Headphone connection

Discussion in 'General Electronics Chat' started by keyboardcowboy, May 31, 2014.

  1. keyboardcowboy

    Thread Starter New Member

    Dec 31, 2012
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    I have this audio system with no headphone jack, I am trying to add one, but the speakers do not have a common ground, some sort of H bridge type configuration. I have the circuit from the service manual. can i get the ground for the headphone jack in the way drawn below

    [​IMG]
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Ha,
    Your service manual has D302 and D304 pointing the wrong way - hopefully they are installed correctly in the amp. If not, current would easily flow from 0v to the negative rail.

    I think you will be ok with your idea. However, you should add a current limiting resistor to your headphones. I don't know what your positive and negative rail voltages are and no idea of the impedence of your headphones so I can not give exact values. You should have no more than 0.08 to 0.10 watts to each headphone speaker. That is both for your hearing and for the health of your headphones.

    Just remember, your peak to peak voltage will be ground to positive rail since you are converting to half-bridge. You may slightly alter the filter network by using headphones and current limiting resistor. It is definately with trying.

    Ohms law works. if you have questions, post your partial solution and ask away.
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The diodes are indicated correctly. They are there to prevent negative voltages caused by inductive kick from blowing up the output stages.

    There is nothing to indicate that this system has bipolar power supplies. This probably is a class D switching audio amp, and we're looking at the output filters. Note that the only things connected to GND are filter components. Both channel "returns" (SPK_L- and SPK_R-) go to circuit outputs, not GND. Assuming a single +5V supply, Each + and - output pair are PWM digital streams, 180 degrees out of phase. The centerpoint of the audio at each terminal is Vcc/2, not GND.

    The connections indicated will work, but the + outputs now will have more loading than the - outputs. This might upset the amp chip, don't know.

    ak
     
    GopherT likes this.
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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  5. keyboardcowboy

    Thread Starter New Member

    Dec 31, 2012
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    I will be using a jack that will disconnect the speakers when the headphones are connected, won't that take care of the loading problem??
    The amp ic is marked NTP7500L, i am unable to find any documentation for it



    The problem is still the GND connection, where to get that from
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Speaker/headphone switching jacks usually are configured for a common GND, L, and R signals, not BTL outputs, so be sure to check that.

    ak
     
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