LM386 - pin 7, inputs, gain

[atferrari]

I just finished the assembling of an amplifier with a LM386. All worked OK when I provided filtering to power supply. Key point was a .01 ceramic cap.

My questions:

A) What is the actual use of pin 7 (bypass)?

In most of the suggested circuits the baypass capacitor is shown as optional. When and how much to use?

A 10 uF cap is shown somewhere but in a circuit provided by Audioguru I recall there was a .1uF used instead. In my case I used 10 uF but without it, I found no difference.

B) What is the correct way to control volume at the speaker? By changing gain (R in series with cap between pins 1 & 8 or increasing/decreasing the input level? The last option seemed easier to me.

C) In what application the specific use of inverting and / or non inverting inputs would be relevant?

Datasheet here: https://www.ti.com/lit/ds/symlink/lm386.pdf

Thanks for any help.

 

[ronv]

A) What is the actual use of pin 7 (bypass)?

It is used to isolate the input stage from noise on the power supply that can be amplified along with your signal.

In most of the suggested circuits the baypass capacitor is shown as optional. When and how much to use?A 10 uF cap is shown somewhere but in a circuit provided by Audioguru I recall there was a .1uF used instead. In my case I used 10 uF but without it, I found no difference.

It is probably more important to have it if your power supply is not very good. For example if you have a voltage regulator like a 7812 it is better than a 9 volt battery. Look at the chart in the data sheet labled power supply rejection ratio to see the effect of the capacitor size.

B) What is the correct way to control volume at the speaker? By changing gain (R in series with cap between pins 1 & 8 or increasing/decreasing the input level? The last option seemed easier to me.

If you use pin 1 & 8 you cannot reduce the output to zero, so the input level will work best.

C) In what application the specific use of inverting and / or non inverting inputs would be relevant?

It can be used like a "power op amp" so where polarity is important like the square wave oscillator diagram in the data sheet.

 

[audioguru]

Usually an inverting amplifier has a fairly low input impedance. But both inputs of the LM386 amplifier have the same fairly high input impedance of about 50k ohms.

A non-inverting amplifier might oscillate at a high frequency if the input wire is near the output wire due to the capacitance between them. But an inverting amplifier probably will not oscillate like that.