LM386 Output Impedance

[fcb]

This thread is bizarre.  Current banking...

So you are trying to build a small "wire-less" amplified speaker.

My suggestion would be use FM over your optical link, much much easier to get a flat response.  Use a 4046 to demodulate the FM carrier (say 40-60KHz) - Philips did a great 4046 app note many years back. Once you've got good audio, then worry about the amplification.

BTW the LM386 is shit.

[Hideki]

I don't think you understand WHY they put a capacitor there in the data sheet. It's a zobel network, and in most of the drawings you may notice that the capacitor is in series with a 10 ohm resistor.

The speaker may present about 8 ohms at lower frequencies (except for its resonance peak), but its impedance will rise at higher frequencies (it is a after all a big inductor) so the capacitor gradually shunts in the 10 ohms in parallel with the speaker to keep the total impedance seen by the LM386 about the same.

It has nothing what so ever to do with "current banking" - whatever that means. It does have something to do with stability.

[w2aew]

Hello w2aewm,

I used the capacitor because it was suggested in the LM386 datasheet.
http://www.ti.com/lit/ds/symlink/lm386.pdf
They are using a 47nF capacitor.

Now that you mention it, I did not take into account the 'sinking' part, and the capacitor
will indeed place an additional load on the output.  Why are they using it, then?

It's there to ensure stability (part of a Zobel network, as mentioned previously http://en.wikipedia.org/wiki/Zobel_network). It does not affect the frequency response.  47nF has a capacitive reactance of about 170 ohms at 20kHz.  This is not in the feedback path, and appears in parallel with your speaker (8 ohms?), so does not affect the frequency response to the speaker really at all, at least not as a low pass filter as you may have been expecting.

I suspect, as the others have stated, that the rolloff you're seeing the frequency response is due either to your sound-light conversion scheme, or your light-to-sound conversion.  Even though you're using a low capacitance photodiode to pickup the light, the circuit configuration will have a huge affect on the achievable BW (photoconductive mode vs. photovoltaic mode, etc.).   I highly suspect that your BW limitations lie in the S2L and L2S circuits...

[hgg]

FCB, I thought of using FM as well but I am a newbie and I though that I would learn much more,
if I try to find the cause of the problem.  If I fail, I will try FM.  (and mess up even more  )

Hideki, very good explanation!  Thank you.  When I was trying to figure out why they were using
the capacitor/resistor to ground, I couldn't find any info.  Then, one website was supposedly
explaining the reason.  http://www.learningaboutelectronics.com/Articles/How-to-connect-a-LM386-audio-amplifier-chip
It turns out that not only the explanation was wrong but the LM386 wiring was totally wrong as well...

w2aew, yes the problem is definitely in the S2L part, because when I used the 555 to test the
frequency response, the result was much better.  (but still there).  I need to make a LED pulser
that will have the same amount of light intensity up to 100Khz and then look at the L2S circuit
for similar problems.

Thank you very much for your help!

[TonyStewart]

I know this is an old thread but this audio amp is intended for lower power from a battery. Yet at max Vout [Vpp] vs V+ in ,  this output stages are based drive limited push-pull Darlington & https://www.wikiwand.com/en/Sziklai_pair  output are limited on the high side which protects it but also makes the Rout very high for a power amp at Vmax out.  My calculations show this rises from 4 to 6 ohms using the method of KVL at 50% load loss rising at maximum output power.  This means damping factor is low in this condition.

It would be better to use complementary Darlingtons driven from a Rail to Rail Op Amp with negative feedback as the excess gain lowers the output impedance reduces the error with effectively a lower regulated output impedance.  Don't be fussy about biasing the 2.4V drop between the bases to prevent shoot thru as the GBW of R2R OA will eliminate any crossover distortion and not be visible or audible.

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