I have built a digital VCO with a dspic30f3013. It is a numerically controlled oscillator (NCO) with the exponential conversion performed in the dspic using a lookup table. The NCO phase is used to lookup a triangle waveform. The triangle wave is output through an 8bit R2R DAC. The normal VCO waveforms, sine, sawtooth, and PWM, are generated using analog circuitry. For this first version, I wanted to create a traditional VCO, but future versions may include additional waveforms.
I have built a simple sallen-key VCF using a miniature tube (1J17B) as the main amplifier. There are a few opamps in the signal path, but they are only used with a gain of 1 as inverting and non-inverting buffers. The circuit is based on Ken Stone’s tube VCA which uses the 1J24B. The cutoff frequency is controlled using a dual vactrol.