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Single ended audio amplifier why need DC block

  1. Apr 22, 2010 #1
    Hello all,

    This is my first post in this forum. Hope can get some constructive help from the community.
    I did some testing with some radios which is actually a double ended circuit (BTL), meaning it has the spk+ and spk- port.

    When I want to connect it as a single ended circuit, meaning the spk - short to ground, the audio is muted. The only way for it to work is by adding a capacitor in line with the spk+ which is the DC blcok. I search through the internet and found similar information saying that the DC block is required in order to work.

    Can I know why ? what is the reason behind?
    Thanks, appreciate the view and help.
  2. jcsd
  3. Apr 22, 2010 #2


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    Any DC current running through your speaker coil is wasted energy, for a start - it's not part of what you want to hear.

    Afaik, most amplifiers - even double ended ones - use DC blocking - or they will use feedback to eliminate any DC current through the speaker. Connecting one amplifier outlet via a very few ohms to ground will totally upset the DC conditions; the two outputs of a balanced output could be varying (in antiphase) between V++ and 0V (or between V++ and V--) and neither of the output halves is expecting to supply a lot of DC current to ground. Give the poor thing a break!:smile:
  4. Apr 22, 2010 #3
    Thanks sphiecentaur.
    The main reason for single ended connection, the audio amplifier sees a low impedance that is going to ground hence causing stress in order to provide higher current. Correct me if I am wrong :)

    For BTL, even without the DC block, the speaker will work, is it been cancel off by the inverting spk- ?

    This is a good information i manage to read through internet, just for comunity sharing :)
  5. Apr 22, 2010 #4
    I hope these were not old valve radios they can present lethal voltages.

    More recent, but still old, transistor radios usually had a transformer coupling to the speaker. In this case a blocking capacitor is not needed.

    If you are talking about muting at the speaker output jack, some of these automatically switch out the internal speaker when you connect an external one.
  6. Apr 23, 2010 #5
    its not old radio :)
    I am not trying to mute the speaker output jack, instead I want the audio. this can be done with the help of the dc block.
  7. Apr 16, 2011 #6
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