AM8400 JP-4 VCO

AM8400 JP-4 VCO

Overview  This is replica of the VCO in the Roland Jupiter 4 (and Pro Mars), their first polyphonic synthesizer introduced in 1978. The VCO was a unique design, based around a 555 timer and a CMOS divider chip, which was not seen in any other synthesizer.

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 4 octave range and 3 waveforms.

This VCO is the sound of early Human League and Depeche Mode albums.

EuroRack Module:   14HP wide
User Manual:              Download PDF


How it works The Jupiter 4 VCO is a unique design, never repeated after 1978, a hybrid analog and digital design. The core of the VCO is a current controlled CMOS clock circuit built from three inverter gates of a 4069 chip. A traditional exponential generator with matched NPN transistors provides the voltage to current conversion and  1V/octave octave calibration. The pulse train is fed to a 555 timer chip with a feedback loop via a two more CMOS inverters. This feedback is added to the original exponential generator current and results is a stable narrow pulse train.

The 555 pulse train is divided by a 4024 divider chip to provide four octaves of range (as square waves), with one half of a 4052 chip selecting which of these square waves is used. The front panel RANGE switch provides a 2-bit binary output to select one of the four octaves. The other half of the 4052 selects one of four different integrating currents, created by a different set of resistors. This current goes into an Op Amp set up as an integrator, with a polystyrene timing capacitor, and is discharged by the pulse train.

This creates the sawtooth waveform. The curved ‘steps’ in the saw comes from the discrete-quantized bits of charge provided from the master CMOS clock and quantized by the 555. The number of steps corresponds to the frequency range, the lower octave at 16′ has 16 steps, and the highest octave at 2′ has just 2 steps. The sawtooth wave is therefore not perfect and is made up of steps, this creates a different sonic shape as you change octave.

Jupiter 4 Sawtooth

The pulse  waveform is created from the steeped sawtooth waveform (and not the square wave), using an Op Amp with a pulse width voltage provided by a set of JFETS that select resistors, and a 2-bit control signal from the front panel. Only four static pulse widths are available (50%, 40%, 20%, 10%). The pulse width becomes narrower as the range increases due to the stepped sawtooth.

The sub oscillator square waveform, one octave below the selected range, is created by a single chip CMOS divider – the DN819. In later Roland mono synthesizers this was created using a 4013 flip flop chip (e.g. SH-101). The sub output has a simple RC low pass filter after it, which reduces the level of the sub waveform at 482Hz (-3dB). This level reduction should be audible in the 4′ and 8′ ranges at the top octaves of the JP-4 keyboard.

The four waveforms (sawtooth, square, pulse and sub) are selected by 4066 CMOS analog switches. The first three are selected by a decoded 2-bit binary signal from the front panel, with 00 being an off status, so that just the sub can be heard. The sub waveform is selected independently of the main waveforms by a slide switch.

The Jupiter 4 VCO is a very elegant design, although it is rather component intensive, so it did not make it into the Jupiter 8 (which is based on the VCO in the Roland 100M). Similarly the voltage controlled ADSR was very complex and was replaced by a dedicated ADSR chip in the Jupiter 8, to save weight and reduce PCB size. The unique Jupiter 4 VCO characteristics are:

  • An accurate square wave
  • A steeped sawtooth
  • A pulse waveform which varies by octave.
  • A sub oscillator (in a poly synth)

VCO Improvements Roland improved the design over time and in the Module Board Rev D they split the power supplies between the digital and analog side and decoupled them. In Rev E they add capacitors on the expo generator to filter out pulses on the KCV signal. Presumably these were coming into the KCV signal from the motherboard. I have not replicated these capacitors in the AM8400.

Pulse Wave

Detailed Description  The AMSynths circuit is very close to the original except for a new Tempco based matched transistor pair rather than the obsolete heated uA726 chip, as the exponential generator. The range has been increased to 4 octaves by eliminating the transpose feature on the original.

There is also a slide potentiometer controlled Pulse Width rather than a 4-way switch, from 50% to 10% and a front panel 1v/octave jack socket for connecting a pitch signal to.

AM8400 Saw Wave

The sawtooth and pulse width waves are quite different to any other synthesizer and contribute a lot to the unique Jupiter 4 sounds, and contain more harmonics and movement. It is a stable design when used with a modern expo generator, with pitch accuracy within 2 cents over 5+ octaves.

Component Upgrades Original components have been used in the AM8400, along with a few careful upgrades:

  • Original DN819 CMOS divider chip
  • Modern 1% polystyrene timing capacitors
  • Metal film 1% resistors
  • Precision matched transistor pair
  • Precision 10V reference voltages

The resistor values in the sawtooth current selector have been retained (736K, 342K, 150K, 39K) although they are not true to the expected 8R, 4R, 2R, 1R values.

AM8400 Square Wave

Front Panel The panel design keeps close to the original with a rather sparse set of controls. Frequency range is selected by a rotary switch from 16′ to 2′, a rotary switch also selects from three waveforms (OFF, sawtooth, square, pulse), and a rotary potentiometer provides fine pitch tuning over +/- 1 octave. There are slide potentiometers for frequency modulation and pulse width modulation amount, with slide switches selecting whether pulse width is manually controlled or from the external input and to select whether the sub oscillator is on or off.

The first version of the AM8400 can be used standalone, or in pairs like the ProMars using the Doepfer bus as the controlled keyboard CV, or four VCO’s can be used with a MIDI to polyphonic converter for the full Jupiter 4 sound. The second version of the AM8400 removed the bus connection and provides a front panel keyboard pitch input.

There are four 3.5 mm jack sockets on the base of the front panel:

  • Keyboard Pitch Input (1V/OCT)
  • Frequency Modulation Input
  • Pulse Width Modulation input
  • Audio Output

AM8400 VCO

Module Availability  The AM8400 module was developed in 2018 – 2019 and a prototype was successfully tested and the circuit adjusted for 12V EuroRack operation. The VCO had an initial manufacturing run in late 2021. The module was further refined in 2022 to use a SMD transistor array and add the keyboard input on the front panel.

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