Hi Fi Speaker Humm when connected to a amp?

Discussion in 'Home Cinema Speakers' started by nearly50, Jan 9, 2008.

  1. nearly50

    nearly50
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    I run a DJ/Karaoke programme from my laptop down to a Skytronic Amplifier for home parties. I run the sound from my speaker socket on my Laptop via a 1 x jack to 2 x RCA cable to 2 jacks on the back of the amp. (The video goes straight from the laptop SVideo socket to the SVideo socket on the TV).

    Out of the back of the amp I then connect my two normal home hi-fi speakers and the wires connect directly to the amp by pushing the wires into two spring loading connections.

    What I am trying to fix is the humm that I get through the speakers when connected to the amp.

    I thought it might be the wireless microphone pickup box that plugs into the amp at the front so I disconnected it, but the humm is still there.

    Has anybody any suggestions that could stop this humm please

    Cheers
     
  2. Mark.Yudkin

    Mark.Yudkin
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    Ground loop (is it 50Hz)? If so, we're going to need a lot more details on the hookup. Do you have cable / satellite TV / radio? If so, disconnect all antenna cables to see if that's the cause.

    Basically, you have to disconnect absolutely everything, then reconnect one wire at a time until you have the cause. Laptops are normally not the issue (as unearthed), but can be.
     
  3. BlueWizard

    BlueWizard
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    Have you tried it with the laptop running on just the battery. Then try it again with the laptop running from an external power supply.

    Also make sure the cables from the laptop to the amp are shielded. Most likely they are, but they could be picking up noise from the air if the shield isn't that good.

    I'm not sure what the electrical plugs are like in the UK, so this may not fit, but sometimes, if you have electrical plugs that are not polarized, it's simply a matter of turning them around.

    Also, you could try making sure that all the equipment is plugged into the same circuit.

    Next, who much wire are we talking about here? How long is the cable between the laptop and the Amp. If it's 3 or 4 feet that's probably not a problem, but if it is 6 ft or more, then it could be a problem.

    It is a tough problem to fix by description alone. To some extent it is like Mark said, remove various connections, one by one, until it stops, then you at least have some idea of where to start to fix the sound.

    Sorry, that probably doesn't help much.

    Steve/BlueWizard
     
  4. Mark.Yudkin

    Mark.Yudkin
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    UK plugs cannot be rotated (except when using two-pin Europlugs through an adapter - mostly shavers have this). However, UK equipment tends to be earthed where continental and US equipment would not be.

    Nearly50 is going to have to give us more information. Long interconnects shouldn't cause a ground loop (RF interference is something else) - I'm running 10m between the TV and the amplifier (shielded of course). Long antenna cables should have ferrite clamps. But let's get some info from Nearly50 first.
     
  5. Nobber22

    Nobber22
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    It's the laptop. I've tried half a dozen different models from work (Tosh, Dell, IBM, HP, etc) and they all put out a nasty hum from the headphone/speaker socket. Possibly caused by the fan & the lack of space inside the chassis for proper current isolation.

    You'd be better off using a PCMCIA sound card or a USB pre-amp.
     
  6. nearly50

    nearly50
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    :clap:Many thanks for your help, and it would appear Nobber22 has provided me with the answer. Now for the solution......

    This is probably the only thing I haven't tried isolating, and as soon as I played the laptop on battery power without mains the hmm became almost silent.

    Many thanks for this info.

    Nowthen, I only do this as a home DJ at my own parties and friends houses, so I don't do it for money (as a mobile DJ), and I have already spent a bit to get up and running.

    Therefore I need suggestions, (cheap ones if possible) about how to overcome this.

    Are the two suggestions from Nobber22 the only solutions?

    My Laptop is a Dell Latitude D830 model if that helps. I notice the two suggestions are not cheap.

    The hmmm is only really loud and noticeable early in the evening when people are talking and the music is low, or no music on, and the speakers connected and amp turned on.

    Once the booze kicks in and I turn the sound up it is not as noticeable as the hmm gets drowned out by the music/karaoke.

    Failing an affordable solution, I suppose I could run the laptop on battery power in the early evening and then switch to mains power when the party gets noisy.

    Cheers
     
  7. Mark.Yudkin

    Mark.Yudkin
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    So we know it's a ground loop. A ground loop has to have two separate earthing paths. The laptop is one of them, but you haven't determined the other. Could you please answer my question on antenna cables, as if that's the other path, it's also trivial and cheap to fix with no change in sound or picture quality.

    Otherwise you're going to need one of these or these. Unlike fixing an antenna problem, this will affect the sound quality.
     
  8. nearly50

    nearly50
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    I have a TV connected to my laptop for Karaoke graphics via SVideo from Laptop to Svideo or Scart connection on TV which has an outdoor ariel connected (antennae)??.

    I also have a wireless microphone box which has two ariels that extract on the back of the pickup box so they pick up the signal from the two different frequency wireless microphones. This connects with a jack plug into the front of the amp (antennae)??
     
  9. Mark.Yudkin

    Mark.Yudkin
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    Does the hum disappear when you unplug the arial cable from the TV?
     
  10. nearly50

    nearly50
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    These are the variables:

    1. Laptop connected to amp - connected to speakers - when mains powered humming - when battery only - silent.

    2. Laptop connected to amp - connected to speakers - connected to TV from Laptop with S Video to scart - when mains powered and TV turned OFF -humming

    3. As 2 but TV turned ON increased very loud humming.

    4. As 2 running on laptop battery only with TV turned OFF - humming

    5. As 2 running on laptop battery only with TV turned ON very loud humming.

    Whenever the TV is turned on and is connected to the amp the humming goes up twice as loud as soon as the TV set is turned on.

    Unfortunately, without a TV we have no Karaoke because we need the graphics displayed on the TV set.

    I tried disconnecting the TV areial and running the Laptop on battery only but the humming is still very loud, so my previous suggestion of running my laptop on battery only would be no good, because I need the TV connection.

    Plugging each piece of equipment into different sockets had no effect either.

    A lot of information I know, but hopefully it might help diagnose the problem??

    Cheers
     
  11. Mark.Yudkin

    Mark.Yudkin
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    Just let me summarize to make sure I've understood:
    1) laptop + amp + speakers, no other connection: hum when laptop plugged in.
    2) Add TV without aerial to 1): hum regardless of whether laptop is plugged in.
    3) Without a laptop, there's no hum.
    Specifically:
    4) Unused devices and all other devices were physically disconnected from the amp
    5) All devices were plugged into the same wall socket (using a simple distributor).

    By the sound of it you have a ground triple (yuck!)

    a) Does the TV sound hum if you connect the plugged in laptop to the TV (no aerial) but not the amp (sound into TV if possible).
    b) Is there a speaker hum if you plug the TV (audio out) into the amplifier, with no laptop?
     
  12. nearly50

    nearly50
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    Mark,

    Points 1-5 all yes.

    A) No
    B) No.

    I must admit that I didn't realise that the TV further compounded the humm and made it worse. I thought it was just the speaker side of my Karaoke set up that hummed through the speakers.

    One thing though, when the TV is connected it doesn't matter if the TV ariel is plugged in or not, it still humms, and even louder when I turn the TV ON.

    Of the two connections from the laptop, one goes into the amp and gives a humm, (headphone on laptop to amp to speakers) and the other bypasses the amp, (SVideo from laptop to TV) but also gives a humm.

    So is the laptop the culprit as both connections start here?

    Am I correct in thinking that a Ground Loop Isolator between the laptop headphone socket and amp would only stop the humm for the speaker side of the set up but not the TV when I connect the TV to it?

    This is getting a bit complicated and would be loathe to spend money on something that might only kill the humm on one, but still be evident with the other.
     
  13. Mark.Yudkin

    Mark.Yudkin
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    Our analyses concur; you seem to have multiple unwanted ground paths:
    1) Amp -> Laptop -> Earth. Can be solved by using batteries or using a phono ground loop isolator.
    2) TV -> Laptop -> Earth. Can probably be solved by using batteries. I know of no S-Video ground loop isolator
    3) TV -> Laptop -> Amp -> Earth.

    Since the amp is what causes the hum, isolating the amp from the laptop may solve the problem (it should fix 1 and 3 and split 2): for that you'll need a phono ground loop isolator. The ground loop between the TV and the laptop should be OK as you're not connecting these up to the amp. Perhaps you can ask your friendly local dealer if you could borrow a phono ground loop isolator?
     
  14. nearly50

    nearly50
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    I can buy a phono ground loop isolator pretty cheaply, so will give it a go and let you know how I went on later in the week. If it doesn't work, I may be able to return it for a refund/credit note and have to either re-think, or just put up with it, (annoying that it is).
     
  15. Mark.Yudkin

    Mark.Yudkin
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    Don't skimp too much, or it won't work all that well. Unlike using a coax ground loop isolator for an aerial, here you're filtering the 50Hz signal that is also part of the audio band.
     
  16. nearly50

    nearly50
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  17. Mark.Yudkin

    Mark.Yudkin
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    The Maplin may work. I wouldn't consider isolators made for cars - cars are not earthed to power points (at least not while driving) so the problem is different - but that doesnt mean it wouldn't work.

    Trouble is we live in different countries and most of these cheaper components tend to have very local distribution. The result is that I can't actually say which phono isolator available in your country will work best for you as I have no experience. That's why I suggested:
     
  18. nearly50

    nearly50
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    :clap:Hi Mark,

    Been away, hence no update.

    I am pleased to say that the Maplins ground loop isolator works fine and only cost £9, (which happens to be the cheapest option as well).

    All references are for using this in a car, yet it kills the humming if put between the laptop and amp. It even kills the TV humming as well - all whilst running the laptop on mains power.

    Thanks for your help in sorting this out.
     

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