# 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?

• 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. Mar 3, 2016 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. Mar 3, 2016 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.

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.