LM386N-4 Amplifier Noise with Raspberry Pi

Hi Sean,

Removing the capacitor on pin 7 (i.e. leave pin 7 disconnected) can help, often it turns the LM386 into a radio receiver when that's there : )

Also, personally, I would try to connect pin 3 to ground via 2.2k resistor, and then change your 0.01uF input capacitor to something larger (try 1uF to 4.7 uF or so).

If you disconnect the left side of that capacitor, you can insert in a ferrite bead inline with the capacitor, and also solder a (say) 10nF capacitor to ground from that left side of the capacitor too. That will act as a sort of RC filter, where the R is the ferrite bead. It will help reduce high-frequency noisy pickup from the audio input. Also, between the supply pins (4 and 6) solder a capacitor (maybe 100 uF) and also a 100nF capacitor in parallel. Change the two left side resistors (1k) to something lower, say 220 ohm.

These are just some ideas, others may have better suggestions, but I think there's a chance it may improve things a bit.

Perhaps try this and see if it helps

Thinking about it a little more, this might be better than what I gave above

Your low value of coupling capacitor [10n], to give you a HPF, means that there's a considerable change in phase around the cut-off frequency [325Hz] (see plot below). If we keep the inverting path the same with a similar capacitor, the two effects will cancel, otherwise we're losing the benefit of the differential input lower down.

BTW this 'solution' might actually make your situation worse rather than better, but at least it will be telling you things about what the noise is and how it's affecting the circuit. Any chance of some 'scope traces?

I see where this one is going.  I'll try it out.

I actually have a big ground plane that the ground of the 3.5mm port as well as the battery ground are connected to.  However, a ground loop is the problem here.

Last night I cut the battery side relying on just the ground plane taking things to ground via the raspberry pi headphone jack.  Call me crazy, but it did get quieter.

I'll get things going and should have something in a few hours.

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

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

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

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