A hybrid synth introduced by Korg in 1979, at the time of its "Greek letter" synths (along with the Lambda and Sigma). Like several other Korgs of the period, it is divided into a "strings" section (a 1970s-conventional string synth), and a paraphonic "synthe" section. It is packaged in a sturdy case with a 49-key keyboard and the typical-for-the-era Korg joystick, which serves the functions of both pitch wheel and mod wheel.
Both sections derive their tones using a divide-down setup; a single high-frequency VCO is divided down to produce all of the tones. Accordingly, all notes can be played at once, but polyphonic playing has the usual problems associated with divide-down frequency ratios being static and unchanging. The string section provides two signals for each note, in two different octaves, designated as 16' and 8'; they can be mixed as desired. A pair of non-voltage-controlled filters, one low pass and one high pass, can be used to shape the string sound. There is one VCA and one envelope generator for the section, which all notes go through; the envelope is ASR-type with user-adjustable attack and release. A switch selects whether the envelope generator should retrigger each time a note is pressed, or only when the first note is played after all notes have been released.
The "synthe" section also has one VCO which is divided down to produce all notes. The VCO produces only square waves, but it does so in four different octaves simultaneously; four sliders labeled 16', 8', 4', and 2' allow the user to vary the proportions. Careful setting of these can produce an approximate triangle wave. (Korg used this trick in a later model, the Poly-800.) There is one VCF, VCA and envelope generator for the section. The VCF is switchable between low pass and bandpass response. As the Delta was shipped from the factory, the maximum resonance setting of the VCF was limited to a low value, leading users to regard it as not very effective. However, users and techs eventually discovered that by adjusting a trim pot inside the synth, the maximum resonance value could be increased substantially, and the VCF's reputation improved after that. Eventually Korg altered the service manual to call out this higher resonance value in the calibration procedures. The envelope is an ADSR type, and it is routeable to the VCF cutoff frequency, in addition to being routed to the VCA.
An LFO in the "synthe" section can be applied to the VCO using the modulation function of the joystick. Its frequency is variable, but it produces only a triangle wave. The pitch bend function of the joystick can be routed to both overall pitch (of both sections) and to the synthe section VCF, in variable proportions as controlled by sliders. The rear panel contains an external control voltage input for the VCF, and an external trigger input for the synthe section envelope generator. (There is no CV/Gate in or out, and no MIDI retrofit was ever produced.) Each section has an individual output.
The Delta is fairly rare today, despite this, prices are not particularly high on the collector's market. It is said by several leading techs to be difficult to repair. A number of the integrated circuits used are no longer available, although aftermarket substitutes can be found for the NLA divide-down ICs. Additionally, some units (it is not clear which serial numbers are effected) apparently had faulty metal-film capacitors installed at the factory, in the VCF and several other places; these can produce hard-to-trace clicking and spitting noises, and make the VCF controls behave erratically.