Technically, the sounds on Baba O' Riley and Won't Get Fooled Again weren't created by an actual "synthesizer". It is believed that they were created by a theatre organ. A synthesizer may have been involved in Won't Get Fooled again to produce the pulse wave to AM the organ, but that's it. Baba O' Riley was probably just created on a theatre organ with the Banjo sound on and the "repeat" function set at a high rate. I can do the same sound on my organ. And a synthesizer was used on the Beatles' Abbey Road in 1971, but not as extensively as the electronically produced sounds on Who's Next.