LM386N-4 Amplifier Noise with Raspberry Pi

I've been fighting a hum in my amplifier circuit on my Raspberry Pi -but only when my Pi and the amplifier are powered by the same battery.

If I plug a speakerbomb into the Pi which is powered by its own battery - it sounds great.

If I power my amplifier with anything other than the Raspberry Pi battery and plug the Pi's headphone jack to it - it sounds great

It only sounds bad when both the Pi's switching regulator and the amp circuit are powered by the same battery. The amp is powered upstream of the switching regulator

I've tested many ranges of caps at the LM386-N4's power pin to no avail. I tried different speakers. I tried different LM386N-4LM386N-4 chips. Same behavior.

I have a cap in the signal line from the headphone jack that helped a little, but not enough.

Any thoughts on what could remedy this behavior.

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jc2048 over 4 years ago

Perhaps try this and see if it helps
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Sean_Miller over 4 years ago in reply to jc2048

This is the one (the first picture you sent) I went with and it eliminated the buzz. I breadboarded it and was happy without that cap at pin 3. After I soldered it on the perf board, I picked up a faint radio station. So, in the morning, I'm going to add back in that cap to see if it kills that. Somewhere along the way, I believe I found that would kill it.

The data sheet simply shows pin 2, 4 and the incoming negative going to a common ground. However, putting the negative from the pi's headphone jack directly to pin 2 definitely killed the hum. I'm thinking "ground loops" was the issue.

Thanks for the tip!

See ya',
Sean
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Sean_Miller over 4 years ago in reply to jc2048

This is the one (the first picture you sent) I went with and it eliminated the buzz. I breadboarded it and was happy without that cap at pin 3. After I soldered it on the perf board, I picked up a faint radio station. So, in the morning, I'm going to add back in that cap to see if it kills that. Somewhere along the way, I believe I found that would kill it.

The data sheet simply shows pin 2, 4 and the incoming negative going to a common ground. However, putting the negative from the pi's headphone jack directly to pin 2 definitely killed the hum. I'm thinking "ground loops" was the issue.

Thanks for the tip!

See ya',
Sean
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Sean_Miller over 4 years ago in reply to Sean_Miller

After I hooked it back to the battery and got it back in my project - the amp worked great. Didn't need to add the pin 3 cap.

The datasheet is very misleading on this one, so I'll be sure to mark up my copy so I don't connect pins 4 and 2 together ever again.

Thanks,
Sean
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geralds over 4 years ago in reply to Sean_Miller

Hi Sean_Miller ,

You can read this datasheet as well. The original datasheet from national.
Best Regards
Gerald
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Attachments:
LM386_nat.pdf
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Sean_Miller over 4 years ago in reply to geralds

Thanks for sending.

What gets me on these datasheets is the #2 pin showing ground just as all the others.

I don't see where it distinguishes this ground to be separate from the other grounds. I found that tying pin 2 to a common ground versus just the '-' of the headphone cord led to the ground loop noise.

See ya',
Sean
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