This started as a classic ringmod project. It's based around a very classic transformer design of a ring modulator. It has a built-in oscillator
providing audio-frequency square and expo-triangle waveforms.
People say it doesn't sound as a classic ring modulator, though! I do not hear the difference, but it definitely makes fun when one of the inputs is
in the inaudable range of frequencies (e.g. an LFO), and sure helps to make a hell of a drone tone out of a squarewave.
HOW TO CONNECT STUFF:
look at the placement pic. Locate the TRI, SQR and MOD IN pads. Either take an ON-ON switch and connect side contacts to TRI and SQ and the middle (common)
pin into the MOD IN if you only want to use the internal modulation source, or connect the middle contact of the switch to a normalized signal of a 3.5mm
jack socket, and the signal of that socket to MOD IN, so the internal osc is normalized to MOD IN if nothing is inserted into the modulation jack, and if
some external signal is inserted, the internal osc isn't passing. AUDIO IN and AUDIO OUT are input and output, respectively. GND pad is used for grounding
the jacks! (do ground them, don't be lazy). WIRE lines are uninsulater copper lead traces made on the component side of the board.
LT44 transformers are easily acquired at eBay.
Actual module photos are of first version: it has a buffering op-amp glued to the PCB and wired to stuff. This op-amp is included in the final board!
Also, if you want one like this, you can try writing to my mail to check the PCB stock, i might still have factory ones! (firstname.lastname@example.org)