Green’s Dictionary of Slang

a synthesiser.

1975		L.A. Times 22 Nov. 69/7: [advert] Farfisa pro organ Moog Synth [...] will deliver.
1975		Age (Melbourne) 9 Oct. 29/6: [advert] The fabulous Synth Aks A3 oscillator synthesiser.
1976		Liverpool Echo 24 Nov. 5/4: [advert] Crumar String Synth, only one year old, perfect working order, cost nearly £500, accept £270 o.n.o. [OED].
1982		Intelligencer Jrnl (Lancaster, PA) 13 May 26/2: A synth sounds cool and silky as opposed to the electric guitar’s hot abrasive edge.
1988		R. Doyle Commitments 4: He owns a synth though, said Derek.
1999		K. Sampson Powder 175: ABC, Spandau Ballet, Depeche Mode and the rest of the synth-pop set of the early eighties.
2000		W. Shaw Westsiders 17: Terence painted the ceiling black and set up his tracks of synths, mixers and sequencers.