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The simplest mixer, which can be done in 20 minutes on the knee shown in the diagram:
The gain of the mixer is determined by the ratio of the resistance of the resistor R7 and the output resistance of the signal source. If you really need, the number of inputs can be increased simply by connecting additional resistors to C1. By the way, is permanent, you can include variable resistors.
The transistors can be replaced by KT315B or CTB. The last is most preferred.