Audio Amplifier for a woofer

Discussion in 'The Projects Forum' started by Luke Morrison, Jun 3, 2015.

  1. Luke Morrison

    Thread Starter New Member

    Jun 3, 2015
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    Hello all,

    I've been trying to replicate the following schematic (with the same parts) so that I can drive a small woofer speaker for a class project: http://www.electroboom.com/?p=243
    Unfortunately, it's not working. I am driving this circuit with a 12V input from a wall wart and the input signal is audio coming from the audio jack of my computer. The speaker constantly makes a high pitch noise. I measured the voltage across it and it showed 0.67V. Also, I measured the voltage from VCC to ground and it showed 1.4V (instead of 12V) even though the wall wart supplies 12V on its own.
    After further investigation, I found out that the op amp doesn't even affect the circuit.

    I have no idea what's going on, does anyone have any insight on what's happening? It would be greatly appreciated.

    Thanks for your time.
     
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Clearly, the wall wart is not supplying 12V (you only measured 1.4V). This may be because it cannot supply enough current or there is a problem with your amplifier.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    The circuit shows a plus and a minus supply.
    That circuit won't work with a single supply since there is no path for common currents back to the power supply.

    You need two wall warts, one for plus and one for minus, with their commons going to circuit common.

    Alternately you can capacitively couple both the input signal and the output to the speaker (speaker requires about 1,000uF), if you bias the op amp (+) input to 1/2 the supply voltage with a two resistor divider.
     
  4. Luke Morrison

    Thread Starter New Member

    Jun 3, 2015
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    Oh that makes sense! So there is no way to "split" my 12V source into two 6V rails?

    Then biasing seems like the easy way out. Where should the biasing voltage divider be inserted? Before the resistor R1? And then the DC blocking capacitor would be between the source and the biasing resistors?
     
  5. Luke Morrison

    Thread Starter New Member

    Jun 3, 2015
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    The wall wart is rated for 1.5A, so I doubt that current overdrawn is an issue. My guess is that since the two transistors are nonlinear devices, they are forcing a 0.7V drop across them (as a diode would) and pulling the input down to 1.4V
     
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    I would advise testing the wall wart by itself to see if it is still putting out a voltage with no load.

    Also, warts come in many types. Some put out a pure AC, some just rectfy the Ac for a puklsing DC. Only some put out a pure DC (and put "pure" in quotes).

    Also, forget about biasing for the split supply: the divider would also have to split the speaker return. That is why no one suggested this way: it just ain’t gonna work.
     
  7. Luke Morrison

    Thread Starter New Member

    Jun 3, 2015
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    Alright, I see. Thanks for your help.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    No, the voltage divider goes to the plus input on the op amp (say two 10k resistors).

    The capacitor does go in series with R1. You don't need to bias that point since the output feedback will bias it to be equal to the op amp plus (+) input bias from the voltage divider.
    The capacitor value depends upon the value of R1 to give a low frequency response of at least 20Hz. If you use an electrolytic connect the plus side to R1.

    The 1,000μF output capacitor goes between the amp output (+) and the speaker with the other speaker terminal going to the power supply minus.

    You wall wart should work okay when you do that.
     
    Last edited: Jun 3, 2015
  9. crutschow

    Expert

    Mar 14, 2008
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    That's why I suggested using a large capacitor at the amp output to drive the speaker (whose other terminal is connected to power supply minus).
     
  10. Luke Morrison

    Thread Starter New Member

    Jun 3, 2015
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    Okay that sounds good. Just for reference though, do you think I would get more distortion from doing it that way than form just using two 6V sources?
    Thanks for your help.
     
  11. crutschow

    Expert

    Mar 14, 2008
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    There should be no appreciable difference.
     
  12. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The consant high pitched noise probably is the circuit oscillating because the dead zone between the two output transistors conducting potentials is causing the opamp to "hunt" for a zero output. As drawn, the circuit is unstable when there is no input.

    ak
     
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