Became well known in the mid seventies because of their String Keyboards R490 and Rhapsody 610. These instruments were used by Klaus Schulze (490 for the strings of the Timewind release) and Tangerine Dream (Rhapsody mainly used by Christopher Franke from 1976 till early eighties for handplayed sequences and riffs). They also built electronic organs of different types and sizes. One of their flag ships was the huge, GX1 sized x705 that was used by Jean-Michel Jarre for example for the famous first China concerts.
The also built real synhesizers. Their first was the preset synth Soloist505. It hides two full Moog clone transistor ladder filters (of cause not licenced!) under its keyboard action while all the other electronics are on the back. Maybe because to for better hidding the circuits ? Anyway, one of the Moog ladder filters is inverted and mixed with the output of the other for bandpass filtering. Well done. In 1984 they built a classic analog polyphonic synth: the Synthex.
It was not sold that much, because nobody expected a good polysynth from an italian organ manufacture. But when they ended the production, even Steve Wonder would like to have one. Jarre used it for his famous Laser Harp sound (first sounds on the 1997 release Oxygene 7-13 on part 7 and all over the release Rendez-Vous from 1986).
They also did synths like an 4 OP FM synth using licenced chip by Yamaha. But their approch tried to get the rather complicated approch of FM synthesis into more substractive synthesis by limiting parameters and coupling a few of them by some clever programming to ad brightness without changig multiple OP's modulation indexes. But it were not succesful.
Today there is still a company named Elka, but they are not a direct left over of the real Elka. It is rather a company that bought the right on the name to sell guitar FX.
Elka at sequncer.de