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Desperate with RFI problem...HELP!

Discussion in 'Pro Audio Equipment' started by Violin Dan, Mar 30, 2003.

  1. Violin Dan

    Violin Dan Active Member

    I've been fighting RF interference in a PA system at our church. The offending station frequency is 1510 KHz (which corresponds to a 1/4 wave distance of 162.91 ft).

    The system is controlled from the rear of the sanctuary and shielded balanced cables run to mic jacks mounted on the floor various distances from the mixer. The cables terminate in XLR connectors on both ends, with the shield attached at each end. There is no earth or AC main ground at the mic end. The speaker setup is a distributed system mounted at the ceiling in 6 positions, three on each side. The building is about 10 years old with wood frame construction. The sanctuary is on the second floor.

    The RFI occurs with certain combinations of mic cables plugged into the mixer with three or four of them being the worst offenders. This can occur with or without a mic attached to the cable. Some introduce no RFI at all. At its worst the station can be heard loud and clear with no inputs turned up. At best, with all the offending cables unplugged, the output is clean. Often the RFI is only heard when the podium mic is up, and is worse with a high gain condenser like the AT mini gooseneck, and not so bad with a medium gain condenser like the Shure SM94, and non existant with a dynamic. It is not always present and is better some times than others. It seems to be the longer runs that offer the worst RFI, but none of them approach being a 1/4 wave on the offending frequency. I'm sure that they are all under 100 feet, but could easily be 50 to 75. The RFI can be introduced by simply touching the shell of the XLR to the mixer ground without plugging in the signal cables themselves. Shorting the signal cables to the shield at the mic end makes no difference.

    The system is powered off AC mains using the main ground as system ground. The breaker box is about 100 feet from the control booth, and could actually approach the 1/4 wave distance taking into account the routing of the cable run. I have no idea what else is on the circuit, if anything.

    I have tried the following solutions:
    1. Lifting the third wire ground from various combinations of power amp, mixer, effects etc. This only made it worse.
    2. Put ferrite beads on the ac to the power amp, on the speaker leads and the offending mic cables. Only slight improvement, the most helpful being on the power amp.
    3. Replaced mixer and power amps with new units. No difference.
    4. Unplug the offending cable runs. This works but defeats the purpose of having microphones in the first place ;>}

    I am considering lifting all the equipment from the AC main ground and running an earth ground which should be closer than the breaker box. It could still possibly be 30-50 feet. Other than that, I'm about out of ideas.

    Sorry this post is so long, but I'm about at my wit's end. I have important services coming up next weekend, so I hope that someone has tackled this one and emerged victorious and can share with me the solution.

    Tired of "Talk Radio 1510",
    Dan
     
  2. HiString

    HiString Guest

    Dan,

    What make and model mixer?

    :cool:
     
  3. Violin Dan

    Violin Dan Active Member

    The original mixer was an older Peavy, sorry don't have the model. The new mixer is an Avlex cmx24/4. The symptoms were the same with the old and new mixers.
    Dan
     
  4. HiString

    HiString Guest

    Hmmmmmm............I had a different make of mixer in mind that had an RFI issue.

    I'm clutching at straws, but have you looked for a common denominator between any offending leads...........similar lengths or multiples of a specific length...........OK, just re-read your post and see that you were thinking along similar lines. What about the brand of cable, could it be that a lower grade cable has been used for some of the leads.

    I think I'd be contacting a pro-audio company for advice.

    :cool:
     
  5. vpoulos

    vpoulos Guest

    The fact that you're picking' up A.M. is telling. A "detector" is necessary for A.M. I believe something in your system is acting as a detector.

    Do you have access to a scope? Connect a scope to one of the main outputs of the mixer and look for evidence of RF ...with and without the mic cables connected. Perhaps there's RF on the mixer at all times and the mic cables just exacerbate the situation.

    It certainly could be the type of cable used for the mics. I've seen that happen before. Have you experimented with ferrite beads on the mic inputs? It's worth a shot.

    I don't think it's an AC issue but anything is possible.

    Call the radio station and talk to their engineer. He/she might be able to help.

    Vince Poulos
    Speck Electronics
    http://www.speck.com
     
  6. lowdbrent

    lowdbrent Guest

    This console is manufactured by Inter M by the way. I hope that you didn't pay more than $1,000. That's how much I sell them for. Anyway, the console has had a good track record with RF.

    You said that you had your "shields connected at both ends." Is your drain wire connected at both ends? If so, it should only be on the output side of the cable.

    It is most likely a combination of the mic cables, just as other have said. Have you taken the "RF free" cables and tested them on each line of your snake? What condition is your snake in? Any little break in the shield will be a contributor.

    You may just have to get an inline RF filter ($50) and be done with it.
     
  7. Violin Dan

    Violin Dan Active Member

    Thanks for the insiteful replies. Any and all help is appreciated.
    I'll try to answer some questions:

    "The fact that you're picking' up A.M. is telling. A "detector" is necessary for A.M. I believe
    something in your system is acting as a detector. Do you have access to a scope? Connect a scope to one of the main outputs of the mixer and look for evidence of RF ..."

    I'll give this a try.

    "This console is manufactured by Inter M by the way. I hope that you didn't pay more than $1,000. "

    Got it direct from Avlex at their wholesale cost...

    "You said that you had your "shields connected at both ends." Is your drain wire connected
    at both ends? If so, it should only be on the output side of the cable. "

    The drain wire is connected to pin one and the shell on both ends of the cable. At the mic (input) end the only connection is to the mic as the connector is floor mounted in a box that is not earth grounded, so there is no chance of a loop here.

    "It is most likely a combination of the mic cables, just as other have said. Have you taken the
    "RF free" cables and tested them on each line of your snake? What condition is your snake
    in? Any little break in the shield will be a contributor."

    Again, there is no snake...these are individual cables run under the flooring when the sanctuary was constructed, using high quality cable designed for permanent installation. It is several of these long runs that are the most offensive. Others don't seem to be a problem, but often it is the combination of several cables plugged into the mixer that causes the worst problem.

    "You may just have to get an inline RF filter ($50) and be done with it."

    I have tried ferrite snap beads on the input lines but this has no effect. Are you talking about a filter inline with the mic cables? Sounds interesting.

    Additonal info:
    I found that touching the shell of the offending cable to the mixer ground would cause the same problem as sliding the shell off and attaching only the signal lines and pin one. This would only be of import if the mixer has seperate signal and earth grounds, which I don't know.

    Thanks again,
    Dan
     
  8. Violin Dan

    Violin Dan Active Member

    SUCCESS!!!???!!!
    So far so good....3 days without radio station with all the offending cables attached and the most problematic mic in the line.
    The culprit seemd to be the vintage Peavy CS800 amp. I had absolved it of responsibility because I turned it off and the RFI was still present. Well, it wasn't until it was entirely disconnected and removed from the rack that I succeeded. Must be a cold solder joint in the speaker output section or something. Will do a post mortem on it.

    In the process I learned a lot about grounding and the "pin one problem". For some good white papers on the subject look at:
    http://www.rane.com/library.html#rnotes

    Thanks for all your concern and suggestions.

    Pray for no talk radio in church.

    Dan
     
  9. Kurt Foster

    Kurt Foster Distinguished Member

    Is that CS800 running in balanced mode? You have to have a balancing transformer to balance those old Peavy's. This is a can that plugs into one of those sockets that looks like a tube socket in the back. If it doen't have the transformer can, it is not running balanced in spite of the balanced XLR connector. Kurt
     
  10. Violin Dan

    Violin Dan Active Member

    Kurt,
    Thanks for the input. I was going into the Peavy unbalanced anyway, but the transformers are there.

    I was a little premature announcing success...I still have a remnant of radio, but only sporadically. I have yet to come up with a theory for the intermittant nature of the problem, unless I'm getting mixing products from signals that are only on during certain times of the day. But the received station is always the same 1510 KHz I think. I have put ferrite beads on everything that I can find. I also dressed the cable runs a little better to cut down on crosstalk. I know that I'm playing with fire using balanced and un-balanced connections in the same system, but the main culprit still seems to be the long balanced mic runs buried in the floor.
    Dan
     
  11. Kurt Foster

    Kurt Foster Distinguished Member

    Dan,
    You should defiantly use a balanced input to the power amp. This is a transformer balancing scheme and physically de-couples the rest of the system from the power amp. In regards to dressing the mic cable runs, often by bundling line and mic level cables a more efficient "antenna" is created. It is sometimes better to have all the cables loose and apart from each other to aid in rejecting RF. You really should contact the radio station. It is strictly against the law for them to be broadcasting a signal that interferes with anyone else. Tell them you have tried everything and you are about to contact the FCC. Perhaps they will send out an engineer to asset you with your problem. Kurt
     

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