I'm designing a very simple circuit which contains a microcontroller, an LM386 and a speaker.

I've previously asked a question:Help in designing a speaker driven by an LM386 and the answers helped me a lot. However, there are still some fine tuning I need to do in order to maximize the performance:

The speaker is 4-Ohm (as I measure using a voltmeter). I don't know it's rating (I persume 1W) LM386 can provide (typically,according to it's datasheet) 325mW.

Now, I just want to make sure the planning I'm doing is correct:

  1. The microcontroller produces 2.4v signal (under 20kHz)
  2. I multiply it by 0.05 (two resistors) and feed the amplifier with it
  3. (I'm using basic LM386 apllication;GAIN=20) the amplifier multiplies it by 20 so we're back to 2.4v
  4. I filter the signal with a simple RC circuit (30-Ohm,200nF, cutoff at 30kHz)
  5. so basically now I have 2.4v on 4 Ohm.

obviously, the LM386 cannot provide so much power! my questions are:

  1. what happens if I try to draw so much power from the LM386? will the signal be distorted? or simply less powerful?
  2. Am I doing the planning right? I mean, in order to draw 325mW I simply need to play around with the resistors until I drop about 1.3v on the speaker,is that correct?

Thanks

  • 1
    Why cut off at 30kHz if you are not interested in anything above 17kHz. Also consider a active filter setup around the opamp. (first hit in search engine, look at the bottom of the page, second order low pass: electronics-tutorials.ws/filter/filter_5.html ) – jippie Jan 5 '13 at 0:11
  • @jippie I wanted to get rid of harmonic aliasing, and since my sampling frequency is 80Khz, I thought, in order NOT to attenuate my desired bandwidth (17-18kHz) I'll set the cutoff frequency such that my the desired bandwidth isn't hurt, while the undesired (above 40kHz - Nyquist freq) is attenuated. I guess this isn't a correct design. I will try designing the active design you have suggested. Just to be sure though, the OP-Amp will replace the LM386! is that correct? Can you suggest a specific part for that matter? Thanks! – Daniel Jan 5 '13 at 7:57
  • 1
    I would design cut off frequency little above the desired frequency so all its harmonics are suppressed. Remember 6dB/octave (or 12dB/octave for 2nd order), so the further apart cut off from your unwanted harmonics, the better the suppression. If open loop gain of LM386 is sufficient, you can simply use that. – jippie Jan 5 '13 at 10:01
up vote 3 down vote accepted
  1. Filter the signal before the amplifier, not after it.

  2. If you overdrive the signal, it will be distorted. enter image description here

  3. LM386 is good when you don't much care about audio quality or power or efficiency. If you do care about any of those things there are modern parts that do a lot better.

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