For the LM386 IC, what is the function of the 10 uF capacitor (between pin 1 and 8).

I know that by placing the 10 uF capacitor at pin 1 and 8, it bypasses the internal resistor and setting the device at 200 gain. But what I want to know is what does the capacitor actually do? Like, what would happen to the audio signal if I replaced it with a 1 uF capacitor?

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Directly from the datasheet:

"To make the LM386 a more versatile amplifier, two pins (1 and 8) are provided for gain control. With pins 1 and 8 open the 1.35 k ohm resistor sets the gain at 20 (26 dB). If a capacitor is put from pin 1 to 8, bypassing the 1.35 kW resistor, the gain will go up to 200 (46 dB). If a resistor is placed in series with the capacitor, the gain can be set to any value from 20 to 200."

As to why a 10 uF capacitor instead of 1 uF, they wanted to make sure the full range of audio frequencies made it through. A 1 uF would cut off to many of the lower frequencies, producing a tinny sound.

For further clarification, Pin 1 and Pin 8 are on either side of an internal 1.35 k ohm resistor that, if not bypassed in some way, sets the initial gain at 20. If you place a 10 uF capacitor across these pins, you provide a low impedance path for the audio freqencies to go around the internal resistor. That effectively removes the resistor from the signal path allowing the first internal 150 ohm resistor to set the gain at 200. If you pull up the PDF, you'll see what I'm talking about. (www.national.com/ds/LM/LM386.pdf )

The value of the capacitor sets the frequencies that pass around the resistor. The smaller the value, the more low end fequencies get cut off. Using the formula f=1/(2 * Pi * C * Xc) where the value of the resistor is substituted for Xc, the capacitive reactance, you get f=1/(2*Pi*0.00001*1350)=~12 Hz. (0.00001 is 10 uF expressed as Farads and 1350 is 1.35k) If you change the capacitor value to 1 uF (0.000001 F), the result is ~118 Hz.

Hopefully this explains well enough how the GAIN pins on a LM386 work, along with how a capacitor/resistor combo control the frequencies in a circuit.

Also, in my previous answer, the second instance of the 1.35 k ohm resistor translated into 1.35 kW resistor. The ohm symbol in the PDF did that and I missed it.

framistan7 years ago
I have an old book that mentions it as controlling the GAIN of the chip.  If you leave the capacitor out... the amp will have a gain of 20.  If you put the capacitor from pins 1 to 8... then the gain will be 200. 
aDimWit (author)  framistan7 years ago
Yeah, but what I want to know is why a 10 uF capacitor rather than a 1 uF.
seandogue7 years ago
 I think the cap should be smaller...more like.1uF, but the datasheet is bound to give the math, so I'll leave it to you to look up the LM386 datasheet.

I believe it provides a low impedance bypass path for the feedback loop and improves stability of the circuit, but beyond that I can't recall