The American DR 330, R 331, and DR 332 ribbon and ribbon/moving coil dynamic microphones
This is a combination ribbon and moving coil microphone made by the American Microphone Company of Pasadena, California. Frequency response is from 40 to 15,000 hertz, with an output level of –86 dB. Polar patterns are cardioid (ribbon and moving coil), bi-directional (ribbon only), or omni-directional (moving coil only). Impedance is switchable for 50, 150, or 250 ohms. Dimensions are six inches high and two inches in diameter. Weight is twenty ounces.
The inside of the cover plate (above) and the actual terminals (below) determine the impedance and sound acceptance pattern of the American DR 330. This microphone is currently configured for an impedance of 250 ohms, utilizing a cardioid polar pattern.
The following three photos show an American R 331 ribbon mic, and are provided here through the courtesy of Dennis Schrank.
The following four photos show an American R 331 microphone, and are provided here through the courtesy of Stephen Sank.
Thanks to Mr. Sank for the following clarification: There was never a model “DR 331.” It was a typo in some of the brochures. The R 331 is the only 331 model, and is purely a bipolar ribbon mic. If you want a bit more info, here is what I know: The DR 330 and DR 332 were made, like the Coles 4033, under license from Western Electric, the company that produced the first ribbon/dynamic hybrid mics. The ribbon is 0.076 of an inch wide, identical to the RCA KB-2 and Shure 300/315/330/333/SM33 mics, and of similar length, but of thicker aluminum ribbon material (estimated at 30 per cent thicker).
The aluminum diaphragm dynamic element has a pronounced “squawk,” i.e., broad midrange peak, to its nature, which was partially (but not nearly enough) corrected by the r/c network across its terminals (which does not exist in early examples of the DR 332 series I have worked on). For this reason, the cardioid mode of the DR 330/DR 332 sounds excessively midrange heavy compared to the largely flat response of the ribbon section. The DR 330 has jumper options under the label plate to select a bipolar “ribbon only” mode, which is quite an advantage, sonically, over the DR 332, which has only impedance selection under its label plate.
That’s about all the trivia I can add at this time. One indirectly related bit of info of which I have yet to find the answer: Did Western Electric do the moving-coil/ribbon hybrid to duck RCA/Harry Olson’s patent on double ribbon pattern creation? I am of the opinion that the answer is “yes,” since I am fairly sure W.E.’s hybrids came after Harry’s 77A mic.
The following schematic is for the American Model DR 332 Microphone. Though similarities exist, corresponding data for the DR 330 and R 331 are unavailable.
Actor Fritz Weaver end-addresses an American microphone during “The Obsolete Man,” an episode of The Twilight Zone.
This is an interior view of a DR 332 with a damaged ribbon element. In the words of Stephen Sank: This was taken before I removed the wiped-out original ribbon, so it may be of some value to show what a ribbon should NOT look like. Also visible is the omnidirectional moving coil element, facing straight up, under the ribbon motor.