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Noise....any thoughts??

Discussion in 'Recording' started by Cucco, Jun 14, 2007.

  1. Cucco

    Cucco Distinguished Member

    Okay, so here's the deal -

    I'm recording a chamber music festival this week and I'm having a terrible problem with noise. Specifically, I'm having a problem in that I'm picking up a spanish radio station as clear as a bell through my recording rig. Here's the problems -

    1 - Location = church in the middle of a mildly heavily populated area. It was built over 200 years ago. None of the outlets are grounded and I can't find anything that will act as a suitable ground so I've had to lift it.

    2 - I have to live with relatively long cale runs.


    Here's what I've done -

    1 - tried replacing cables using different shielding methods. (no luck)
    2 - tried placing a better power conditioner in line (no luck)
    3 - tried running off of battery (no luck - indicating to me that it's not the power)
    4 - Made sure all cables are straight with no loops (no luck)
    5 - used a 3 foot cable with and without load on the preamp (no luck)
    6 - used NO cable and simply turned up the gain on the preamp (no luck - Millennia and Grace)


    Any ideas?

    I already recorded a portion of the concert on Tuesday. I have more to record tonight and Friday.

    BTW, I recorded in this church around Christmas and had the exact same problem.

    Thoughts, suggestions, ideas???

    All are welcome!

    Cheers,

    J.
     
  2. Kev

    Kev Well-Known Member

    long shot
    but
    can you try a battery driven system
    car or truck battery then an inverter

    at least it will seperate you from local ground and power

    the inverter may produce a new noise problem
     
  3. Cucco

    Cucco Distinguished Member

    Hmmm...tried that already (it's above in the original post, but perhaps a little buried.) I used a UPS and simply unplugged it.
     
  4. Thomas W. Bethel

    Thomas W. Bethel Well-Known Member

    Some where some how you have a connection that is acting as a diode and it is a radio reciever. I think it is probably a cable that is not soldered correctly or is what is sometimes called a "rosin joint" and it is not making contact correctly.

    A couple of years ago while recording a children's choir I was picking up the local C&W AM radio station quite loudly on the microphone inputs. We tried EVERYTHING and it was still there. Later on I found one of my microphone cables with a badly soldered FACTORY connector and that turned out to be the problem.

    Best of luck those things can drive you C R A Z Y in a hurry!
     
  5. Cucco

    Cucco Distinguished Member

    That's a brilliant thought Tom! Thanks. The funny thing is, there is (at this point) only one cable in common with the last gig which I did in the same location where I had the same problem. It just so happens it's my main snake. However, I can yank it in a hurry and see if that fixes the problem.

    I'll let you know.

    If it does, I owe you BIG time!
     
  6. Cucco

    Cucco Distinguished Member

    unfortunately, the problem is still here. I replaced every cable 1 at a time with nothing else plugged in. no luck.

    is it possible with all of this stained glass, lead paint and ungrounded wiring that I'm just doomed?
     
  7. BRH

    BRH Active Member

    So, if you tried sans cable and still got the RF, wouldn't you think it's going right into your pre?

    So, no cable, but mic plugged into pre, is this what you are saying?
    Try different mics. Maybe it's coming in right thru the shielding around the capsule.
     
  8. Cucco

    Cucco Distinguished Member

    I'm actually picking it up with no mics at all and no cables (did not figure this out til 5 min before the downbeat).

    I'm on location right now and the good news is that, with the cable change outs and the gain set as it is, I'm gettint the noise at -68 dbfs at max so it's likely to go mostly unheard on the cd. radio may be different given the limiter factor, but we'll see. fortunately npr doesn't push the limiter too hard - at least not around here.

    sorry for the bad typing...on a pda.
     
  9. Link555

    Link555 Well-Known Member

    Ok here is werid thought you say the the outlets are not grounded correct?

    So is there a decent copper pipe around? Maybe you could connect your EARTH connection of your set up to this pipe. A simple jumper wire will confirm.

    Floating grounds are tricky but my guess your shileding is not connected to earth, which means the noise has no where to go.
     
  10. Cucco

    Cucco Distinguished Member

    I did actually try that. fortunately the church is being renovated soon since the only pipe I could find was a support beam for the railing and it was painted. I used my trusty leatherman and got a good bit of the paint off. later I was able to confirm that I did get a ground however later even still I found a grounded outlet (the only one in the old section of the church) and plugged in there. it doesn't appear to be a ground problem anymore.
     
  11. Link555

    Link555 Well-Known Member

    Nice work, but Damm....if its not a ground problem and not a cable problem, is it your connectors on your gear? One time I had a mic that the ground tab liffted off when a customer knocked it over (DON'T ASK still bitter). It picked up RF really well after that. I opened her up and added some copper tape and the issue went away. Grasping at straws I know, but somewhere there is a bad connection.
     
  12. Thomas W. Bethel

    Thomas W. Bethel Well-Known Member

    Not right now put when you get some time put some ferite beads on each wire from the XLRs to the circuit board. They will STOP any RF. You can get them here http://www.palomar-engineers.com/Ferrite_Beads/ferrite_beads.html

    More about there uses here http://www.bytemark.com/products/ferbead.htm

    Hope this helps. I did this to all my microphone preamps AFTER the problem with the Children's choir. They are very inexpensive but work GREAT!
     
  13. Cucco

    Cucco Distinguished Member

    Thanks guys!

    I should mention I've only ever had this problem in this one location.

    I will check into the ferrite beads. I use some audiophile (snake oil) ones on my hifi speakers. they seem to work well...black is black through them - never a noise issue.
     
  14. Cucco

    Cucco Distinguished Member

    okay...something interesting now...
    it's intermission and I powered up the house mixer and the house mics. I'm getting the radio on their system too.

    also, I checked the voltage coming out of the wall and I'm getting between 106 and 108.

    I switched to a poeer conditioner...getting 120 now and the radio is all but gone (barely there).

    I'm frikkin stumped!!!!!!!!!!!!
     
  15. BRH

    BRH Active Member

    106-108 not good.
    Maybe it's time to put together an all battery rig...all DC.
     
  16. Link555

    Link555 Well-Known Member

    Just a quick note on ferrite beads. They typically act like LC filters usually in the MHz Range. If you select the right frequency range you can attenuate that frequency, not remove, but attenuate. Just like any filter.

    From what your saying there is noise on the power lines, the hot and neutral. The low line voltage isn’t necessarily directly related to the noise problem, it sounds more like the neutral line (or possibly the hot) in the outlet may have some parasitic impedance, just like a cold solder joint on a mic cable. That building may have very old wiring that has corroded. The power conditioner helped attenuate but not remove, so the filtering that the power conditioner provided helped attenuate the signal. Add two in series and you might reduce the noise more.
     
  17. Boswell

    Boswell Moderator Distinguished Member

    If this is breakthrough from an AM station, then small ferrites aren't going to make a lot of difference. You need quite big lossy ferrites to do much in the 1-2 MHz range.

    I remember having a similar problem a long time ago and traced it to RF being picked up on the headphone leads and being injected back into the mixer I was using at the time. If I monitored from the recorder output rather than the mixer phones jack, the problem went away. Your trouble sounds rather different if the house system suffers from it as well.

    You don't say if the amplitude of the breakthrough is affected by the position of the gain trims, just to get an idea of whether the injection is happening at the pre-amp inputs or later. Experience from designing for EMC immunity would suggest taking a thick ground strap (preferably braid) between the chassis of the pre-amps and the ADC. You have to try to make all the bits of gear one big Faraday cage, so the signals are sent from one to the other via screened cables acting as pipes with a surrounding low impedance path for the grounds. An actual connection to mains earth will not make a huge difference in this case.

    Good luck!
     
  18. BobRogers

    BobRogers Well-Known Member

    The subject on the "radio" isn't your immortal soul by any chance? ...mysterious ways....
     
  19. Thomas W. Bethel

    Thomas W. Bethel Well-Known Member

    If this is a really old church they may have power problems that you are not aware of and cannot simply fix. They may have some outlets that are not wired to code specs or they may have an overloaded circuit feeding the main hall.

    We did some recording in a small church and every time the organ started up the power dropped from 115 VAC to about 108 VAC. Later in the day when we turned the lights on so the performers could see the line voltage dropped to about 105 VAC so the church was UNDERPOWERED to say the least.

    We also did a recording session in a church and we were having all sorts of noise problems. I finally got very curious and pulled one of the AC power outlets to see how it was wired and got the shock of my life (thankfully not literally). The outlet had one wire running to the outlet and the neutral and ground were tied to the box. They were using the conduit for the return/ground/neutral and this was done, according to the rector, to "save money". How they ever got passed the electrical inspection is beyond me. Then there is the time that we were given a feed from a wall outlet out of reach to us and the extension cord was wired up backward so that the neutral and hot were reversed and for some reason our equipment went bonkers......Not fun!

    We have started carrying a TOPAZ ultra isolator transformer with us on remotes. This seems to knock out a lot of hash that gets onto AC lines from light dimmers and any RF that is present on the AC wiring. It is heavy but well worth the extra weight.

    Try the ferrite beads and if you can carry a TOPAZ transformer with you.

    Best of luck!
     
  20. Cucco

    Cucco Distinguished Member

    Hey Boswell -

    You're right, I didn't mention whether it's affected by gain. Yes - they higher the gain, the higher the noise - indicating that the noise is at or before the preamp.

    I'll try the hard grounding.
     

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