The Americans are coming

Here is a trio of American model R331 ribbon mics...

American R331 microphones

These rather pretty looking mics made a brief appearance in Back to The Future (at the end, where Marty sings Johnny B Goode....)

Is this the DeLorean of microphones? No.

The mic can be set to either low (50 ohms), medium (250 ohms) or high impedance using jumpers and clamps hidden behind the badge. The inside of the mic looks like a bowl of spaghetti where all the wires from the transformer are run out to the panel.

Impedance settings for the R331

The mic also has a block of wood in it! In fact American Microphones made a range of three ribbon mics, and the sister models - the DR330 and DR332, both employed a dynamic capsule as well as the ribbon motor. In the R331, the wooden block fills the space which would otherwise be occupied by a dynamic capsule - a low tech but effective solution to the problem.

The motor is simply made of two magnets screwed and welded to a metal frame, and the ribbon itself is about 2.1 mm wide. This one needed some welding and a new ribbon, but once repaired sounds nicely balanced for a vintage ribbon, with the transformer rolling off some of the bottom end to compensate for the proximity effect. 

American R331 ribbon motor assembly 

There is a lot more information about the R331, DR330 and DR332 at Coutant.org

Thanks to Myles Davis