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Old 14th April 2012, 03:08 AM   #1
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Default Noise unless I touch audio jack

I built a circuit that runs from my amp to my computer. It steps down the large speaker output voltage to line level. There is audio hum that is fairly loud, but whenever I touch the phone jack that connects the computer to the circuit, the noise goes away. How can I get rid of the noise without being there to touch the circuit?
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Old 14th April 2012, 03:14 AM   #2
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It also stops the noise when I touch the power cord connecting to the wall.
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Old 14th April 2012, 03:19 AM   #3
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Maybe post a drawing of your setup, then someone can recommend how to ground it.
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Old 14th April 2012, 03:20 AM   #4
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sounds like a grounding problem,you could take a volt meter and put one probe on the computer and one on the amp and see if there is reading there ,if so you could run a ground between them or use a matching transformer ,and if you do use a transformer ,maybe a line transformer it has alot of taps you could use to step down the voltage ,Have you tried the headphone jack from the computer for signal to your amp it's already low level,,,,,good luck
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Old 14th April 2012, 03:34 AM   #5
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EDIT:I forgot to draw the transformer from 120V AC to 12V. It immediately precedes the diodes.

Schematic

Here it is. I blackboxed some of the simple resistive networks. The circuit works perfectly (aside from the noise). The capacitor has about 12V DC across it and I change that to +/- 5 and ground using regulators. The voltage is 4 or 5 volts peak at the isolation transformer and about 100mV at the voltage buffer. I'm pretty sure tying the audio output ground to AC ground would be a pretty bad idea because of the 5V difference (and possibly because it is AC ground).

Last edited by Littlebilly91; 14th April 2012 at 03:51 AM.
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Old 14th April 2012, 04:28 AM   #6
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Quote:
Originally Posted by Littlebilly91 View Post
EDIT:I forgot to draw the transformer from 120V AC to 12V. It immediately precedes the diodes.

Schematic

Here it is. I blackboxed some of the simple resistive networks. The circuit works perfectly (aside from the noise). The capacitor has about 12V DC across it and I change that to +/- 5 and ground using regulators. The voltage is 4 or 5 volts peak at the isolation transformer and about 100mV at the voltage buffer. I'm pretty sure tying the audio output ground to AC ground would be a pretty bad idea because of the 5V difference (and possibly because it is AC ground).
If your schematic is accurate, you have mistakenly used a 7805 twice, for both +ve and -ve supplies.
This is impossible:

You must use a 7905 for the NEGATIVE rail!!!!
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Old 14th April 2012, 04:46 AM   #7
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Good save...sorry, I wrote that up in a hurry.

Right now I have the power cord taped to the metal leg of my desk to keep it quiet. I would assume that neither my desk nor I are grounded properly. Why does the noise go away?
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Old 14th April 2012, 06:57 AM   #8
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Is the opamp unity gain stable.
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Old 14th April 2012, 07:18 AM   #9
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LT1013CP Datasheet
I am reasonably sure that this is the right datasheet. The label on the IC is near impossible to read.

I believe it is unity gain stable. I haven't had trouble transmitting the signal. If I grab the power cord coming from the outlet, the noise goes away, so I don't think that is related to the op amp's stability. I'm almost positive it's a grounding issue. If I tie the signal ground to AC ground, I am worried that I will get a ground loop when I plug into my computer.
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Old 14th April 2012, 08:31 AM   #10
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It sounds to me like an oscillation problem , the hum is being carried by RF . If you havent got something like 100 ohms in series with the output of the IC i suggest you try it . Also is the IC fully decoupled with 0.1uf ceramic caps on each rail pin etc and have the voltage regs got all the required decoupling .

Last edited by epicyclic; 14th April 2012 at 08:36 AM.
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