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LM386N-4 Amplifier Noise with Raspberry Pi

Sean_Miller

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.

 

image

Parents
  • in reply to jc2048

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

     

    image

     

    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.

     

    imageimage

     

    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?

Reply
  • in reply to jc2048

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

     

    image

     

    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.

     

    imageimage

     

    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?

Children
  • in reply to jc2048

    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.