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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

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  • 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.

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  • 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.

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