Two TDA7056A, one "negative" cable.

Discussion in 'General Electronics Chat' started by alan1975, May 9, 2012.

  1. alan1975

    Thread Starter Member

    Jan 1, 2012
    42
    0
    Hello,
    I have problem with two TDA7056A, one is used for left chanel and another to right chanel.

    Everything working fine when i'm using speakers what have separated wires.

    But sometime i need plug headphones, what have common ground.

    How safe can i tie together negative (-) output of TDA7056A ?
    When i short them, together, everything woork fine, but when i play loud not... it's make horrible sound... i have disconect them from each other and reconnect...
    Is there are any way to fix this ?



    Regards
     
  2. #12

    Expert

    Nov 30, 2010
    16,346
    6,833
    These are internally bridged amplifiers. I can't see any way to make a common ground for stereo headphones.
     
  3. alan1975

    Thread Starter Member

    Jan 1, 2012
    42
    0
    Hello,

    Quite sad, but thank You for answer.

    Regards
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    It is simple to connect headphones:
    1) Use a coupling capacitor connected to the positive output of the left channel TDA7056A amplifier and feed it to the left earphone.
    2) Use another coupling capacitor connected to the positive output of the right channel TDA7056A amplifier and feed it to the right earphone.
    3) Connect the common ground of the headphones to the amplifier's ground.
     
    #12 likes this.
  5. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    The value of the capacitor will depend upon the impedance of the headphones. If they are 32Ω, then you would need at least 250μF to achieve a 20Hz -3dB low frequency response. For a lower impedance the required capacitor value would go up in proportion. You can use a polarized capacitor with the plus side to the amplifier.
     
  6. #12

    Expert

    Nov 30, 2010
    16,346
    6,833
    Good answer. The output voltage will be half of what it could do with 4 wires to the headphones but that probably won't matter in this case. Strapping the speakers to your ears reduces the amount of power needed by quite a lot.
     
  7. alan1975

    Thread Starter Member

    Jan 1, 2012
    42
    0
    I run simple test following your idea, i connect 470uF, and... looks good.
    I will eatch pcb and then do more tests.

    Can You explain in more detail how select right capacitor?

    You say it's depend on headphones impedance, but it's hard to do this for just one type.
    Creating a project what will work ony with just one type of headphones look wrong.

    You said, at least. So what will happend if i use a quite big capacitor.
    For example 1000uf * 2 in pararell ?

    Is this make any change to sound quality ?

    Regards
     
  8. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The formula for calculating the value of a coupling capacitor includes the impedance of the source in series with the impedance of the load and the frequency where the output is -3dB (0.707 times).

    The formula is 1 divided by (2 x pi x R x f).

    If the capacitor value is too big then very low frequencies are passed and it might take too much time for the capacitor to charge.
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Too much time when? Are you referring to when power is applied to the circuit?
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    896
    If the coupling capacitor takes too long to charge when the power is applied to the circuit then the sound might be distorted while it is charging and the headphones might be damaged.
     
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