building a power amplifier

Discussion in 'The Projects Forum' started by williamthomas, Jul 29, 2013.

  1. williamthomas

    Thread Starter New Member

    Jul 26, 2013
    17
    0
    Hi..

    am building a power amplifier and i have some questions due to my design.
    You are welcome to comment any mistakes found in my circuit.

    1) i have a 10k pot to use as volume control, where and how should it be connected? in the input stage or?
    2) how can i calculate the correct resistors for the vbe multiplier, because i have a lot of distortions on the output.
    3) what should i take into consideration when setting the gain for the entire circuit.
    my power supply is +-24v.
    thanks.
     
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  2. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    I don't design amplifiers, but I do work on them. First thing that jumps out at me is R14. I think it's a bit too small The voltage across it needs to be 1.2-1.4 volts or so before the outputs can begin to turn on.
    There should be some form of adjustable bias using Q1 (not R14). Q1 will need roughly 4 diode drops (~ 2.4 to 2.8 volts) across it to start turning on the outputs slightly.

    The power transistors need to have a small base current to just turn them on or you will have crossover distortion.


    Can you please post your asc file and/or simulation results.
     
    Last edited: Jul 29, 2013
  3. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    One more thing. If you are using a +/- 24 volt supply, using 4 pairs of Mj15024/25 transistors is overkill. 2 pairs properly heat-sinked would be more than enough. I calculate ~ 70 watts at 4 ohms.

    How much current can your power supply provide ?
     
    Last edited: Jul 29, 2013
  4. williamthomas

    Thread Starter New Member

    Jul 26, 2013
    17
    0
    My power supply can provide 14 A for 2 secondary.. i have thought about adding a potentiometer with the vbe but i cant find a potentiometer in the SPICE library.
    i used 4 output transistors because i would like to add a more bigger transformer in the future..
    the asc file included.
     
  5. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    I have no argument against overkill. "Too many" power transistors will run at a higher current gain and lower temperature, reducing the demand on the driver stage and making their response to the the Vbe multiplier circuit more stable. You want to over build your amp? Go ahead.
     
  6. donpetru

    Active Member

    Nov 14, 2008
    186
    25
    I recommend you choose the amplifier input: a smaller capacitor C8 and resistor R28 higher. Then, the new value of R28 must be equal to R7. Then you will have to recalculate the value of C9 (which was originally 400p). The same should be done with resistor R8 (originally 160 ohm - must be higher).

    To minimize output distortion amplifier I recommend:
    1) to increase the value of R8 = 390 Ohm;
    2) to increase the values ​​of R9 and R10 to about 330 ... 470 ohms;
    3) decreased values ​​of R3 and R4 at 22 ... 100 ohms.
    Then, you must replace Q1, Q7, Q8 with a transistor in TO-126 capsule, such as BD139 or MJE340.
    After that, the resistor R14 is too small. I would try something in the range 47 ... 100 ohms.

    Amplifier gain is influenced by the ratio of resistors R7 and R8. Response at low frequencies is influenced by capacitor C2 and limiting high frequency capacitor is made by the 400p capacitor (I can not see the number of that capacitor. Amplifier scheme should provide more readable).
    Amplifier supply voltage could be even greater, that if not intend to use the amplifier output impedance too low.
     
  7. williamthomas

    Thread Starter New Member

    Jul 26, 2013
    17
    0
    i have tried what you wrote but it's still not working
     
  8. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    C'mon william, show us the schematic after modification so we can see whether you understood correctly. This is not a simple circuit that we can do verbally. It's basically a hand made, power op-amp, and they will lock up over the simplest mistake.
     
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