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Old 2nd December 2003, 06:47 PM   #1
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Default System humm

I going mad

I've finaly build my own active high pass XO and want to use it with my DAC, but that's a no go

Signal chain:
DAC (homebrew with a nice +4V offset, I know it is a hughe offset but with a nice big input cap in the preamp that haven't been a problem )
Preamp (4.7uf input cap followed by a 20k ohm attenuator and then a buffer).
then my own active high pass xo and an ashley active xo (used for lowpass).
Lastly to 4 gainclones.

My problem is when using my DAC I get big time humm in both chanels, but when using my computer as source there is no problem (and it sound way better than using the ashely alone).

When only using the ashely xo there is no problem.

So I figured that it must be my own active xo that does something stupid , but why does it work when my computer is the source

My active xo has 3 ground stars. One for the power, and one for signal ground for each chanel. From signal input star there is a wire to the power star ground.

I've tried countless different ground options . The biggest difference I made was a nice offset of my xo (at this point I switched to a test speaker and no I didn't damage my primary ).

My xo is a 6 db filter (a cap and a resistor) before an opa627 with unitygain.

Any ideas?
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Old 2nd December 2003, 07:37 PM   #2
jwb is offline jwb  United States
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Quote:
My active xo has 3 ground stars. One for the power, and one for signal ground for each chanel. From signal input star there is a wire to the power star ground.
I'd say that's your problem right there.
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Old 2nd December 2003, 07:41 PM   #3
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So you say go for one common star ground?
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Old 2nd December 2003, 07:52 PM   #4
jwb is offline jwb  United States
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There's two things you should try. First, connect all your signal shields (RCA ground, XLR screen) to the chassis wherever they enter. Ground everything else at the 0V point of the filter caps in your DC power supply. Connect earth to this point, and connect the chassis to this point as well.

Second thing you can try: isolate the input shield/screen from the chassis, and ground it directly to the star ground at the filter caps.

Remove the input jacks from your case and experiment with grounding them in different places. Do not do this with your good speakers attached.
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