# Simple LM386 Circuit RC Branch Question

Please take a moment to checkout at the LM386 circuit below. The circuit comes directly from the LM386 specifications document:

My question revolves around the output branch consisting of the 0.05 microfarad capacitor and the 10 ohms resistor.

At first I thought that this branch acted as some kind of RC filter but then I realized that on all RC filters (as far as I know) filtering is achieved by taping into the node between the capacitor and the resistor (which is not the case on this branch).

So what is this branch supposed to do? Is it for filtering? Also, what effect does the branch consisting of the 250 microfarad and the speaker has on the output signal? It looks like at the end both branches will combine in parallel and I am having trouble understanding how all these component work together.

Thanks.

It's called a Zobel Network. The purpose is to neutralize the effect of the inductance of the speaker.

The 250uF simply blocks DC from the output bias point of the amplifier output stage, which normally sits around half the supply voltage with no output, and swings from close to GND to close to the positive rail when it is providing full output.

• I was once asked if Zobel was spelled with one l or two. "One, of course" since I'd always seen that way. Why the "of course"? I was asked ... without thinking I answered ... "to stop it ringing" – Brian Drummond Sep 15 '15 at 10:32

I didn't read the datasheet, but the 50 nF and 10 Ω on the output are almost certainly because of a stability requirement of that particular amplifier. The amp must need the 10 Ω for stability, at least at the higher frequency end of its range when assuming the worst case characteristics of the actual load. The rolloff frequency is 320 Hz, so a octave or two above that and beyond, the cap and resistor combo look like just 100 Ω.

The 250 µF in series with the speaker is just to block DC. That reduces the current requirement on the amp, and operates the speaker at a better point anyway.