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[HELP]Audio power amplifier analysis

Discussion in 'Audio' started by foolah44, Nov 15, 2012.

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  1. foolah44


    Nov 15, 2012
    I have a pre-designed audio power amplifier to analyse and test. The circuit is attached.

    I have got several values but during test some of them were not quite right.

    My results

    differential (input) stage of about (-) 65.2 - that is for Q1
    second stage gain of about 1224 - that is for Q4
    feedback ratio, about 1/25 i.i 0.04
    Overall gain of about 26

    Input impedance of about 4.4KΩ
    Output impedance of less that 1Ω

    Low frequency limit of about 32Hz due to C2
    High frequency limit of about 133kHz due to C3

    at the output stage, we assume a load of 8Ω
    Maximum operational output voltage was about 3V peak (positive)

    which gave me an operational power delivered of about 0.56W

    Power supply is about 1W
    so effieciency gives me about 56%

    The maximum power dissipated at each output transistor was about 0.51W
    So heatsink not really needed.

    When testing my amplifier, The low and high frequencies limits were not right. I got about 25Hz and 4MHz for low and high frequency limits.

    Can you please help me check my results. Especially for high and low frequency limits, and also power analysis?

    Attached Files:

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    C3 may act to reduce the gain of the amplifier, but that capacitor is within the feedback loop.

    I think you'll find that the open loop gain may roll off as you expect, however the closed loop gain will hold up much longer (essentially until the closed loop gain approximates the open loop gain).

    The low frequency rolloff is done in the feedback loop itself, and I suspect the differences may be due to component tolerances or a slight miscalculation (I have neither done the maths nor built your circuit)
  3. FutureFuzz 26

    FutureFuzz 26

    Nov 15, 2012
    im not trying to step on toes here, but whats the point of having frequency respnse that high? most music takes place from between 25-10khz

    and why limit your self to 8ohm on the output, 4,2,1 and even .5 is pretty common in the audio indistry these days
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