RCA's 77 line of ribbon microphones comprised Harry F Olson's migration from the large ribbon, figure 8 systems of the 44 series, to a small ribbon, multipatterned device. The 77DX was the higher output version of the 77D which predated it. One can easily look up the published performance specs of the 77 series, but what really sets this line apart from the others was the attempt to produce a true multipattern ribbon microphone using a rear lobe acoustic suppressing system involving a variable acoustic shutter. In addiiton, a selectable shunt reactance across the output could be switched in or out to reduce bass. The complexity of this arrangement, which I think represents a penultimate design for the time, is apparent to anyone who takes a 77 apart.
We took one 77DX completely apart, analyzed the design, even made detailed drawings of all of the components, and did experiments to understand the operation and potential tradeoffs in the 77DX design. For this forensic examination of the 77DX species, we bought two units in as original condition as possible, one to disassemble, and one as a working reference mic. Every screw, bracket, washer and spring was carefully measured, tagged, and drawn into Solidworks. We were fortunate to find a beautiful pair to work with. Before doing anything, we compared the two, recorded the results, and noted any differences.
Written by Bob Crowley