What kind of capacitor is that? Answered

Oh yeah, the standard LM386 schematic. The decoupling "output" capacitor on these amplifier ICs is there to block DC from reaching the speaker. But it serves another purpose...

The value of that cap isn't a single, universal value, it's dependent on the impedance of the speaker. Together with the speaker, that cap forms an RC filter. Do a simple RC filter calculation: an 8 ohm speaker w/ a 220uF cap has a cutoff frequency of about 90Hz, which is pretty typical. It also matches almost all LM386 schematics you'll find (since all "official" schematics always show a load between 4 and 8 ohms).

However, headphones often have an impedance that might run between 32 ohms, on up to 600 ohms. For those, similar cutoff frequencies occur at:

32 ohms: 55uF

600 ohms: 3.3uF

A polarized electrolytic is almost always used here, but for the "lesser" load (lesser meaning greater resistance, less electrical load, i.e., 600 ohms) a non-polarizing cap can be used instead.

220uF at 25v is a common electrolytic cap. Electrolytics are polarized. The negative lead is shorter and there is a - sign down that side of the cap. DO NOT install an electrolytic backwards. It will explode when you power up. An electrolytic smooths power supply ripple and acts as a battery in the circuit.

You can get a 35v cap for $1.5 at Radio Shack.

http://www.radioshack.com/220uf-35v-20-radial-lead...

http://www.ebay.com/itm/like/251538284691?lpid=82 Exact match on eBay. Low ESR is high quality for audio use. The voltage rating can be higher, but not lower. Don't change the capacitance [220uF] unless you know it doesn't matter. Axial leads are on opposite sides, radial leads on the same side, if it matters how the cap will be mounted.

Must be a very old schematic....

Polarised caps are in most cases electrolytic caps, the standard kind you find everywhere.

The label in the circuit is not to todays standards.

The modern symbols look like this (seems not to work, where it might end up, the first image):

And here to compare your symbol with the more modern ones: