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I'm trying to add a bluetooth audio receiver to an old 12V battery-powered boombox. The receiver board takes 5V and shouldn't draw much current, but I'm trying to understand which type of DC-DC converter would introduce the least amount of noise in the audio frequency spectrum. (And since it will be battery operated, efficiency wouldn't hurt either.)

The reason I ask is that I've noticed a very audible buzz on my car stereo when my phone (connected by an aux in cable) is connected to an automotive USB charger. I want to avoid this terrible noise.

A voltage divider seems like the simplest and quietest possible option, but certainly not the most efficient, and definitely not the wisest.

I looked at this similar question and I get the sense that both zener circuits and linear regulators are also going to be very inefficient for this application.

This question on buck converters suggests that they are very noisy.

Is SMPS the best approach? The circuits I've seen online seem like extreme overkill for getting the ~100ma that I'd imagine would be required by the board I have.

Is there a different design that I should be looking at? Should I be dealing with the noise a different way? Is the noise that I'm trying to avoid a non-issue given the relatively small load I need to power?

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Yes, a linear regulator won't be the most efficient for this application. It will, however, be the least noisy, especially if you select a regulator designed for low noise such as the LP2985-50. And at 100mA there will only be 700mW loss, which may be considered acceptable for the application.

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For this kind of application a linear regulator is your best choice. The circuit below is simple and cheap. If you don't have the parts, you can get all of them at some place like Fry's Electronics, or still some Radio Shacks. LM317 12Vin, 5Vout

12V in is on the left, 5V out is on the right. Ground is on the bottom. The resistor values aren't that critical, but their ratio is. The ratio should be as close to 3 to 1 as you can get.

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