problem with designing an audio amplifier

Discussion in 'The Projects Forum' started by inkosi, Nov 13, 2014.

  1. inkosi

    Thread Starter Member

    Dec 10, 2013
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    Hi,

    I've made an audio amplifier on breadboard using this schematic (courtesy of circuitstoday.com):

    [​IMG]

    Before I made it I tested it on MultiSIM... this is how my circuit looks like (I replaced the audio in jack with a function generator and also TIP41C is a smiliar NPN transistor as 2N3053, makes very little difference)

    [​IMG]

    My output is amplified but the signal isn't of the same shape... why is that? The left pic is the input and the right one is the output.

    [​IMG]

    When I made it on breadboard it works fine but there's some static noise generated at the loudspeaker (I'm going to guess this is because I have a cheap 8ohm 0.2W one).

    But the main problem is the audio isn't really that loud. I assume this is because of the values of resistances/capacitors. I tried a few different values of resistances but it didn't make a difference. How would I calculate or know what values of the resistors/capacitors would give me a louder output here?

    Help would be appreciated.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    You are overdriving the input. What the amplitude of the input?

    Is the loudspeaker on a baffle?
     
  3. inkosi

    Thread Starter Member

    Dec 10, 2013
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    It was 2Vp. I reduced it to around 100 mVp and the waveforms are similar now. However now I've noticed the waveform is inverted. Is this normal? (it was also inverted when I checked my actual circuit on an oscillscope in the lab).

    The loudspeaker isn't mounted on anything. I just connected one input of it to ground the the other to the -ve leg of the 1000uF capacitor.
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Also the use of complementairy transistors will help.
    The tip41 is way off from the 2n3053.
    Better use the combination tip41 / tip42 or 2n5679 / 2n5681.

    Bertus
     
  5. crutschow

    Expert

    Mar 14, 2008
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    With a 9V power supply the peak voltage into an 8 ohm load is about 3V, giving a maximum output of about 0.6W. That's why it's not very loud. An increase in the supply voltage will allow a higher maximum power.
     
  6. inkosi

    Thread Starter Member

    Dec 10, 2013
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    Hmm. Yeah I increased the input voltage to around 15V and it's definitely a bit louder. The only problem now is the noise which right now I am assuming is because the loudspeaker is a simple, cheap one (that's my circuit with the loudspeaker on the left... I know it looks messy but I was just testing it there).
     
    Last edited: Nov 13, 2014
  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    A speaker alone will not produce much sound.
    When mounted on a board or box it will sound much better.

    Bertus
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    That is a reason why it sounds tinny, and not very loud.

    It is an inverting amplifier.
     
  9. MrChips

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    Oct 2, 2009
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    If the wave is inverted don't worry about it. Your ear wouldn't notice the difference.
     
  10. bertus

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  11. crutschow

    Expert

    Mar 14, 2008
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    A speaker will not produce noise so getting a more expensive one likely won't change that.
    What type of noise?
     
  12. inkosi

    Thread Starter Member

    Dec 10, 2013
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    I made a video to make it more clear (please excuse the hammering sounds, construction was going on next door :p)



    You should be able to clearly hear the buzzing/extra distortion as soon as I increase the audio... but when I lower the volume you don't really hear it. This happened with the MultiSIM circuit too (the waveform started to deform a little when the amplitude was high...), but I'm not sure how to fix this.
     
  13. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    At what input level starts the distortion?
    Is the distortion on the top or bottom side in the simulation?

    Bertus
     
  14. inkosi

    Thread Starter Member

    Dec 10, 2013
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    It starts at about 800 mV on the simulation. I took some screenshots showing the amplitude and the input (left) and output (right) waveforms.

    https://imgur.com/TXFPWWF,BPWbGsz,E1XBCrX,ap93jZ6 (shows 100mV, 250mV, 500mV and 1V).

    If you want I can send you the simulation file.
     
  15. bertus

    Administrator

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    Hello,

    The first image the output is correct.
    The second is already clipped, already to much signal at the input.
    The third and fourth are overdriven, a foldback at the bottom is seen.

    Bertus
     
  16. inkosi

    Thread Starter Member

    Dec 10, 2013
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    So I take it that it is a drawback of the circuit that the amplitude can't be too high? Otherwise distortion will occur?

    And if this is true can it be fixed by changing any resistor/capacitor values?
     
  17. MikeML

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    Oct 2, 2009
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    I think you are confused because this amplifier has quite a bit of gain (about 20). If you operate it on a 15V supply, the largest output signal that it could deliver without distortion is about 10V peak-to-peak. If the gain is 20, then that means that the largest allowed input signal is 10/20 = 0.5Vpp.

    What are you driving it from?

    Can you just turn down the input level until the distortion goes away?
     
  18. inkosi

    Thread Starter Member

    Dec 10, 2013
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    I just connected it to an MP3 player. It's not exactly a big problem but I was wondering if I could keep the input high without the distortion occurring.
     
  19. MikeML

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    Output from a Mp3 player is likely to be about 1Vpp or more. You need a volume control between the output of the MP3 and the input of the amplifier (or turn down the MP3).

    You are using a Stereo (tip, ring, and sleeve) plug when connecting to the MP3? If your plug is mono (just tip and sleeve), you will have severe distortion because you are shorting out one channel inside the MP3.
     
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