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RF Noise - a big problem?

Discussion in 'Recording' started by Jeemy, Feb 15, 2005.

  1. Jeemy

    Jeemy Well-Known Member

    Hi,

    When I opened my studio about 8 months ago, I noticed a very small problem - or so I thought.

    When recording an acoustic guitar using a pickup, direct into the desk in the control room, you heard a repeating, high-pitched, chirping noise.

    So I just decided not to do it! However after micing and recording a few bands, I noticed that when you connected a bass guitar to an amp in the live room, via a Tech21 DI box, which connected balanced to the desk in the control room, you heard the same noise.

    This time I fixed it by lifting the earth wire out of the power supply to the bass amp. Seemed to cure it, the earth still ran through the desk, so I checked everywhere thoroughly for surface voltages, and put a skull'n'crossbones on the plug.

    I then decided it was an earth loop problem and thought I would fix it by checking every fuse in the building and see if I could remove the offending circuit.

    By this time I was noticing it more & more - with keyboards and single-coil guitars.

    After a lot of trial and error with fuses which didn't work, I put in a UPS to give me some clean power. That didn't work so I tried a power conditioner.

    Finally I had the bright idea to try a battery-powered practice amp. Horror upon horror - the noise is still there! It seems to get worse with my test 'aerial' - a custom US Strat - when the guitar is horizontal. i.e. moving the guitar around makes it better or worse.

    Whatever the preventions, it looks unprofessional. I can remove it by careful placement of the keyboards and use of a balanced signal converter right by the keys. So its only single-coil guitars that are the problem. The trick at the top for the bass amp still works.

    And this is what confuses me - if its an RF noise, what possible reason can there be for the bass amp trick working?

    And if it is an RF noise, what do I do - wrap the building in foil?

    We are looking for a temporary solution here - I am going to be moving to a purpose-built property where I will obviously need to build proper prevention in, but in the meantime I've not 9-12 months of my lease left before the building is demolished and turned into a shopping centre!

    Anybody can help I will be very grateful,

    J
     
  2. LittleDogAudio

    LittleDogAudio Active Member

    This sounds more like EMI, not RFI.

    I had a similar problem and found out that it was a large transformer going bad on a pole out on the street. In fact, it wasn't the transformer that fed our building, it was an older one that was feeding the building across the street. Apparently, the bad transformer was sympathetically transmitting EMI into our transformer.

    The bad Trans. was replaced and the EMI went away.

    I tried everything before we found the real issue.

    Good luck,

    Chris
     
  3. Jeemy

    Jeemy Well-Known Member

    I guess I'll speak to SCottish Power. Its their old offices I am in, and the wiring is atrocious. Was your noise similar?

    i.e. if it might be radio waves, might need a different approach?
     
  4. LittleDogAudio

    LittleDogAudio Active Member

    My specific problem was an incredibly loud hum when using single coil guitars. Way more than the usual. It also made it's way into certain preamp inputs on the console.

    When I've experianced RFI it was usually a radio station coming through the system. This was almost always a result of a bad mic cable or a shotty solder point somewhere in the signal chain.

    Chris
     
  5. assis

    assis Guest

    you have a big problem.
    you can try: to change the position of your equipment; to use expensive and short cables; one very good pre amp can help too.
     
  6. LittleDogAudio

    LittleDogAudio Active Member

    Nah, none of these things will help.
    Why would a different preamp help? It may make your hum sound nice and tubey, but it will do nothing for the issue.

    Get an experianced electrician or pay a line man from the electric co to come in and spec out the problem.

    Chris
     
  7. Jeemy

    Jeemy Well-Known Member

    i've got no hum, quite a low noise floor in fact. but the chirruping is infuriating.

    i can experiment with recording duplicate hum out of phase i guess. i just wondered if any clues in the bass amp thing might suggest a cure.

    will speak to a pro, cheers anyway Chris
     
  8. Dave62

    Dave62 Guest

    Are there computers in your system? I had a problem years ago with a hard drive in a sampler causing spurious noises through the elec distro system. It took a while to figure that one out. More recently I have had the odd noise pollute my Digi 002 Rack if there is a cell phone on within 50 feet so that can also cause problems. Make sure all mic wire is of hi quality i.e. braided shield versus wrap shield.
     
  9. Jeemy

    Jeemy Well-Known Member

    Hi,

    I am going to double-double-check, but I have tried everything.

    As a battery-powered amp still displays this, its not coming through the wires (I think, otherwise its doing both).

    I am gonna power down the entire building and try with the battery-powered amp. If not, I am gonna walk around like a troubador with the amp until I track it down!!

    But if any thoughts, like why the bass amp trick removes it, and how to replicate this 'trick' with keys and guitars, I'd be grateful.

    Thanks,

    Jamie
     
  10. Kev

    Kev Well-Known Member

    IF this is something leaking into the sytem via the ground ....

    A fully floating battery driven UPS just might give some relief.

    One cool test would be to get a section of the studio fired up off the UPS with the fault ... then remove the power and let the Battery UPS take over.

    If the fault goes away , you have the begining of a solution.

    IF this noise is ... EMI or RF ... and simply just TOO close to your building you could be in real trouble.
    As with acoustic noise, you can only go to the source and STOP IT.

    :(

    I have know of studios being forced to close down because of nearby building works that went on for too long.

    good luck
     
  11. Jeemy

    Jeemy Well-Known Member

    I've tried a power conditioner. Not quite a UPS, but no alt whatsoever.

    But surely if it is coming through a battery-powered amp nothing I do to the mains will affect it?

    I am wondering if tinfoil behind the walls would help......also extra ground wires on guitars, I could develop 2-core standard cables using 3-core with a long piece of the third core attached to metal parts.

    I just need a workaround, I am gonna purpose build in September, hopefully in an area with none of this - don't worry - I will check!

    J
     
  12. Kev

    Kev Well-Known Member

    The battery powered amp is a fine test and does show you have a problem.

    It does however involve a Strat and as indicated by the 'aerial' statement ... a problem of this nature is likely to appear on a Strat.

    Back to the UPS idea ...
    Do you have a battery powered Synth ?
    Does the battery synth and the batt amp have the same problem ?

    you could be in trouble :(

    Just as a mobile phone on an audio desk will make that noise and the solution is to throw the phone out the door .... :)

    Here, if you have an external problem you need to find the source and turn it OFF.
     
  13. Jeemy

    Jeemy Well-Known Member

    I know, doesn't seem like I can block it, does it.

    That said it is only Strats and keyboards it happens with, thats why I am using one as an 'aerial'.

    And mics at very high gain in the wrong place, it happens so slightly as not to be noticeable.

    I can move keyboards into the right place and use a 20cm cable and then convert to balanced and reject it completely.

    I just need a trick with Strats. Given the way guitar input voltages work i don't think putting a second blank signal with the RF out-of-phase will work to cancel it, it might muck the tone up too much.

    Will look into a battery powered synth.
     
  14. jdsdj98

    jdsdj98 Active Member

    I'm the last person to offer any expertise here, but I will say that from what you've said, it DOES sound like an RF or EMI issue, since you still have the problem with a battery powered amp, AND you say that converting keyboard to balanced connections eliminates the problem. That, to me, is as big of a clue as the battery powered amp. That's what a balanced connection is supposed to do when any noise or interference is inducted along the cable run. So, I'd say don't bother with a UPS or calling the power co. Your problem is somewhere in the air. Just my very cheap $.02 worth.

    Can you say with any degree of certainty if there are transmission towers/antennas of any kind within close proximity to your building?
     
  15. Kev

    Kev Well-Known Member

    The UPS idea was totally about the chance it was on the earth and or power.

    As has been said, it is likely to be in the air and due to proximity.

    Find the source and turn it off.
     
  16. Jeemy

    Jeemy Well-Known Member

    I shall carry the battery powered amp around the locality and sabotage whoever is doing it. shall also keep you posted.
     
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