The Sony C37a needs no introduction. The one pictured here is a late 50’s “Old Logo” c37a paired with a CP3 PSU which was made for a short time between the CP2 and the more common CP3b. The CP3 differs from the CP3b in a few cosmetic ways, and also has the larger Fuji output transformer.
The C37a uses Sony's C3 capsule, which is known for it's neutral and realistic response. The capsule has a mechanical control accessible from the back of the mic with a flathead screwdriver that opens and closes the vents in the back of the mic, changing it from cardioid to omni.
The 37a is an incredibly simple design known for its smooth and natural sound which is largely due to its proprietary capsule design. It uses a 6AU6 tube in a cathode-follower circuit which results in a low output but also makes it a great fit for loud sources. The mic itself only contains the capsule, tube, and a few resistors and caps, but the rest of the circuit (filters and output transformer) are located in the PSU, which is why it is very important to have the original PSU with any vintage Sony tube mic.
