Mixer output too low

Hello synth enthousiasts,

I have built a mixer some time ago. It was one of the first working modules I designed myself. It works, but the output volume is too low. What I would like to have is that when you only use one channel of the mixer and turn it all the way up, it would be as loud as directly plugging that signal in. Unfortunately, its not loud at all.

Here is the stripboard layout. So I have an inverting adder and another inverter to get the original sound back. Those together give me inverted and non-inverted outputs.

(FIXED: Previously uploaded an outdated version)

The green cables go towards 50K potentiometers that act as attenuators. I suspect that I picked the wrong value for the resistor that is marked in red in the picture.

I looked at this page when making this stripboard layout. Which contains the following formula

So my guess is that when I turn the volume of one of my channels all the way up, I would get a single term inside this equation and have a gain of 1 because numerator & denominator are both 100k. All the other input voltages are zero because I have nothing plugged in and then an attenuator is simply a variable resistor to ground.

Am I missing something here? In a strange case, it is a bug in my output module, but I think that is unlikely.

Thanks in advance.

The labels “output” and “inverted output” should be switched around, and the layout on the right side is messed up — there’s nothing connecting to pin 5 (IN+), while pin 6 (IN-) connects to ground and the input (yellow jumper) is connected to pin 7 (OUT).


Oh my apologies! :flushed: I checked the mistakes and apparently I have uploaded an outdated version of my stripboard. I fixed the labels on the wires as well as the yellow & brown wire on the right side. This now reflects the actual build.


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I’m not sure what exactly happened, but the problem seems to have disappeared. The 100k resistor is actually the right value after all. It might be me being silly and having made a mistake with the setup, causing the problem. Nothing to see here.