Requisite Audio OS1 Preamplifier MK II MICROPHONE PREAMPLIFIER The OS1 was developed around the 6SN7 vacuum tube. The 6SN7 was developed in 1939 and, over the years, has become my personal favorite and often considered king of the general-purpose audio triodes. High headroom and broad frequency response are the hallmarks of the OS1. The OS1 [...]