Dave posted this vid along with a write-up, covering the ins-and-outs of the unbalanced diode mixer circuit -

This circuit uses the small non-linear response area of a single diode to create combinations of sum and difference frequencies of two input signals (or one input signal containing multiple overtones). Radio designers use this type of circuit to “downconvert” received RF signals to a lower intermediate frequency, which makes it a lot easier to design the radio’s signal processing circuitry. We can use the same circuit for electronic music to generate non-harmonic overtones. (In the RF circuitry literature, there is a class of related circuits that all use diodes to do frequency mixing functions. What we call a “ring modulator” originated as a more sophisticated version of the circuit presented here.)

Much more info + schematic available on his blog entry.

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Make presents: The Diode