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This question is actually connected with this one: Getting 1.2V from USB

I've successfully connected my FIIO E3 headphones amplifier to USB (so that I don't have to use batteries). But I've notices few things:

1) I connect it to computer USB port. I get some strange noise - like humming and crackling. Actually, when I do something on my computer (e.g. I copy something to HDD), that noise gets louder.

2) If I connect it to the computer USB port with USB extension cord, I don't hear noise from 1), but I hear very loud white noise

3) Then I've tried to connect it to my iPhone charger (it gives 5V from 220V). It works perfectly (no humming or other noise).

So anyone got a clue what's happening ?

Thanks :)

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I don't know what you mean by "connected" your headphone amplifier to USB. I'll assume you mean that you are powering it from the USB. In that case, the noise on the USB power is getting into the headphone signal.

Since the amp apparently isn't designed to filter the power input, you'll have to add that. Put a small inductor in series with the USB power line, then add a capacitor to ground on the amp side of the inductor. You probably don't need much of a inductor. Even a ferrite "chip inductor" would probably help. The most important point is to put a few 10s of µF of ceramic capacitance immediately accross the power input to the amp. A few 100 µF electrolytic in addition couldn't hurt.

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  • \$\begingroup\$ Yeah, I'm powering it from the USB. I already have electrolytic capacitor across the power input to the amp (100 µF), so if I add inductor it should remove at least some of the noise? What value should I use for inductor ? \$\endgroup\$ – xx77aBs Jan 15 '12 at 15:46
  • \$\begingroup\$ @xx77aBs: Go back and read my answer more carefully. \$\endgroup\$ – Olin Lathrop Jan 15 '12 at 17:23
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Computer power supplies are typically noisy. And that noise is getting into your headphones. You need to add some sort of filtering to eliminate that noise before it gets into your amp. Typically you would use some sort of LC low pass filter (LC = Inductor + capacitor). In some cases you might need several LC filters in series.

I can't tell you exactly how to design this filter, since I don't know what frequencies the noise is at, or what the power consumption of your headphone amp is. This is a good place to start. I should also point out that a T or PI filter is very similar to just putting several LC filters in series.

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    \$\begingroup\$ Did you miss part of a sentence? "Odds are that an" \$\endgroup\$ – Kellenjb Jan 14 '12 at 19:19
  • \$\begingroup\$ @Kellenjb Hah! I should never write an answer just after waking up! I'll fix it. \$\endgroup\$ – user3624 Jan 14 '12 at 23:22

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