LM386 audio amplifier

[rmarchesi]

I'm trying to make a small audio amplifier using an LM386N-3 IC from National. 700 mW max output.
I have a small loudspeaker, 4 ohm, 700 mW max power.
I connect the IC like indicated in the datasheet, with a gain of 20. I get lots of distortion, even at low levels.
I'm using the schematic indicated in the datasheet as 'Amplifier with gain 20 minimum parts'.
I found out that with earphones (135 ohm) there is no distortion, so I put a 56 ohm resistor in series with the load and it works ok, but the sound level is dramatically reduced. If I go lower then 56 ohm I get distorsion.
Any idea?

 

[geekEE]

How much power is "low level"? Have you measured it?
What are the frequencies that you are trying to reproduce? The 250uF cap with the 4-ohm load creates a high-pass filter with a knee at about 160Hz, so your gain below that is going to drop off rapidly.
Have you scoped the power supply pin to see if it is being pulled down by the load?

Glenn

 

[rmarchesi]

Thank you, Glenn. In the meantime I've solved it. It was probably 50 Hz noise coming from power lines. I put a small cap between positive of power supply and ground and everything disappeared. I don't have testing instruments, so it was hard for me to understand where it was coming from. A fact that still I don't understand is that it was doing it also when I was using a battery to power it, probably it was getting the noise from the socket power lines.

 

[geekEE]

My guess is still that the power supply pin was getting pulled down by the load. That would happen whether it was powered by a power supply or a battery. In fact, it might be worse for a battery because the battery might have a higher series impedance.

Anyway, glad to hear you solved the problem.

Glenn