Vintage Neumann SM69 Stereo Large Diaphragm Tube Condenser Microphone with original NSMa 23 power supply, and original wooden box. The SM 69 was Neumann’s first large-diaphragm stereo microphone. Its descendents include the solid-state SM 69 fet, and later the USM 69.
All three microphones contain two large-diaphragm K67/K87 center-terminated capsules, mounted one atop the other. The top capsule assembly can be rotated 270° (presumably, -90°–+180° like the USM 69). The K67 capsule uses dual 26mm diaphragms on dual 34mm backplates. The same capsule was used in the U67 and U87.
Both capsules could be set, via a pair of rotary switches on the external power supply, to Omnidirectional, Cardioid, Figure-of-8, or any of the six intermediate patterns.
This feature, combined with a rotating top capsule assembly, allowed the SM 69 to support a variety of single-point stereo pickup patterns:
X-Y - With both capsules set to Cardioid, rotate the top capsule 45°–120°.
Mid-Side - Set the lower capsule (“Mid” channel) to Cardioid; set the top (“Side channel”) to Figure-8 and rotate it 90°.
Blumlein - With both capsules set to Figure-8, rotate the top capsule 90°.
The mic contained two separate amplifier circuits, and two separate Telefunken AC701k triode tubes. The output stage was transformer-coupled.
The mic’s output impedance was 200?, but could be reconfigured for 50? by “connecting the secondary windings of the output transformer in parallel” — which would reduce the mic’s output by 6dB.