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Old 18th June 2010, 11:57 PM   #1
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Default Hum from PC

I just built a Chipamp.com LM3883 Mono Block I tested it on the Television rca out to Amp rca input sounded great.

Then my son used his Mbox 2 (Protools) head phone out to PC sound card input(3.5mm) we hear a hum.

Then just the PC sound card out (3.5mm) to Amp rca input and the hum is a little worse.

Are their any sound cards that elininate hum? would s/pdif eliminate hum? (I would have to add a dac.


What could be causing this hum and how is it eliminated?
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Last edited by ppcblaster; 18th June 2010 at 11:59 PM.
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Old 19th June 2010, 12:55 AM   #2
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Sounds like it could be a ground loop..If it is a ground loop,using the OPTICAL spdif output would eliminate it..the coaxial type are still an electrical connection,and the ground loop would still be present.
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Old 19th June 2010, 01:00 AM   #3
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Sounds like it could be a ground loop..If it is a ground loop,using the OPTICAL spdif output would eliminate it..the coaxial type are still an electrical connection,and the ground loop would still be present.
The coaxial type of what? I don't understand, the Mbox2 has spdif out in that look like RCA connections, would that eliminate the hum?

What type or model of dac should I use maybe add a dac to my Pass B1?
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Old 19th June 2010, 01:55 AM   #4
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Originally Posted by ppcblaster View Post
The coaxial type of what? I don't understand, the Mbox2 has spdif out in that look like RCA connections, would that eliminate the hum?
The spdif RCA is commonly known as a digital coaxial.
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Old 19th June 2010, 05:27 AM   #5
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Just a thought....Impedance miss match?
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Old 19th June 2010, 05:42 AM   #6
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Quote:
Originally Posted by ppcblaster View Post
I just built a Chipamp.com LM3883 Mono Block I tested it on the Television rca out to Amp rca input sounded great.

Then my son used his Mbox 2 (Protools) head phone out to PC sound card input(3.5mm) we hear a hum.

Then just the PC sound card out (3.5mm) to Amp rca input and the hum is a little worse.

Are their any sound cards that elininate hum? would s/pdif eliminate hum? (I would have to add a dac.


What could be causing this hum and how is it eliminated?
Yep, you found a ground loop. The SPDIF optical interface will certainly cure your problem at the expense of the added DAC. Most folks try the 'brute force' approach with ground loops -- fatter cables, special grounds and such. An 'elegant' solution is to cancel out the hum by adding balanced inputs to your amp. 'But I don't have balanced outputs from my PC'. You don't need balanced outputs to drive your balanced inputs. You do NOT connect the grounds between the units. You DO connect both channels '-' inputs to the ground of the source unit. Then connect the '+' input of the left channel to the left unbalanced source and the same for the right channel. The ground differential will be subtracted out in the opamp and Voila - no hum. A transformer would do nearly the same thing but you'd have to be listening through the transformer so you'd want a pair of good ones - pricey. I have even added a 'hum input' on a signal source - essentially putting the balanced 'input' as part of the signal source. It's exactly the same thing just mounted on the other unit. You're just adding negative hum onto the output. Works fine.

G
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Old 19th June 2010, 02:47 PM   #7
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Audio Component Grounding and Interconnection
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Old 19th June 2010, 03:06 PM   #8
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I too would guess it's a ground loop...
Try running all the equipment from the same power strip, this reduces the loop size and sometimes helps.
Lifting the circuit ground via a thermistor above earth-potential is also an option (like in the F-series amps, http://www.firstwatt.com/pdf/prod_f1_srv.pdf f.e.). Removing the earth connection from the chassis is potentially lethal, so don't.

http://www.passdiy.com/pdf/ground-loops.pdf
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