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This is not supposed to happen

Discussion in 'audio' started by adamdea, May 23, 2016.

  1. adamdea

    adamdea You are not a sound quality evaluation device

    Ok I was delighted with my new Atc scm 40s which were installed while I was away last week.

    They have power and signal running together through a conduit, which I was a bit concerned about. This does not so far seem to be a problem, although I haven't had the chance to run a test yet.

    They are hard up against a wall but this doesn't seem too much of a problem. In fact they sound pretty great to me. I have put in a bit of correction from the antimode which I think might be slightly over correcting the bass- I will have to fiddle a bit further.

    So far so good. At night I noticed though that there was a funny noise coming out of one tweeter (if I put my ear fairly close). I couldn't hear this during the day. Probably the ambient noise level masking it, I mused. Could it be picking up from mains to signal cable on that side? Err no it seems because the noise is the same when I pull out the xlr in the back of the speaker.

    Now the installer has used very audiophile looking screened mains cables with a fancy iec socket. The power come out of one socket in the wall, coming out to a sort of block with two cables coming out and into a conduit on either side to the speakers. I did wonder whether the super duper screened cable might be the problem, if for some reason it was working as an antenna.

    Then after random fiddling, poking about and gentle swearing I somehow established that the funny noise from the tweeter comes out when I turn the lights on. Or rather one lighting circuit on. This circuit operates lights around the room, but most obviously an uplighter which is about three feet above the ATC in question. There is a similar uplighter above the other speaker (on the same circuit), but that speaker doesn't make any noise when the light is on. The lighting circuit is on a dimmer switch, but turning the dimmer doesn't seem to affect the noise. The switch is, however, very near to the one which does make the noise.

    Any suggestions as to cause or solution? I suppose I could try taking the light bulb out, powering the speaker from another socket using an extension lead (experimentally only- I would be skinned alive if I tried it permanently), and possibly try moving the speaker further out into the room (ditto).
  2. dogbait

    dogbait pfm Member

    What kind of lighting are you using?

    Any lighting tranformers or drivers fitted? Quality of drivers and trannies varies widely - some of the newer PWM ones especially can be quite noisy.
  3. mattgbell

    mattgbell Help the elderly and unwell!

    The finger of suspicion points at RFI from the dimmer switch, I'd have thought.

    Glad to hear the ATCs are performing well otherwise.
  4. adamdea

    adamdea You are not a sound quality evaluation device

    I'm not sure what's in that particular uplighter. I am at work but will investigate tonight. From memory that particular circuit is not the one operating the halogen low voltage bulbs but operates ordinary AC- will check though.
  5. adamdea

    adamdea You are not a sound quality evaluation device

    They do sound lovely. They remind my of my proac tablettes in clarity, but they seem to fill up the room. Department of Aesthetics is very happy too, even to the extent of telling me how great they sound.
  6. adamdea

    adamdea You are not a sound quality evaluation device

    I'm puzzled though as to why that particular switch makes the speaker squeak, when the others don't.
  7. AndrewM

    AndrewM pfm Member

    I think you are going to have to work through the possibilities methodically. The uplighter does seem to be the obvious culprit. Dimmable LED or long life bulbs can be quite noisy. If replaceable, try an older filament bulb instead. Also you could remove the dimmer unit and temporarily replace with a basic off-on switch to see if that's the source.
  8. adamdea

    adamdea You are not a sound quality evaluation device

    Good idea. I hate dimmer switches. Can't see the point.

    Still, equally I am a bit disappointed that ATCs can't manage to be dimmer switch proof.

    The other possibility which had occurred to me was that the routing of the wiring going to the light in question might somehow be responsible
  9. Purité Audio

    Purité Audio Trade: Purite Audio

  10. awkwardbydesign

    awkwardbydesign Officially Awesome

    There is some info here. If your dimmer is leading edge, then changing to trailing edge might improve matters.
    But dimmers generally are suspect.
    And I listen with the lights out, mainly, so don't have to worry. :)
  11. Eyebroughty

    Eyebroughty JohnC

    If not already suggested try the following.

    A) Change the uplighters around, (if possible) if the noise then comes out the other speaker it must be the uplighter.

    B) Change the speakers around with the uplighters in their current position, if the noise is then heard in the same way then it is not the speakers but the uplighter circuit/light itself.

    C) If nothing stops it and it is RFI getting into that speaker then don't use the uplighters.


  12. zippy

    zippy pfm Member

    I have a dimmer switch on an uplighter, and I can't use it at all when I'm using the hifi, not only because the dimmer switch itself makes a very loud buzz, but I'm sure it's very likely to cause RFI too.
    As with yours, the dimmer on the main lighting circuit doesn't seem to cause any problems.
    As awkwardbydesign says, they're all suspect..
  13. mattgbell

    mattgbell Help the elderly and unwell!

    Job done.
  14. Jim Audiomisc

    Jim Audiomisc pfm Member

    My impression is that you're dimmer is injecting RF back into the mains circuit. Having a 'screened' mains cable running to the speaker won't necessarily fix that as the crap is already being guided by the wiring. Indeed, a screen might help keep the RFI running along the wires. Its an example of the 'Russ Andrews flaw'. :)

    Dimmers are supposed to pass specs on RFI injection back into the mains. But there will be ones about that simple have CE marks stamped on them and the makers never bothered to actually test. And a duff solder joint or connection may ruin performance anyway as they can make nice harmonic comb generators.

    Because mains wiring and equiment PSUs are *not* 'matched' transmission and load systems as per standard transmission line theory, the amount of crap carried and getting into a device will vary with all kinds of variables. Quite possible that even plugging in something else changes the behaviour because you altered the RF 'circuits' between sound and speaker mains inlet.

    So as soon as the mains leads differ - length, type, one being coiled, etc, etc - you can find an effect on one speaker and not the other.

    If trying either RF filters and/or a ferrite 'block' it is usually best to try these near the source end.
  15. martin clark

    martin clark pinko bodger

    Although the mains leads are 'shielded', how about the interconnect leads?

    I bet the speaker signal input is where the noise is making itself felt.
    Even if using the balanced inputs, have you tried swapping the interconnects side -to side, and seeing if the noise follows (yes, channels will be temporarily reversed!) If it does, then the interconnect lead on the 'noisy' speaker may be worth taking a look at, to ensure that the screen connection hasn't broken internally in the connectors at one end or the other.

    Triac light dimmers are the worst kind of noise source for interfering with audio - lots of di/dt.
  16. Whaleblue

    Whaleblue Southbound

    Evidently not.
  17. sbgk

    sbgk pfm Member

    Separate the power and signal cables, that's just a lazy design.
  18. westsea

    westsea Retirement present

    Just a thought, I would be suspicious of the power and signal cables in the same conduit, (as well as the dimmer switches), particularly if it is a longish run. As a simple Engineer it is best avoided.
  19. AndyU

    AndyU pfm Member

    Dunno if one or more of these will help, but might be worth a try ..

    And it should be simple to mess around with some temporary, separated cabling to see if there are issues with the stuff buried in your conduit. Something in the back of my mind tells me you aren't supposed to mix signal and mains anyway from a regulations pov.

    Also worth phoning ATC - they're always very helpful ime.
  20. London Lad

    London Lad pfm Member

    Temporarily change the dimmer for a normal switch. 10 to 1 its the problem. they are evil when it comes to sending noise down the mains.

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