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
Thanks
Attachments
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.
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.
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.
Thanks
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.
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.
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?
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?
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
Otherwise adding a parallel 100pF cap with the 100K resistor does not solve anything.
Does anyone have a clue?
Thanks
Have you actually pulled R9 from your circuit and measured the noise coming from X2 output, and X1 output?
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.
Question: What type of input coupling cap is being used? If metallized polypropylene, or something similar, with the volume up tap on the cap gently, or even just move your finger towards it. If it picks up the tapping, or starts to hum as your finger gets closer, remove the cap and reverse the leads. Then try the same test again.
Also, you mention 7812, and 7912 voltage regulators. Do they have the recommended bypass caps? What is the DC voltage feeding them?
Don Taylor
Also, you mention 7812, and 7912 voltage regulators. Do they have the recommended bypass caps? What is the DC voltage feeding them?
Don Taylor
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