AM8400 JP-4 VCO

Description 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.

The AMSynths circuit is close to the original except for a new Tempco based matched transistor pair rather than the obsolete heated uA726 chip, as the exponential generator. This improves warm up time and precision. 

The panel design keeps close to the original with a Pitch potentiometer without center detent, Range selection from 32' to 2', a choice of three waveforms (sawtooth, square, pulse), frequency modulation depth and pulse width depth.

The rotary control knobs are Roland type with an inlaid orange position bar and the slide potentiometers have orange LED's. The pulse width can be varied from 50% to 10% using the manual slider or a 0 to +10V control voltage. 

• Sawtooth is 0 to +10V
• Square is 0 to +10V
• Pulse is 0 to +10V

There are slide switches to turn the sub oscillator on, as well as change PW modulation from manual to external control. We suggest using the VCO in pairs, even though there is a sub oscillator on board, to provide a thicker tone without  the need for a chorus.

This is a Version 2 module with an external KYBD input socket for connecting a pitch CV and orange panel graphics and control knobs with an orange bar. This module does not use the Pitch CV signal in the Doepfer bus.