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I am trying to make a simple audio mixer with several audio jacks as input and one output to headphones.

I built a circuit on a perfboard using an LM358A with only a positive power supply. It seems that to make it work I should feed half of my input voltage to the op-amp's "+" wire.

It works, but I have a strange problem: when I connect an audio jack to one of inputs, I hear nothing in the headphones, but after 5-10 seconds the audio slowly starts to play with some crackling and then continues to play fine. I can't understand what causes this several seconds of silence.

If I quickly reconnect the audio input the audio plays fine. But if disconnected for a few seconds and connected again - same behaviour, a few seconds of silence and then audio starts to play. What can cause this?

I tried another LM358A - same problem. I built a very minimal setup on a breadboard without potentiometers and capacitors to test it out, but the problem persists.

Schematic

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Try adding a 100nF capacitor in series with the input.

There may be a large value capacitor in the source that has to charge in order to allow your op-amp to reach bias, since your amplifier is DC-coupled and it sits at 4.5V for the input.

LM358 is not a very good op-amp for audio- it has visible crossover distortion for even very low frequencies.

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    \$\begingroup\$ I agree that the LM358 produces poor audio. \$\endgroup\$
    – Audioguru
    Sep 16 at 0:09
  • \$\begingroup\$ Your circuit is missing a series input capacitor, series output capacitor, power supply pin to ground capacitor and +input pin to ground filter capacitor. The second opamp must be properly used or properly disabled. Your circuit has a gain of 1 like a piece of wire but its series 1k output resistor attenuates signals to headphones. \$\endgroup\$
    – Audioguru
    Sep 16 at 0:23
  • \$\begingroup\$ I think 100nF capacitor did the job! Thank you very much. I read that LM358 is not good for audio, it just was in the schematic I wanted to replicate. I found another one with TL072. \$\endgroup\$
    – Alex_G
    Sep 16 at 1:09

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