Preventing DC to pass through the speaker wires.

Discussion in 'General Electronics Chat' started by antonioqbelo, Jan 21, 2014.

  1. antonioqbelo

    Thread Starter New Member

    Jan 21, 2014
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    We have about 60 pcs tiny speakers strung up to 800+FT in our church Monastery. They were installed ages ago. Badly wired because the speaker wires are bundled with the 24 VDC wires. Changing wires will be prohibitive due to labor and cost. Can we use a Bi-polar capacitor in each speaker to block the DC in case the DC wires gets shorted to the speaker wires ?
    Powered by a high powered car amp.
    DC wires are about 3/4 " thick and the audio wires are #10 AWG or larger.

    Using matching transformers to block DC is also labor and cost prohibitive.

    Anybody have a suggestion other than replacing the wires and separating the 24 VDC wires ?

    What do you think about about using Bi-polar capacitors to block the DC ?
    Value will be around 100 uF @ 50 VDC to pass audio 400 HZ - 8 KHZ.
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    How are the 60 speakers wired? Are they somehow wired together?
    Do you know the wattage of the car amp?
    Did you try the 100μF capacitor?
    You can put two 220μF in series wired in opposing polarity.

    Depending on the car audio amp, it might already contain capacitors.
    Also depending on the circuitry of the amp an electrolytic capacitor might work if you get the polarity right.
     
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  3. crutschow

    Expert

    Mar 14, 2008
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    Bipolar capacitors at each speaker should work fine for that purpose.
    You might also consider a fuse or circuit breaker such as one of these.
    What is the speaker impedance?
    Do you need to protect the amp also?
     
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  4. MrChips

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    Oct 2, 2009
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    Some folks use incandescent bulbs in series as fuses.
    Once when I was setting up for a gig the amp went into feed-back mode and I saw a speaker light up. It scared the dickens out of me then I realized it was the bulb lighting up.
     
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  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Chickens? Ha!;)
     
  6. antonioqbelo

    Thread Starter New Member

    Jan 21, 2014
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    Thank you very much.
    We plan to use a bi-polar with 50 volts rating.
    Will the poly-switch be fast enough ? will it reset when triggered ?
    Speaker impedance is 11 ohms, some are 8 ohms.
    We definitely need to protect the amp too, we have blown 2 already.

    Prayers,
    Tony
     
  7. antonioqbelo

    Thread Starter New Member

    Jan 21, 2014
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    Thank you very much.
    Most speakers are strung in parallel wired together.
    Car amp is a very big Russian made ???? No markings made about 20 years ago.
    We have not tried the capacitor yet, the value of 100 ufd is just an estimate from what we have seen in cross-over or tuned circuits.
    We plan to use a 100 ufd @ 50 volts, but if they are too expensive, we will use 2x220 ufd.

    Thank you again.

    Prayers,
    Tony
     
  8. antonioqbelo

    Thread Starter New Member

    Jan 21, 2014
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    Your suggestion placed a smile in our faces.
    Reminded us when we were young a hundred years ago.
    I do not know if this will work with protection from DC.
    The voice coil might be finer wires than the bulbs filament.
    If the bulb blows, the speaker will be out of commission and replacing the bulb could be a hustle.
    Repairing speakers located even just 8' poses accident to happen.
    Would send old priests or nuns to the chiropractor, the least.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    Yes, the poly-switch should be fast enough to protect the speaker. They also reset automatically when the overload is removed.

    The easiest way to protect the amp is likely with an inline regular (not slo-blow) fuse. The current rating should be determined from the amp power rating using P = I^{2}R or I =\sqrt{P/R} where R is the amp load impedance rating (typically 8 ohms).
     
  10. MrChips

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    Oct 2, 2009
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    60 speakers wired in parallel to the output of the amp is going cause a serious overload problem.
    What is the impedance of each speaker?
    Wire the speakers in series/parallel combinations to give a net impedance that matches the recommended speaker impedance which is usually 8Ω.
     
  11. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Mr Chips is correct if the speakers are all parallelled to the speaker output of an amplifier.

    The proper way to distribute to many speakers, which would also protect against unwanted DC etc is to use a 100 volt line.

    http://forum.allaboutcircuits.com/showthread.php?t=66599&highlight=100+volt+line

    However since you are using pc speakers and supplying them with dc power they may have their own internal amps and you may be connecting to the line out.

    Please confirm what you are doing.
     
  12. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Sounds like your monastery needs to break down and hire a professional audio system designer or at least find a willing local to help you out on designing a proper setup. :(
     
  13. MrChips

    Moderator

    Oct 2, 2009
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    So let's understand the situation correctly.

    You have a very large monastery and you need to pipe sound to many distant parts of the facility. There are preexisting #10 cables which you would like to use in order to connect to small 8Ω speakers.

    As tcmtech says, this is not as simple a job as connecting a bunch of speakers to an audio amp. This requires some knowledge of audio distribution systems.

    There are many references available on the internet. Here is just one of them:

    http://www.rane.com/note136.html

    btw, you can see how a supposedly innocent question about DC blocking capacitors can turn towards a completely direction once you tell us what you are attempting to do in the first place.
     
    Last edited: Jan 22, 2014
  14. ian field

    Distinguished Member

    Oct 27, 2012
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    If your wiring is that dodgy - you may soon not have a building to put speakers in!
     
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