op amp Humming and Transmission issues

Posted on Feb 7, 2014

When connection to PC speaker is plugged in and Telephone connection(bottom-left) is plugged in, I hear humming in the headphones. When only one is plugged in I hear it without distortion. Sound files showing humming. If you criticise my circuitry, add something to act as a replacement, too. Just saying that it won`t work because of this and that is not as

op amp Humming and Transmission issues
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helpful as saying: "It won`t work because of X, which is why I recommend doing Y". I try to update this question with the current circuit so you don`t have to scroll through all replies to get the current status. PS: Circuitry for telephone connection taken from a recording interface from Tekniikan Maailma magazine. I chose it to be able to use the circuit in parallel with my current phone. Regarding your power supply. You have created a dual supply with two batteries. Batteries have internal resistance and so should be bypassed by capacitors. Any resistance in your return circuit can create ground loops. When a dual supply is created using batteries, there is a risk that it will go lopsided if one battery loses voltage faster than the other one. You have a number of sloppy choices of part values in your circuit. For example at the bottom, you have a virtual ground mixer whose input impedance is is only 100 ohms. First of all, that is a low input impedance to ask the other op-amp to drive. Secondly, it loads down the 10K pot that you have in front of it. Weird things will happen. When the pot is turned all the way up, the impedance is basically just the 150 ohms. The pot`s impedance to ground doesn`t matter because it is much larger. When you turn it halfway, the impedance goes up because now you have a 5K branch of the pot in series with the input impedance. If you want a potentiometer to be linear, it should have an...

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