Re: LM386 Audio Amplifier

From: John Popelish (jpopelish_at_rica.net)
Date: 02/16/05


Date: Wed, 16 Feb 2005 09:36:10 -0500

zero wrote:
>
> I am trying to build an LM386 audio amplifier on a breadboard but am getting
> "motorboating" oscillations.
>
> The layout on the breadboard is:
>
> At left is a Radio Shack condenser mic, biased with a 1K resistor, the
> output coupled via a 10uF capacitor to an inverting amplifier based on an
> LM741 op-amp.

This provides a path between the positive supply and the high gain
input, with only a 1k and 10 uf low pass filter. That pair has a
corner frequency of about 16 hertz, not counting the parallel load of
the microphone. This is probably the main feed back path affecting
your oscillation. Try using a separate 9 volt battery for the bias
source and see if the effect changes.
  
> The inverting amplifier of the LM741 has a 1K input and a 220K feedback
> resistor for a gain of -220. 1/2 the supply voltage goes to the + input. The
> output is coupled via a 10uF cap to the LM386.

This provides an input impedance of about 1k ohm, which seems very
low, for a microphone biased through a 1k resistor. I think I would
change this to a non inverting amplifier with the + input biased
through a 100k resistor, so that the full microphone output voltage
appears at the amplifier input.
 
> The LM386 on the right of the breadboard is set up for a gain of 20. Its
> output goes to the snubber and is coupled via a 220uF cap to the 8 ohm
> speaker.
>
> Questions:
>
> A. Does this layout make sense? Is the 220 gain in the first stage and the
> 20 gain in the output correct for audio signals and this circuit?

There is no right answer. Does this arrangement provide the needed
gain?
 
> B. To avoid ground loops, should I try to tie ALL grounds to the same point,
> including the mic's, the LM741's, the LM386's, and the speaker's? Or should
> I separate some components from others?
>
> Right now the mic, the LM741 and the LM386 have their grounds go to the
> breadboard's ground line. I've tried to keep them as close as possible.
> Also, I wired the speaker's and the snubber's grounds to the LM386's 4 pin,
> not the ground line.

You want ot keep the big speaker and LM386 currents out of the ground
line to the input section. That means that the power negative and the
speaker return and the supply bypass for the 386 should go to the
amplifier pin, as directly as possible. Then tie that node to the
input group with a wire.

> C. Bypass caps: where on the breadboard should I put them? Close to the 6
> pin of the LM386, the speaker, or the battery connections? I put a 220uF cap
> directly between the 4 and 6 pins of the LM386 but it didn't help. I
> paralleled it with a 0.1uF ceramic cap but it also didn't help.

That is still the right place for those components, so their currents
do not produce I*R drops in any other paths.
  
> Any help will be appreciated.

-- 
John Popelish