Back when, I bought several of the Byrds' LPs and read on the back of one that Roger McGuinn was at a concert of some kind and during intermission he was sitting in the lobby playing his guitar and this guy - Dave Crosby - came along and they sang a song together, and Crosby was doing this "4ths harmony" and it was really great. So they decided then and there to form a band, which they named the Byrds.

Roger ran the line from his guitar thru a compressor and then into the amp. Then he put a 2nd compressor in, and kept adding more until he had SIX in series and that's what you hear, for instance in Turn! Turn! Turn! The compressor subdues the loud sounds and promotes the little ones. There's also a treble booster.

Later, Rickenbacker created a guitar for him with the circuits built in.

After the Byrds, Crosby joined another band called Crosby, Stills & Nash where you can hear more of his harmonies.