# What is the purpose of the 250uF capacitor in the LM386 circuit?

The basic amplification schematics shown in the LM386 datasheet shows a 250 uF capacitor in series with the output. I don't have any 250 uF, but I do have plenty of 470uF. If I understand correctly, is this capacitor used as a high pass filter, so should I be able to use a 470 uF instead of a 250 uF?

Also, is the resistor (10 ohm) and capacitor (0.05 uF) going to ground a low pass filter?

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No. It is not a high pass filter. It is to protect the speaker from DC. Look at the datasheet for the "offset voltage" and consider that the speaker is probably 8 ohms or maybe less. So if the offset voltage is 20 mV, divide that by 8 and you have 2.5 mA running through the speaker. That's not good. Alternatively you could use a "servo" circuit to drive pin 2 so that the offset voltage is corrected. Then you could ditch the cap. But the larger cap is fine for blocking DC. Just make sure it's in the right voltage range. – squarewav Mar 3 at 0:38
It is there to protect the speaker from DC, but it is also a high pass filter, with the speaker's reactance as the shunt leg. 220uF or even 470uF is rather mingy in conjunction with the speaker's 8 ohms, giving you -3dB points at 90Hz or 42Hz respectively. I would use at least 2200uF (9Hz), unless you're only driving an earpiece. – EJP Mar 3 at 2:40

Yes, 470uF will work fine, it will just extend the bass response somewhat. It's a coupling capacitor (or a blocking capacitor, to block DC, if you prefer).

The 10R + 50nF is a "Zobel Network", intended to cancel out the reactive portion of the speaker impedance.

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Figure 1. Typical LM386 amplifier circuit.

For an audio application driving a loudspeaker alternating voltages must be generated. A positive voltage on the output drives the speaker cone one direction and a negative the other. At zero volts the speaker suspension will restore it to the mid-stroke position.

Figure 1 shows that the LM386 is typically powered from a single ended supply and can not drive negative. The chip is designed so that in the quiescent state (no input signal) the output rests at half the supply voltage - 4.5 V in this case. Positive-going signals on the input will drive the output up towards +9 V and negative-going will drive it down towards 0 V.

By adding a capacitor in series with the output the DC is blocked. When the amp is powered up pin 5 will rise quickly to half supply as will the right-hand side of the 220uF capacitor. The right-hand side will very quickly discharge through the low impedance of the loudspeaker so after the initial audible thump or click we will have pin 5 at 4.5 V and the speaker at zero. Note the orientation of the polarised capacitor.

The 4.5 V across the capacitor will remain. As the left side jiggles up and down with the audio signal but centred about 4.5 V the right side will also jiggle up and down around zero volts.

In effect this is a a high-pass filter and will attenuate the bass frequencies. Adding a 470 uF capacitor will improve the bass response.

The series RC filter is another high pass filter but with a smaller capacitor and is intended to shunt very high-frequency noise to ground.

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