Hi all.
I have a project with a 3S lipo powering two components. I have a branched power cable coming off the lipo. One end goes straight into a MAX9744 audio amplifier. The other end goes into an UBEC buck converter, to provide 5V for a 3W smart RGB LED. They are properly grounded. The problem is that when I turn on the LED, it provides lots of distortion and high frequency noise in the speakers.
At the moment I have them powered by separate 3S lipo batteries to avoid this problem, and it is working perfectly. But I’d like to go back to using one battery for both, for obvious reasons.
Being totally new to all this, I learned too late that buck converters can create noise in your circuits. Is there any way to put a filter or something on the +ve/gnd lines going to the amplifier to eliminate the noise?
Thanks in advance for any ideas you might have.
Hey Sneaky,
I think that you will need a low pass filter to reduce the high frequency noise. This is a pretty tricky problem to solve. You would ideally know the frequency of the unwanted noise, and calculate your low pass filter to reduce that frequency. Here is a link to a site that explains the low pass filter, gives a circuit diagram, and gives you the formula to calculate what you need:
https://www.electronics-tutorials.ws/filter/filter_2.html
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Looks a bit complicated for me atm, so looks like I’m using separate lipos for awhile. Thanks for the info.
Hey sneaky,
Because you are filtering the power input and want to remove anything above DC this actually becomes fairly simple. Put a fairly large value capacitor (100uF+) from the positive output of the DC-DC converter to ground that should remove most of your noise.
I guess that would be simple enough to try, thanks for the suggestion. 
The fact there is no noise when the two divices are running off separate batteries suggests that this is not electromagnetic interference (EMI). Shielding therefore won’t help. Without being able to see your circuit diagram or probe it with an oscilioscope, I’d have to guess. I imagine the noise you hear is caused by ripples in the Lipo caused by spurious demands of the buck converter as it continually switches current. If you have access to an oscilloscope, probe the lipo when you hear the noise. Set the timebase to see audio frequencies (around 1ms/div) and look for ripples in the supply.
I would try using a power diode for isolation and a big capacitor for storage. Put the diode from your Lipo’s + terminal to the amplifier’s +Vin terminal (the stripe goes to the amplifier). The capacitor goes from amplifier’s Vin terminal to GND (obviously the negative goes to GND). I would use an electrolytic capacitor valued somewhere between 100uF and 1000uF.
I cannot be sure this will work because I haven’t tried it
but the idea is that the diode allows the capacitor to charge, then if the supply dips even to 0V the diode will prevent the supply from stealing current from the capacitor, leaving your amplifier to enjoy all the power for itself (only a small amount really). Now I have no idea whether the electrolytic will be fast enough to charge and discharge or have enough capacity to supply enough current. Let us know if it helps
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Thanks Lindsay. I’ll source those parts and give it a go. I switch the LED on and off rapidly, to simulate flashing, so the current demands will be going up and down quickly, plus whatever the UBEC does internally. The diagram really helped, cheers.
Hey Sneaky,
I know that I gave you a really technical reference for that Low Pass Filter, but its just a resistor and a capacitor. The math is pretty complex, so just do a little experimenting with different sizes and see what happens
I would try Lindsay’s suggestion first though, maybe try adding a resistor next to the diode and you’ve basically made a low pass filter!
