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Amplifier died after working for a while!!!!

Discussion in 'Electronic Design' started by Himlam8484, Jul 17, 2007.

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  1. Your time? What about everyone else's time you waste?

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida

  2. Jamie belongs on alt.kooks

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  3. Eeyore

    Eeyore Guest

    Jamie is a clueless twat. He's certainly no design engineer.

    His comments are invariably some of the most idiotic / uneducated I've ever come across
    almost anywhere.

    The 'free energy' clueless fuckwits on the energy groups do outrank him however.

  4. Himlam8484

    Himlam8484 Guest

    Yes I have one, but I think the problem may be there. When the heat
    increases more and more then the amp die. Today i will make the board
    with the enough big.
  5. Eeyore

    Eeyore Guest

    Yes, that will happen.

    That means you probably haven't set VR1 correctly.

    You have *refused* to explain how you measured the 'quiescent current'. I
    suspect that means you know you made a mistake and think you can 'save face' by
    avoiding talking about it (an Asian attitude).

    Let me tell you, behaving like that will get you fired in the West. It's best to
    'own up' to your mistakes and learn from them.

  6. joseph2k

    joseph2k Guest

    The design is ok, but the execution must be dreadful (resulting in multiple
  7. Eeyore

    Eeyore Guest

    I'd say the design was barely adequate. Look at the bias voltage generator for
    example. Now tell me how many Vbes it's trying to thermally compensate for.

  8. Fred_Bartoli

    Fred_Bartoli Guest

    Eeyore a écrit :

    Care to elaborate please?
  9. Eeyore

    Eeyore Guest

    It's a Vbe multiplier type, yes ?

    So why does the trimpot adjust the multiplication ?

  10. Fred_Bartoli

    Fred_Bartoli Guest

    Eeyore a écrit :
    What does it multiplies?
    By which factor? Why?
    How many Vbe are to be corrected?

    Then what's the error order of magnitude, its tempco and finally the
    bias current tempco?
  11. Eeyore

    Eeyore Guest

    The Vbe of Q9

    By the factor ( R16 + VR1 / VR1 ) for most of its operation.

    2. Q5 and Q6.

    Eh ? Why are you asking ME ? Whatever the answer is, clearly it shouldn't be a

    Do you understand what I'm driving at ? I'm trying to point out a design defect to
    you. I was hoping you'd understand.

    Do you not know what I mean by 'Vbe multiplier' ?

  12. Fred_Bartoli

    Fred_Bartoli Guest

    Eeyore a écrit :
    Come on. That wasn't the question. Which ratio is it? It isn't set randomly.
    What you don't understand is that it is *not* a variable.

    To me? Oh that's kind of you. Thanks.
    What I understand is...
    that you don't understand how it works.
    A bit like shot noise :)
  13. Eeyore

    Eeyore Guest

    It answered the question " By which factor? " perfectly.

    It shouldn't be. Since you seem to having trouble understanding, let me spell it out for

    There are two transistors in the output stage whose Vbe need thermal compensation.

    Instead of combining the trimpot function of adjusting the bias voltage to the output
    stage (to adjust the quiescent current) together with the Vbe multiplier, the correct
    cicuit should have a *fixed* Vbe multiplier of 2 times and then a trimpot unrelated to
    that process to set Iq.

    Is that clear now ?

  14. Fred_Bartoli

    Fred_Bartoli Guest

    Eeyore a écrit :
    You aren't obliged to be stupid. The question was related to what is
    this ratio when the bias point is tuned.
    No trouble, thank you.
    Of course it is, as it was since the beginning.

    Now, the amp is biased at 75mA, which is 100mV IR drop.
    This is 1/13th of the 2 drivers' Vbe, meaning that the Vbe multiplier is
    14/13 it's 'perfect' 2 times compensating value (which is already BS
    since the tempcos aren't necessarily equal between the small sig
    multiplier and the power drivers).
    This means that the bias current will have a tempco =
    2x2mV/°C*1/13/100mV = 0.31%/°C.

    15%=11mA bias increase for a 50°C temperature increase.

    Wow! That's a real design flaw that makes, quoting your words,
    "the design barely adequate" and it sure is just "trying to thermally
    compensate for the Vbes".

    Sorry but this won't make the amp fail, and there're other things to
    complain about in this amp well before the Vbe multiplier.
    In fact it is perfectly adequate for this kind of simple amplifier.
    And at least they've not put the bias pot on the wrong side.
  15. Eeyore

    Eeyore Guest

    Your analysis is wrong since it makes a wild assumption about the actual setting of the bias
    pot and indeed whether there's even any thermal coupling at all.

  16. Fred_Bartoli

    Fred_Bartoli Guest

    Eeyore a écrit :
    QED: you're definitely an idiot.
  17. Himlam8484

    Himlam8484 Guest

    ok. That is such as you said. I measured it again and the result is
    near !
  18. Himlam8484

    Himlam8484 Guest


    I will face up with my mistake. Avoiding talking about my mistakes is
    not my way.
    I just read all message so late due to the difference of time zone.
    I measure quiescent current by measuring the voltage between two
    transistor, then calculate the current .

    And the current at the V+ or V- at that time( no load) is about 80ma.
  19. Phil Allison

    Phil Allison Guest


    ** ????

    0.075 x 0.66 = 49mV.

    ** Completely satifactory result - far more stable than needed or typically
    achieved for such designs.

    ** Only in the mind a raving paranoid, wog lunatic.

    ........ Phil
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