Right when the legendary professional instruments were developed, Sequential Circuits decided to make a counter move and try to take a niche of budget synthesizers for learning and for churches. In order to implement the intention the brand turned to Siel, the Italian manufacturer of electric organs. The result of this intercontinental collaboration was Sequential Circuits Prelude, also known as Siel OR400 "Orchestra 2".

The synthesizer is a completely polyphonic "orchestra" consisting of four sections: piano, organ, wind and string. Polyphony covers 49 keys allowing two sections to be triggered simultaneously. For each section, a small number of controls are available to optimize the sound.

Sounds from the sections are fed to a common 7-band graphic equalizer and an effects unit that contains a flanger and chorus.