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I've builded a simple amplifier (just stereo audio in, two speaker outs) and I'm building a mixer for it.

There are some audio inputs, which goes through stereo potentiometers (volume for each input) and then all into main potentiometer (master volume). It looks like this: schematics

EDIT: The schematics is wrong. To wiper is connected output, not ground. To resistive strips is connected ground and input.

But when I connected, I find out how stupid this is. The potentiometers are influecing each other, so when I lower one of the outputs by turning potentiometer, other inputs's volume's are also lowered. I tried to solve this by putting diode after each potentiometer, before the wire are joined, but this extremely lowered audio quality (the speakers were just buzzing the melody).

How to solve this? Is there any diode made for audio?

And my second problem is, I have mono wireless microphone with mono outpout (output from lm741 based preamplifier) and I want to make it stereo, but if I will connect the output to both L a R signals and joining them, as far as I know, the stereo inputs will no longer be stereo.

How to make mono signal (one input) playing on both, L a R signals, without destroying the stereo on other inputs?

Thanks.

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  • \$\begingroup\$ The simplest way is a "virtual earth mixer" (which uses another opamp) - searching that should give you plenty of information. \$\endgroup\$ – Brian Drummond May 27 '16 at 21:48
  • \$\begingroup\$ The simplest way is to add isolating resistors of say 10k in series between each input and its potentiometer. You have them connected wrong. The far end should be earthed, and the output should be taken from the wiper. \$\endgroup\$ – user207421 May 28 '16 at 0:47
  • \$\begingroup\$ As to why diodes don't work, audio is AC, and diodes are extremely non-linear (that's kinda their defining factor). Rectifying audio ruins the audio, and the small amplitudes involved in audio also disagree with the forward voltage of the diode. \$\endgroup\$ – uint128_t May 28 '16 at 2:32
  • \$\begingroup\$ @EJP I tried this, the influence is smaller, but is still there, and the volume really dropped. \$\endgroup\$ – Adam Ježek May 28 '16 at 9:48
  • \$\begingroup\$ The voltage can only drop 10% with those values if you did it as I described, and that is only a fraction of a decibel \$\endgroup\$ – user207421 May 28 '16 at 12:38
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You need a simple mixer circuit based on the summing amplifier.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Stereo mixer basic schema.

As Brian Drummand says, you need a virtual earth mixer. Here's the basic schematic showing how to handle the mono input.

This diagram does not show power-supply and capacitors. You'll find details in a web search.

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As the circuit is drawn, you are grounding the wiper of the potentiometers. This is wrong. To wire a potentiometer as a potential divider, you need to connect the input to one end of the track, ground to the other end of the track, and the output to the wiper.

As wired, you are dragging the outputs down to ground. Which is why the pots are affecting one another.

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  • \$\begingroup\$ I just realised that the schematics is drawed wrong, it is connected as you are saying. \$\endgroup\$ – Adam Ježek May 28 '16 at 9:46
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    \$\begingroup\$ @AdamJežek So please fix it. \$\endgroup\$ – user207421 May 28 '16 at 12:39
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The active circuit as shown by @transistor is certainly the preferred method. But if for some reason you need a PASSIVE solution, then a circuit like this is typical....

Three-input passive stereo mixer

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