USB noise filter

[Thomas]

Well, I took the big step and upgraded my computer. This meaning I also upgraded the sound.

So, I got the Audiophile USB from M audio. It features its own seperate PSU, but somehow noise seems to find its way thrugh the USB connection anyway. Strange thing is, the noise go away then I start playing or do any activity on the computer.

The noise is a continous buzz at a couple of KHz's.

The big question is, can I get a filter that cleans the USB signal. I can make one myself if someone got a schematic for it?

[ukram]

I tried measuring with digiscope the voltage at computer case. If i use peak detect (in Tektronics TDS 210), i see 150 mV peaks at about 8 ms intervals, probably coming from switching power supply. This causes some audible background noise.

I used 10 ohm resistor in my USB DAC to get some isolation between digital and analog ground. This made the noise inaudible. I guess you could try some kind of inductor in series in GND and +5V USB lines. This might cause problems with USB communication...

http://hepso.dna.fi/misc/pcm2902/PCM2902_usb_dac.html

[MWP]

Are you sure its not just a simple ground loop?

I would think the "Audiophile USB" would have some kind of USB ground isolation built in.
If it doesnt, you would have to be a little dissapointed.

Do you hear the noise in headphones plugged into it?

[Thomas]

Originally posted by MWP
Are you sure its not just a simple ground loop?

I would think the "Audiophile USB" would have some kind of USB ground isolation built in.
If it doesnt, you would have to be a little dissapointed.

Do you hear the noise in headphones plugged into it?

Ground loop you say?

The noise was'nt in the headphones, so the problem must be in the amp, then. I removed the cable from the cheap integrated soundcard to the amp and this solved the noise problem. Just like that! I don't realy need that cable anymore now the 'audiophile usb' is playing beautifull music.

Thanks.

UKRAM:

Nice project you have there. Now I solved the problem, there is realy no need to build a filter right now. But, I will try out a few posiblities anyway. It certanly won't hurt the soundquality to try. Maby I could isolate the usb line from the computer totaly, using optocouplers for the data transmision and make a whole new clean 5 v supply and only use the 'real' usb 5v to sense the connection?