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Old 8th July 2008, 03:06 PM   #1
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Default opamp hum and buzz problem

Hi i would be glad to receive help on the following issue. I have attached circuit built on PCB. with no source connected and the pot set set to full attenution there is no noise. But with the pot set to max volume there is an audible hum and slight buzz with no source. I've changed amps/regulated 7812/7912PSU still same issue. Is there a way to solve this issue ? I've traced faulty circuit as being encircled section.

Thanks
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Old 8th July 2008, 03:55 PM   #2
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Default circuit?

Do you have a picture of the circuit? Top and bottom? This'll make it easier for everyone to see how you built the circuit. Something may be connected wrong on the board.
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Old 8th July 2008, 05:29 PM   #3
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What is this circuit for?

It has no voltage gain...
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Old 8th July 2008, 06:08 PM   #4
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With no source the input impedance is the 100k resistor, so it will easily pick up hum unless completely shielded. As asked for above, a person would have to see how it was laid out and built to offer any more thoughts.
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Old 10th July 2008, 06:35 PM   #5
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Hi thanks for your answers. Firstly this circuit is used as an impedance match in between input/pot and amp. I will post the layout soon.
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Old 12th July 2008, 03:26 PM   #6
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Default Layout

Sorry weekend here i can DIY. I've attached a section for the circuit layout am in process of taking pictures. Will post them shortly. The section denoted in the Orange box is the section causing noise.
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Old 15th July 2008, 11:34 PM   #7
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The layout looks ok. We have no indication of what your actual circuit board looks like. Are all of your solder joints good?

One problem I had was with a pot once. The pot would pick up noise when it was at minimum attenuation. Sounds just like the problem you have. I grounded the metal shaft of my pot and the noise went away. Have you tried to ground the metal shaft of your pot?
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Old 16th July 2008, 05:59 PM   #8
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Hi thanks for your reply. Well no its not ungrounded pot. the layout does not show it but i have a ground lead soldered to the pot. Like i mentionned the issue is coming from the non inverting buffer. Setting the pot to full attenuation cuts all noise. The opamp is bypassed by 100nF ceramic caps paralleled with 10uF caps. The only thing that really decreases this noise is lowering the resistor value from 100K to <10K.

Otherwise adding a parallel 100pF cap with the 100K resistor does not solve anything.

Does anyone have a clue?

Thanks
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Old 16th July 2008, 06:07 PM   #9
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Have you actually pulled R9 from your circuit and measured the noise coming from X2 output, and X1 output?
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Old 17th July 2008, 07:46 AM   #10
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Hi agossage thanks for your answer. Thats what i did though i don't have any scope but i've plugged it into an amp and listened to noise, all was coming from the non inverting buffer opamp and no the second stage alone.
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