AM8400 VCO

VCO from the Jupiter-4 with sub-oscillator

The AM8400 module is a replica of the VCO in the Roland JP-4 polyphonic synthesizer that was introduced in 1978. The original VCO is a unique design that uses a 555 timer chip and a CMOS divider chip to create square waves which are then shaped into sawtooth and pulse. Most VCO’s use a sawtooth or triangle core.

This unique design makes for some very different sawtooth wave shapes and tonality. The Jupiter 4 is famous for its powerful sounds and efficient controls. The constraints of microprocessors in 1978 mean there were only 16 sliders that could be memorised, which limited the functionality of the VCO to a 4 octave range and 3 waveforms.

The JP-4 has only one VCO per voice, so Roland added a sub-oscillator which generates a square wave one octave below the VCO frequency. The sub can be switched ON or OFF, and the waveform switch has an OFF setting so that just the sub oscillator can be heard.

https://www.amsynthstore.co.uk/AM8400_JP-4_VCO/p1461448_21433917.aspx