Saturday, November 04, 2006

Findings: RCA 77DX Ribbon Microphone


The RCA 77DX that we took apart, documented, put in Solidworks files and generally examined in minute detail is now back together.

We learned a lot from the investigation of this incredibly historic and iconic microphone. Here are a few of the things we learned:

1. The local field geometry and acoustics around the ribbon determine the character of the sound.The ribbon material makes virtually no difference in sound character, but does affect signal level and noise. The ribbon geometry plays a significant role in output level.

2. RCA's transformers are adequate and are well shielded, but we thought they were a little lossy so we did some more work and came up with this..

3, The shunt reactance method used to tailor bass response is very reliable but is not well understood by users. The method (screwdriver) used to select shunt reactance leads to microphone damage.

4. The rear acoustic loading scheme which is also known as the acoustic labyrinth is effective, but not well understood or utilized in practice. The phenolic used for the labyrinth is brittle and easily damaged. The change in pattern from figure 8 to a "hypercardioid" is effective but drastically changes the overall sound character of the mic, from an open sound to one more blocked. The frequency response changes significantly with changes in the shutter position. The method (screwdriver) used to select patterns leads to microphone damage. An acoustic tube was built and tested which resulted in a somewhat flatter response when the rear lobe is presented with a linear attenuative load.

5. The grille pattern and acoustic properties of the grille are at least partly responsible for the aggressive sound of the 77DX. The "gravel" sound in bass frequencies, noticed on male voice, is possibly also due to some nonlinear movement of the ribbon, which is small and narrow within a relatively large, undamped space.

6. Too many parts are used for a reproduction model to be economically viable product if made in the US. It is also a relatively fragile device compared to what we regard today as a current, professional quality ribbon mic. One very dedicated person has reproduced the 77DX shell and has done a fine job, but it is very labor intensive and expensive.

7. There seem to be significant factory variations between early style and later style 77DX mics.

8. The overall design violates the so-called rule of thirds, yet is nonetheless pleasing and emblematic of all microphones.

9. The 77DX is a favorite of ours, signifying the apex of RCA's efforts in the field of ribbon microphones, and a culmination of many years of evolution. It is an important historic microphone as much for its style which has become an icon for anything having to do with microphones, as it is for the engineering that went into it. Many studios seem to have a 77DX on hand, though a number tell us that it doesn't get used very much. Plenty of 77DX microphones were made so the supply of these mics seems assured for the time being. A cottage industry of people providing restoration parts and services exists today

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