Simple Audio Amplifier Help

Discussion in 'The Projects Forum' started by majbb, May 4, 2011.

  1. majbb

    Thread Starter New Member

    Mar 9, 2011
    I am trying to build a simple three stage audio amplifier, as shown in the attached schematic. The problem I am running into is that my prototype is giving significantly different results than my simulation.

    I am trying to drive 4ohm speakers, and have at least a maximum gain of 15, though slightly higher would be better. When I test my prototype the maximum gain I can get is about 13, rather than the 21 the simulation shows. Also, the prototype has much more distortion than the simulation.

    I was wondering if someone else had experience with this sort of thing and could point me in the right direction.
  2. k7elp60

    Distinguished Member

    Nov 4, 2008
    I think that the design has some faults. R4 serves no purpose. Based on the values of the resistors in Q1 circuit the gain of that stage is close to 100. I also think R12, 10 is way too low in value. I calculate the current thru it is close to 235mA. This assumes the Vcc is equal to 5V
  3. majbb

    Thread Starter New Member

    Mar 9, 2011
    I forgot to clarify a few things, sorry about that.

    R4 and R5 are being used to model a potentiometer, its currently at .01 to represent the potentiometer being turned all of the way to one side.

    While the gain of the first stage would usually be approximately, Rc/Re. In both my simulation and on my prototype I'm getting significantly less when under load.

    I set R2 low to try and decrease the output resistance of that stage, but I agree that it is drawing too much current, especially to be USB powered. I'm rather new at this though, so I'm not quite sure how to fix it.

    The source voltage is 5V, it is supposed to be USB powered.

    Any help on improving this design is appreciated.
  4. nigelwright7557

    Distinguished Member

    May 10, 2008
    I would move to a better design with a differential input, VAS and output pair.
    Its easier to control.