http://online.wsj.com/news/articles/SB10001424052702303497804579238550068715652

Sometime in early '68, I took the Philips recorder into London's Olympic Sound Studios and had Charlie [Watts] meet me there. Charlie had this snap drum kit that was made in the 1930s. Jazz drummers used to carry around the small kit to practice when they were on the bus or train. It had this little spring-up hi-hat and a tambourine for a snare. It was perfect because, like the acoustic guitar, it wouldn't overpower the recorder's mike. I had Charlie sit right next to the mike with his little kit and I kneeled on the floor next to him with my acoustic Gibson Hummingbird. There we were in front of this little box hammering away [laughs]. After we listened to the playback, the sound was perfect.