Amplifier Driving 8 ohm speaker

Discussion in 'The Projects Forum' started by jimmyho08, Oct 14, 2013.

  1. jimmyho08

    Thread Starter New Member

    Oct 7, 2013
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    There is audio signal which I want to amplify because when the audio is on, the signal is about 2V p-p, but when it is connected to the load, the signal drops to 400mV. This makes the volume too low.
    I used the lm386 ic, and here is the schematic. The sound is loud now, and sounds fine when the audio is playing, however, when I turn off the audio sound, and it is idle, there is a terrible static/rattling sound. I've tried different things such as change the values of the caps, and resistors, but nothing changes.

    Has anyone have experience in smoothing this out?

    I also tried to just use a regular op amp to drive the audio signal, but the op amp does not amplify the input signal at all.
     
  2. jimmyho08

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    Oct 7, 2013
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    Amplifier.JPG

    here is the circuit
     
  3. ronv

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    Did you use the decoupling caps you show in the schematic? If so try a 10Ufd from pin 7 to ground. Is it running on small batteries?
     
  4. Brownout

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    Jan 10, 2012
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    You might need to shield the input. Connect the shield outter conductor to circuit common.
     
  5. shteii01

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  6. jimmyho08

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    Oct 7, 2013
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    Yeah, I did use the decoupling caps. I did try adding a 10uF from pin 7 to ground. It didn't improve anything. The 5V supply is from a voltage regulator, 12V to 5V regulator. The 12V is from a power adapter plugged into a wall outlet. I used the oscilliscope to see the 5V supply, the signal seemed to be pretty smooth with no distortion. The input audio line without audio is showing noise on the line when no audio is playing. I've tried to filter this noise, but doesn't help any.
     
  7. jimmyho08

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  8. THE_RB

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    I think you are running too much gain in your LM386 circuit.

    Since your input signal is about 2v you only need a gain of 5 or 6 in the LM386 amp. Check the datasheet for the low gain amp circuit (I think it has a gain of 20?) and either use that or change the parts values a bit and reduce the gain to about 6.
     
  9. GopherT

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    Nov 23, 2012
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    The old LM386 is a noisy chip and there is no gettin around it. I like RB's idea and it will work well with some audio amps. However, I think the minimum gain of the LM386 is 20. Look for a different audio amp chip.
     
  10. jimmyho08

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    Oct 7, 2013
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    I decided since when there is sound when there is audio coming through, I want the signal to go through if only there is an audio signal. I tried to create a switch at the input to only close when it sees an audio signal. Nothing has worked. Any ideas on how to make a switch at the input? The signal when there is audio varies from 1V to 2V p-p. Idle it's 400-600 mV.
     
  11. ronv

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    The RB is right. Even with the volume control on the front it is probably still higher than you would like. You can just remove R3 and C6.

    Your input with no audio should be zero. What is the source of your signal? Is the cable connecting it shielded?
    You can test by grounding the input and see if the noise goes away.
     
  12. THE_RB

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    Yep, remove R3 and C6 to make the gain much lower at 20, that will really reduce the noise when there is no music.

    And also make sure the 3 pot wires (R2) are correct, so that when the pot is turned all the way down it shorts out the amp input, cutting the noise level even further.
     
  13. jimmyho08

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    Oct 7, 2013
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    I've changed the gain to 20, no dice. My audio signal is about 80mV without sound. This is because of the noise on the line. The cable connecting the from the 3.5mm headjack audio out into the amplifer is not shield. I basically cut a 3.5mm to RCA connector, and stripped the wire on the RCA side to get the + and - for the audio to be connected to the amp, and the 3.5mm is plugged into the audio out.

    I have been for the past couple of days trying to find a way to create an audio switch. I've had no luck in this. I wanted to create a circuit where if there is audio coming in, then pass the signal through to the amp. I tried using transistors, because I know that a signal of 80mV won't turn on the transitor, but when there is audio, there can be about 1-2Vp-p. Am I approaching this the right way?
     
  14. ronv

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  15. jimmyho08

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    Oct 7, 2013
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  16. ronv

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    Does your cable have a ground shield on the outside? If so did you connect it?
     
  17. THE_RB

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    Sounds like you are using a bad cable!

    You need a proper "stereo" cable, that has two internal wires AND an outer shield mesh.
     
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