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I have been struggling with this for a while.

I am trying to create a simple stereo system with the PAM8403 amplifier, using a 3.7v lion battery and a 5v charger (both from a powerbank). This is the schematic I used for hooking it up:

Circuit

This works pretty well, but when I plug in a micro usb charger there is a pretty loud hum noise and sometimes other strange noises. Seems like some ground problem or interference or something. I have tried adding capcitors and resistors to the power line but that didn't help. Can anyone explain to me whats happening and how to solve it?

Thanks.

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    \$\begingroup\$ Is the noise present when you charge from a power bank instead of from a USB phone charger or PC ? If the noise is present independant of the way you charge it is probably the charger circuit. It could be a switching charger. Show a photo of that charger circuit to confoirm this. \$\endgroup\$ – Bimpelrekkie Sep 17 '16 at 17:27
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    \$\begingroup\$ Your schematics does not look right in charger area - it creates an impression that 3.7V is shorted with 5V charger port. Could you please correct your picture? \$\endgroup\$ – Ale..chenski Sep 17 '16 at 17:27
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    \$\begingroup\$ You can not safely connect a 3.5 V battery directly across the terminals of a 5 V power supply. You need a charger circuit for your battery and some safe way of switching between the two. The hum and noise is the 5 V power supply trying it's best to blow up the battery. It is overloaded and will not be giving out 5 V and the resultant power line will be full of high-frequency switching noise. \$\endgroup\$ – Transistor Sep 17 '16 at 17:36
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    \$\begingroup\$ @winny, What ground loop? The power source is already floating... \$\endgroup\$ – Ale..chenski Sep 17 '16 at 18:13
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    \$\begingroup\$ Which PAM8403 amplifier board do you have? Is it powered directly from the 3.7V battery, or upconverted to 5V via the powerbank circuit? \$\endgroup\$ – Bruce Abbott Sep 17 '16 at 18:51
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The hum is caused by battery-to-5V upconverter. It is a switcher with variable ripples, since its main function is charging, which does not care much about ripples.

To reduce the noise you should try a filter, like this, something like two 3.3mH wirewound inductors with a 100uF 6.3V electrolytic capacitor:

enter image description here

Bigger inductors are better.

ADDITION: It could be that the noise/hum appears when the audio amplifier has no signal and the whole circuit consumes almost no power. In this case the powerbank will operate at very unfavorable low-output conditions, and switching mode may enter some intermediate random pusling state, such that the noise spectrum gets into audio range. The simple solution could be to add a constant resistive load, 100 Ohm or something, to get the powerbank back into normal (fast-switching) mode. Of course, it will drain extra power, but the hum might disappear.

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  • \$\begingroup\$ Wow thanks for the respones. It indeed uses the circuit from the powerbank to charge the battery. Ali Chen, could you be so kind to integrate your solution in my circuit. That would make more sense to me. (eg where the inductors should be placed and where the capacitor should be placed). Thanks. Oh, and would there be a better solution, for example replacing the charging circuit with something else? \$\endgroup\$ – Thomas van Gerven Sep 17 '16 at 18:41
  • \$\begingroup\$ Voltage symbol on the left represents your powerbank. R1 load represents your amplifier. You probably can find a powerbank with better switcher quality, but I am not sure if this would be a better solution with regard to your budget, since you might need to try several of them. \$\endgroup\$ – Ale..chenski Sep 17 '16 at 18:46
  • \$\begingroup\$ Thanks! So that would look like this in the end? oi65.tinypic.com/6qk4zp.jpg \$\endgroup\$ – Thomas van Gerven Sep 17 '16 at 18:47
  • \$\begingroup\$ Yes, like that. But you still didn't fix your short between the 3.7V battery and 5V charger output. \$\endgroup\$ – Ale..chenski Sep 17 '16 at 18:51
  • \$\begingroup\$ I'm gonna try this shortly. Thank you very much. So in the end, with charging circuit this should be correct; oi63.tinypic.com/296p37t.jpg \$\endgroup\$ – Thomas van Gerven Sep 17 '16 at 18:57

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