Try /synths or/manufacturers. To get a specific use the following format /synths/1 or/synths/Roland Jupiter-8.
You could use &yearProduced=1980 to get synths produced in that year. Or get synths from a manufacturer: &manufacturer=Roland. You can also change default pagination options &limit=20&offset=0. Chain queries together with '&'. For more API exploration, read the doc's!