amplifieraudiolm386
I've built an amplifier with an lm386 but when I connect a speaker (8ohm) to the output I get a very distorted signal. The problem appears only when the speaker is connected. If I connect an external amplifier to the lm386 output, the audio is right.
The issue appears also without gain capacitor between pin 1 and 8 and with the most basic configuration from the datasheet (LM386 with Gain = 20, page 8 on TI datasheet)
How is it possible? 
In designing an electronic music box, I found that the LM386 would behave very badly if it tried to drive some kinds of speakers without a series resistor. At very low output levels the thing would sound fine, but at a certain output level the thing would start self-oscillating during certain parts of the input waveform. Adding a small resistor (about an ohm, if I recall) solved the problem, allowing the amplifier to usefully produce a much greater volume (probably more than 10dB improvement) than it otherwise could.
Filter the signal before the amplifier, not after it.
If you overdrive the signal, it will be distorted.
LM386 is good when you don't much care about audio quality or power or efficiency. If you do care about any of those things there are modern parts that do a lot better.
Best Answer
Compare your circuit with an example from the datasheet:
You have the + input grounded rather than the - input as in the datasheet. The LM386 only works properly when the - is grounded and signal goes in the +. You have the connections to pin 2 and pin 3 swapped.
You have some strange feedback from the output to the gain input. Don't do that. The gain pins should only be connected together (through a capacitor or a capacitor in series with a resistor.) Remove R18 and C9.
The LM386 is not an opamp. It is a power amplifier whose symbol happens to resemble that of an opamp.
It doesn't work like an opamp, and you can't treat it as one.