The GMSN! Pure Sine VCF is a 2 pole or 4 pole OTA filter based on the LM13700. It has control over the audio in level, cutoff frequency, feedback and cv in level. The CV In controls the filter cutoff and the feedback can be increased up until the filter goes into self oscillation, creating a sine wave at the output. The feedback is fed back out of phase, hence the self oscillation.

Temperature compensation is provide by a tempco in the same thermal mass as the matched exponential transistor pair, there’s also a 24 turn trimmer to calibrate frequency to 1 volt per octave.

The circuit board is laid out cleanly to make component population and soldering a breeze and the build documentation includes step by step instructions, as well as fault finding hints and tips. There is also a full explanation of how the module works, broken down into the basic building blocks. This isn’t just a kit, it’s a comprehensive introduction to analogue VCF circuit design.