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My flower pot speakers have blown

Discussion in 'Home Cinema Speakers' started by la gran siete, Aug 8, 2005.

  1. la gran siete

    la gran siete
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    or at leat the midrange drivers seem to have done as all I'm getting is a very trebly weak sound from each speaker. My guess is that the supplied cables at 20 mtrs long and very thin indeed are the culprits. Cant see its the amp ( HK PS650) as its plenty powerful enough wih plenty of current too- (they drive my 4ohm Kef Duette quite comfortably) .. I never played them loud either so its a complete mystery to me?? :confused:
     
  2. Ian J

    Ian J
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    Welcome to the big wide world outside of General Chat :D

    20 metres of cable is a heck of a lot. Have you tried testing them using a "normal" length of cable just to see whether it is the long cable run that is causing you the problem.
     
  3. Troon

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    If it's weak enough that you suspect a blown driver, I'm sure the cable is not the problem. High-resistance cable may screw up the freq response a bit, but the mail problem is self-heating non-ohmic problems if you try and push too much current through them. If you're still seeing the problem at low volumes, it's not the cables.

    That's not to say 20m of thin cable is a good idea, of course...
     
  4. la gran siete

    la gran siete
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    They are out door speakers and as such I cannot help but have such a long length.What does such a long length do that is detrimental?
    The manufacturer did suggest that I should cover the cable with sylicone or ducting which I havent got round to doing yet( that in itself would be difficult because mywalls are covered in climbers etc). I suspect he'll probably tell me the that was the cause although he has offered to replace them with a one mtr lenght of cable attached which I could then join to my existing cable. The speakers are sealed units with the cable connected within.
     
  5. la gran siete

    la gran siete
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    "but the mail problem is self-heating non-ohmic problems"
    not sure what you mean by that. :confused:
    Anyway the good people at MJB audio :thumbsup: who make these Tubthumpers will be replacing them .I''m now wondering whether its worth my while replacing the cable with a thicker one. Maybe some that is rersistant to UV rays
     
  6. Troon

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    Yeah, I'm not surprised. I meant "main", not "mail".

    The power dissipated in any resistive element, like a length of wire, is equal to the square of the current times the resistance: P=I²R. In thin wire, a fairly small current will cause rapid heating.

    Most (all?) materials change resistance with temperature: the temperature coefficient of copper is about 0.4%/°C, so running variable high currents through the cable - causing it to heat and cool rapidly - will alter its resistance and make it act like a weak compressor.

    I doubt this is noticeable except in extreme cases though.

    However, that's not the main effect, I've just realized. Most speakers don't have constant impedance with frequency, and the resistive speaker wire forms a frequency-dependent potential divider. Not good.
     
  7. la gran siete

    la gran siete
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    MJb Are going to fit me a short length of thicker cable which they reckon will make a small difference. I will then try to match it and make the necessary attachment.They also told me that increasing the impedence was no bad thing anmd was preferable to decreasing it and overdriving the amp.They a;lso tell me the new speakers have much better drivers so we shall see.
    Must confess this is all rather confusing to me a humble non techie person
     
  8. Troon

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    That's because what they are telling you is wrong. Speaker cable should be as low resistance as possible. The speaker itself will have a very uneven impedance response, like this one, for example.

    If your cable has a constant resistance of 5 ohms connected to that speaker, at 200Hz, half the amp's output will go to the speaker and half lost in heating the cable (both cable and speaker at 5 ohms). Between about 66 and 82Hz, more than three-quarters of the amp's output will make it to the speaker due to the higher impedance. That doesn't make for a good frequency response...

    Overdriving the amp will only occur if the speakers they use are extremely low impedance, which is unlikely. If that *is* the case, compensating with crappy speaker cable is just bad design.
     
  9. roversd1

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    you could simply measure the impedence at the amplifier end of your 20m cable run, if its less than 2 ohms, change the cable.
     

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