If you're building an oscillator that runs through a speaker, chances are, you're using a 386 to amplify it. Why not? They're cheap, general, and easy to use.
Well, the fact is, they suck. For the most part, they usually work fine, but there's the occasion where you get unwanted oscillation when you want an amplifier. BUT, there are a solutions for when this happens.
We design circuits with ideal behavior in mind, but build them in a much less tidy, but ultimately more beautiful world, and old adage in electronics reminds us that
"oscillators amplify and amplifiers oscillate".
Assuming you use the common gain of 200 from the LM386 datasheet:
Reducing the overall gain is an option. This is accomplished by removing the capacitor between pins 1 and 8. This drops the gain down from 200 to 20.
Be advised that this will also take away some of the pleasant harmonic distortion possible with higher gain.
Because of this sacrifice, I found a solution that is much better.
In my personal device in which I had this problem, the oscillations occur only when driving a speaker, not when using the output directly at the line output. The fix is partially to stabilize the "load" of the speaker. This is done by adding a resistor and capacitor to ground at pin 5, also known as the "zobel network."
Another possible solution if is maybe even adding another larger capacitor to ground at pin 7.
These two solutions are illustrated here:
I hope this helps some people out.
Last edited by hotmessization (Sep 7, 2010 6:56 pm)