Maximum Wattage of LM386 amplifier

Discussion in 'General Electronics Chat' started by Yaşar Arabacı, Dec 3, 2014.

  1. Yaşar Arabacı

    Thread Starter Member

    Nov 11, 2014
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    Hi,

    I have been playing with LM386 audio amplifier IC and I was wondering how much output power it can give me. For example, I have tried using it with 8ohm 0.5W speaker and it seems to work. But I don't suppose I can drive 100+W speakers with it ( -- to be honest, I think that because chip looks so tiny and fragile:) --)

    I am trying to figure out how big a speaker I can make it work with this chip.

    I have also another question about gain. http://www.ti.com/lit/ds/symlink/lm386.pdf says that I can set the chip to give me 26dB to 46dB gain compared to input voltage. I assume that computer or phone jack gives me around -/+ 5V signal (I approximated this from couple of things that I saw on the internet). 5V signal with 8ohm resistance uses 3W of power. This makes me think that standard phone or computer output is more than enough for my speaker.

    On the other hand, when I connect this speaker to my phone, I get very little volume. I guess I am making a mistake somewhere. How should I be calculating the gain I need for a specific speaker?
     
  2. #12

    Expert

    Nov 30, 2010
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    You don't calculate the gain for a speaker. You calculate the gain needed to get an output signal that uses up most of the power supply voltage. You can connect it to a 30 inch, 300 watt speaker, but it won't make 300 watts.

    A telephone line has a large DC component that reverses, depending on whether the phone is waiting or active. Be careful with your input capacitor.

    Add a 100k audio taper volume control at the input side of the LM386.

    Edit: The datasheet shows 10k audio pot. The chip might not like the 100k I suggested.
     
    Last edited: Dec 3, 2014
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    The data sheet you linked to has a power output vs power supply voltage and speaker impedance numbers. AFIR, it is only rated at 1W.

    Line Output level from a computer sound card is about 1Vpp.
     
  4. Papabravo

    Expert

    Feb 24, 2006
    10,144
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    The power output is given in the datasheet for several combinations of voltage and speaker impedance.
    The package can dissipate only 1.25 watts.
     
  5. Yaşar Arabacı

    Thread Starter Member

    Nov 11, 2014
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    So, something like 0.3535V RMS. In that case, do I need 28dB gain to use all 9V battery? http://www.sengpielaudio.com/calculator-amplification.htm says 0.35 to 9V is 28dB gain.
     
  6. #12

    Expert

    Nov 30, 2010
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    No, you have to use peak to peak voltage to see how much gain will get to 9 volts.
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    If it was a perfect amplifier, the best it could do would be to put 9Vpp into the speaker. If the input is 1Vpp, then the required gain is only 9 (19db). You will need to strap it for 26db, and then put a pot in the input. Again, I think this is shown on the data sheet in the applications section...

    Yep:

    amp.gif

    Or more simply, use the slider in the computer sound card interface to slightly reduce the output level.
     
  8. Yaşar Arabacı

    Thread Starter Member

    Nov 11, 2014
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    Ok, so my speaker can handle 2V because V^2/8=0.5. So, I need 0.1V as input because it gets to 2V after amplification. therefore, I need to set upper leg of potentiometer to 9k and lower leg to 1k, am I right?
     
  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    2Vrms = 5.6Vpp. If the line-out level from the computer is 1Vpp, then 5.6Vpp reduced by-26db (1/20) = 5.6/20 = 0.28Vpp, so the attenuator (pot) needs to cut the input by ~1/4.
     
  10. #12

    Expert

    Nov 30, 2010
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    You are mixing peak to peak voltages and RMS voltages again. The speaker power is by RMS voltage. The amplifier voltage is by peak to peak voltage. Peak to peak into the amplifier, peak to peak out of the amplifier. Convert to RMS, square that and divide by R.
     
  11. takao21203

    Distinguished Member

    Apr 28, 2012
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    TDA2003 would be more appropiate perhaps- the legs also arent as fragile.
    It is appropiate for 300W speakers- and will give decent volume.

    with a larger speaker you get a better perception, if you crank them up, they will sound as crappy as cheap phone speakers, or you need a very expensive circuit to seperate bass/treble.

    Now, you can seperate the signal before amplification actually. Its cheaper a lot. And then you use 4 seperate TDA2003 ICs, or 6 of them. You only need high power for low frequency, for 1 KHz, 10W will result in very high volume.

    Or you can try Class D- which is more efficient, you can use smaller power supply.
     
    absf likes this.
  12. Yaşar Arabacı

    Thread Starter Member

    Nov 11, 2014
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    Ok, I have another question, do I need a second 9V battery in order to make stereo amplifiers. I already have a second speaker and stereo jack that is set up.

    Edit: also second LM386
     
    Last edited: Dec 3, 2014
  13. takao21203

    Distinguished Member

    Apr 28, 2012
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