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I have an audio application scenario here. The Idea is to power a device (that later produces sound) with USB.

Unfortunately, I have massive static and what sounds like pink (Brownian) noise on GND. I also have a lot of static and other artifacts on the +5V lead, but I figured I can use a low-dropout voltage regulator to get a nice, smooth +3V out of it.

But how do I filter out GND?

Right now, I was experimenting with all sorts of low-pass and high-pass filters, constructed from caps and resistors. The caps, however, introduce artifacts themselves, it it's like two steps ahead, one step back.

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  • \$\begingroup\$ But your 3V rail is stable in reference to ground, correct? \$\endgroup\$ – Kortuk Jan 31 '12 at 11:45
  • \$\begingroup\$ I haven't implemented the solution with the regulator yet. The regulator is gonna arrive next week or so. I was asking ahead, since I get the noise even when I connect only GND to the device. when only +5V is connected I also get some noise, when both are connected, it's a combination of those. \$\endgroup\$ – polemon Jan 31 '12 at 12:06
  • \$\begingroup\$ @polemon, the regulator is designed to make the potential between ground and 3V rail always the same, it will bounce the 3v rail to match the ground rail. This makes anything using both not see noise. \$\endgroup\$ – Kortuk Jan 31 '12 at 18:49
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    \$\begingroup\$ Possible duplicate of Solving input noise \$\endgroup\$ – Nimjox Jun 14 '17 at 21:25
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You don't filter ground. "Ground" is the reference voltage all other voltages are measured to, so it has no noise by definition. (There are cases in distributed systems where what exactly "ground" is is not clear, but that's another issue)

USB power can certainly be noisy. You would definitely want to filter it before it powers audio circuitry, since audio generally requires high signal to noise ratio. Just a LDO to make 3V is probably not good enough. The LDO will reduce some of the noise, but some will be too high frequency for its active circuits to deal with properly. You need to put some low pass filtering before the LDO. This filter needs to eliminate the frequencies the LDO can't handle properly.

I would start with two ferrite chip inductors in series and a 20 µF cap to ground after each. Then connect the input of the LDO to that, and the output should be pretty clean. You can also just low pass filter the 5V and use it dircectly to power the audio circuit. A typical audio "line" out level is around 1 V, but you need headroom for higher peaks. Even a 5V supply doesn't leave much headroom.

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  • \$\begingroup\$ As for low pass filtering, I use the standard approach with a resistor and a cap: en.wikipedia.org/wiki/… I can definitely hear the higher noise being filtered out, however I can't get rid of the hum that is on much lower frequencies. \$\endgroup\$ – polemon Jan 31 '12 at 13:14
  • \$\begingroup\$ Low frequency hum is probably not coming from the USB power. That sounds like power line pickup. It could be due to capacitive coupling from the environment or possibly from a ground loop depending on what you are connecting this USB-powered circuit to other than the USB. \$\endgroup\$ – Olin Lathrop Jan 31 '12 at 13:27
  • \$\begingroup\$ It is not the usual 50Hz hum we have here in Europe, it is some other hum, something like pink noise. \$\endgroup\$ – polemon Jan 31 '12 at 13:31
  • \$\begingroup\$ I ordered a fixed voltage regulator, once this works OK, I'll accept the answer \$\endgroup\$ – polemon Feb 6 '12 at 13:23
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Ground can't have noise, it's clean by definition, like Olin says. Suppose you wanted to filter its "noise" with a capacitor, what other node would you connect the capacitor to? Vcc? Then you're filtering Vcc's noise, not ground's.

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The noise from USB power comes mainly from cheap powerstation which powers your motherboard. There is many ways to clip that noize by: (1) Checking that your powerstation has ground wire, checking that your ground in house is real ground, checking that your powerstation has connected middle point into ground.

(2) Next, could be good idea to swap your cheap powerstation with real powerstation made from voltage transformer. Our modern day chineese trash powerstations are made from ferrit-type conversions to high frequencies, so even by using such powering you are basically loosing all opportunities to power audio production - all microphones will catch that high frequency trash.

First, need to use lowering transformers to obtain proper +12, +5, 0 and so on to power your motherboard. Our modern day powerstations cheat with that to save a dollar, they all have trash outputs which will cost you hundreds of dollars next.

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