The ETI 4600 synthesiser was the brainchild of Trevor Marshall.

Trevor Marshall was interested in electronics from the early age of 9, when he was given a crystal set to play with. At 15, Trevor was the youngest person in Australia to be granted an amateur radio operators licence.  He studied Electronic Engineering as a student at Adelaide University, and in 1974 achieved a Bachelor of Engineering degree. At the age of 24 he had completed the complex designs of 2 music synthesisers - the ETI 4600 and the ETI 3600. Trevor designed them with the prerequisite that they had to be able to be constructed from kit form, were high quality, and were able to be assembled by any average person.
 

The intention was to make available to pop groups a synthesiser that would cost less than the average of around $1,400 (Australian Dollars). The magazine Electronics Today International approached Trevor and featured the design in a number of articles.

Barry Wilkinson of ETI was responsible for the packaging and kitting coordination. It was adapted for Europe by Maplin Electronic Supplies of Rayleigh, Essex who produced full construction details, and were able to supply all the components for the project including PCBs, printed panels and case.

Trevor Marshall at the age of 24 in 1973 

The information provided here was kindly provided by Trevor, and from newspaper articles from The Adelaide News (5th October 1973) and The Adelaide Advertiser (25th February 1974). I have provided the 2 articles here which you may like to read in their entirety:

'Trevor's Invention a Winner' Adelaide News 5th October 1973

'The Wizard Wears Ear Plugs' Adelaide Advertiser 25th February 1974