A 4-voice polyphonic analog synthesizer with a 49-note keyboard. The first synth in the Jupiter line, manufactured between 1978 and 1981.

The JP-4 offers 1 VCO per voice, switchable between sawtooth, square and square with PWM waveforms. A sub-oscillator and a noise generator are included, but can only be switched on/off with no level control. All 4 oscillators can be stacked together for a thick monophonic sound.
In the filter section, the Jupiter-4 offers a hi-pass VCF, as well as a resonant low-pass one. There are two ADSR envelope generators - one (invertable) for the VCF and other for the VCA.
The LFO features sine, square and ramp up/down waveforms, with sample/hold. It can be routed to the VCO, VCF, VCA or PWM. The range on the LFO is quite exceptional, with what has said to be the lowest rate found on any production keyboard. The highest rates are around 100Hz, making the LFO very flexible.

An arpeggiator is also available, but sadly can't be synced to an outside source. It features up, down, up/down and random modes. Other features include a chorus effect, and most notably an internal memory for 10 user programmable patches! The JP-4 was one of the very first commercial synths with this feature.
For external control, a CV/gate interface is included. MIDI retrofit kits are easy to install on the machine.

On a personal note, I must say I really love my Jupiter-4. Sure, its keyboard isn't that good, most of the 10 preset sounds are totally useless and the unit is quite monstrous in size and weight. But the modulation possibilities are nice, allowing very nice and exciting analog sounds to be created.
The JP-4 is often overlooked with the later Jupiters getting much more attention due to having more features. This is too bad, since they sound completely different from the first one.

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