Darlington Audio Power Amp

Discussion in 'General Electronics Chat' started by Joster, Dec 2, 2013.

  1. Joster

    Thread Starter Member

    Jun 12, 2013
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    Hi Folks,

    I made a short video outlining my troubles getting this circuit going. Any tips would be awesome!!

    http://youtu.be/CdUqb_PwcHU

    Thanks,

    Joe

    Troubleshooting Class AB Darlington Audio Power Amp
     
  2. crutschow

    Expert

    Mar 14, 2008
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    3,232
    What is the bias current through the diodes?
    What type of diodes are they?
    Are you absolutely positive that everything is wired correctly including the transistor pinouts and/or interchanging the NPN and PNP (?) since it really sounds like something is miswired.
     
  3. Joster

    Thread Starter Member

    Jun 12, 2013
    95
    0
    Thanks for the reply!

    I'll have another look at the wiring tonight. I think I'll spread it out on the breadboard so I can make it easier to wire and to show the connections. The diodes are 1N4001. I'll measure the current through the diodes aswell. I would assume it would be the same as the current through R1? i.e. (Vcc-2.8)/R1? I suppose aslong as I measure a 2.8 drop across all four diodes I can assume that the current through them is efficient.

    Am I right about setting Vq to 1/2Vcc. If so, how is this done?
     
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Diode current is equal to
    Id = (Vcc - 4VD)/(R1 + R2) if we ignore Darlington base currents.
    And Vq = 1/2Vcc only if R1 = R2
     
  5. Joster

    Thread Starter Member

    Jun 12, 2013
    95
    0
    Awesome! Thanks! Can't wait to take another stab at this circuit tonight!

    If everything is working correctly what would the standby current draw be. I assume it would be a lot less than the 5 amps I'm drawing currently. Or would it be equal to the current through R1, D1-4, R2 plus yet base currents?
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    To be honest you should never try to build this theoretical circuit in real life.
    This circuit is a disaster waiting to happen. Firs add some emitter resistor.
    Re1 and Re2 = 0.1Ω ... 1Ω. Next replace D1 with a potentiometer 1KΩ and set the standby current in range of 0.1A.
     
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  7. studiot

    AAC Fanatic!

    Nov 9, 2007
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  8. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    Adding a pot is a good idea but you want to retain the diode. You need four diodes to help provide temperature compensation for the four transistor base-emitter junctions. In practice these diodes should be thermally glued to the transistor cases (two on each case) to improve this compensation effect.
     
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