from your station in my equipment and I would like a call from the station engineer". Nothing else. When the engineer calls, tell him what is going on and ask if he has a field strength meter to make a signal measurement at your location. The magnitude of the signal will determine what you need to do to get rid of the interference. Most of the engineers will cooperate with you. There are a handful that won't, but you may find most of the uncooperative ones with larger corporate chains with an attitude. (That is why there is the FCC!)

    If your equipment is located within the 1 V/m contour (1,000 mV/m) of the AM station, this contour is considered the "blanket contour" and AM interference to electronic equipment should be expected.

    Have you tried an isolation transformer on the AC power to your studio gear? RF carries well over the AC mains, especially with overhead service. The equipment grounding prong, the AC neutral (typically grounded at the electric service panel) often cause a "ground loop" and the isolation transformer may help break this type of ground loop and still maintain equipment safety ground. Are any of your cables (or the sum of the cable lengths) anywhere near a quarter wavelength of the AM station's frequency? 1/4 wavelength at 1 MHz is 245 ft, and at 1600 kHz, 1/4 wavelength is 154 feet, so you may have a resonance in AC or equipment cabling and not know it. Unshielded monitor speaker leads may be another RF entry point. Have you paid attention to single point grounding, and removed any other troublesome ground loops?

    For your information, there are three AM stations licensed to Lexington KY. They are WVLK (590 kHz 5 kW day, 1 kW night, different directional patterns), WLAP (630 kHz 5 kW day, 1 kW night, different 4-tower directional patterns), and WLXG (1300 kHz 2.5 kW day (omni pattern), 1 kW directional night (3 towers). Before making any calls, be sure you have the right station identified as the interference source.

    Interference from FM and TV transmitters may require other approaches to eliminate the interference. Identifying the transmitter is the first step. If you have a GPS, get the geographic coordinates of your location (in NAD27 CONUS datum) off the GPS. You can get coordinates for your location off Street Atlas map software in degrees, minutes, seconds N Latitude, W Longitude. Take this information and go on the FCC web site (Home | FCC.gov) and search for AMQUERY for AM, FMQUERY for FM, or TVQUERY for TV. Put in 10 km (6 miles) for the search distance. The output will identify transmitters within the specified distance of your location. For blanket interference work, the distance to the transmitters will be much less than 6 miles but it is good to know the radio/TV stations operating nearby.

    Good luck...

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