LM386 - what is effect of reversing pins 2 & 3

Hello Mike,

I've also builded a power amplifier with two LM386 to drive 8 ohm speakers, that was my first audio power amplifier, but I've learned several issues that we have to have in account, for example you should use two capacitors between pins 6 and 4, one with a value between .01 uF and .1 uF polyester to avoid oscillations (very important without this capacitor you'll get strange noises at the output), and the other one of a larger value (aluminium electrolitic ) between 1 uF and 100 uF this capacitor stiffens the power supply improving the amplifier's response to high amplitude signal pulses (this one is optional) the + terminal of the capacitor should be connected to pin 6 (Vs).

Other important consideration is that sometimes it's necessary to connect the unused input (pin 2 or 3) to a capacitor with .1 uF (polyester) and then to ground instead of simply connecting the unused input to ground this could be useful to prevent oscillations if your gain is very high, without the capacitor you can have strange noises coming from the output.

The bypass capacitor (pin 7) is used to reject the ripple noise coming from the power supply, if you use a battery as supply I think that's not necessary.

The .047uF (polyester) resistor in series with the 10 ohm resistor is the Boucherot Cells it helps to reduce oscillation at high frequencies.

The output capacitor is indispensable, it's used to block the DC offset present at the output of the amplifier, you should use 220 uF, or parallel 220uF with 33uF (this capacitor is alluminium electrolitic).

About your question the only difference between connecting the input signal to pin 2 (inverting input), or to pin 3 (non inverting input) is that if you connect to the inverting input (pin 2) you'll get the output sinewave inverted (with a 180 degrees phase shift) at the speakers, if you connect the input signal to pin 3 you'll have the output signal as you have at the input without any phase shift.

I don't know much about guitar power amplifiers because I've only builded audio amplifiers for other applications (to connect to MP3 players, computers, etc...), but getting the signal inverted could be good if you want some different effect at the output that I don't know but could be good for guitar amplifiers.

You should first test the amplifier in breadboard and then you can easily make the PCB layout.

I think the amplifiers you have in the links should work but you should take some ideias in account specially the polyester capacitor between pins 4 e 6, and the .1 uF polyester capacitor from pin 2 or 3 to ground, this capacitors don't degrade sound quality and only helps preventing undesired oscillations, sometimes the oscillation could not be perceived during some tests.

I hope that this information helped you,

The best luck for the project,

Best Regards,

Daniel Almeida