Searching the web for a simple solution I found this circuit, published by Peter N. Petersen:
The circuit is a traditional opamp based mixer. The opamp suggested here is a TL072, unfortunately that chip was out of stock here. I found a nice replacement in my junk box: the LF356, although it has a different pin layout. Most opamp’s with FET inputs will probably do the job.
R1 47k R2 47k R3 27k R4 10k R5 10k C1 1uF / 16V C2 1uF / 16V C3 3.3uF / 16V C4 10uF / 50V U1 TL072, OP27, LF356, etc... (check the pin layout of the selected opamp)
Check out Peter‘s website for more details.