In my mind there are about three microphone designs that most people would recognize as iconic. The RCA 44, which we covered in the first of this series, the RCA 77 and the Shure Unidyne model 55. This session we will take a look at the RCA 77 series of ribbon mics starting with the very first model, the 77A.
Dr. Harry Olson was RCA Labs expert audio designer. Dr. Olson was responsible for developing the ribbon microphone . The basic ribbon mic has a bidirectional pick up pattern, that is it picks up sound from front and back and is relatively “dead” to sound arriving from the sides. On May 1, 1931 Dr. Olson delivered a paper entitled “A Unidirectional Ribbon Microphone” at the meeting of the Acoustical Society of America in Cleveland, Ohio. The unidirectional microphone, as described in this abstract, consisted of the combination of velocity and pressure ribbon microphones which produced a heart shaped, or “cardioid” directional pattern. Simply stated, the mic picks up sounds from the front side and rejects sound from the rear. This microphone was commercialized in late 1933 as the RCA 77A Unidirectional Microphone. The microphone was an instant success. The RCA 77A was followed by the RCA 77B, 77C, 77D and finally the 77DX. The 77A was a huge mic which had a rounded top and a flat bottom. The 77B and the following models were much reduced in size due to stronger magnetic material available at the time. All of the 77 models used a rubber shock absorber in the stand mounting connector.
The internal workings of the mic consisted of a straight bidirectional ribbon unit in series with a pressure ribbon unit (omnidirectional). In the straight ribbon mic the ribbon is open to the atmosphere on both sides resulting in a bidirectional pickup. The pressure ribbon mic is achieved by leaving the front of the ribbon open to the atmosphere but closing off the rear side of the ribbon by a tube connected to an acoustical space within the mic body that is filled with sound absorbing material. This results in the nondirectional or omnidirectional pattern. Connect the output of the two-mics-in- one in series and the geometric result is the unidirectional pick up pattern.
The first two mics in the series, the 77A and 77B, had a fixed unidirectional pattern. The 77C gave the three basic patterns: unidirectional, bidirectional and omnidirectional selected by a screwdriver switch on the bottom of the mic which electrically selected the mic elements singly or in combination for the uni pattern. All three of these mics used a double ribbon element.
The last two models in this series the 77D and 77DX were called “polydirectional” due to a more effective means to adjust the pickup pattern. A screwdriver adjustment on the back of the upper screened part of these mics let you choose unidirectional, bidirectional, omnidirectional and also three additional variations of the unidirectional pattern. This selection was not electrical but by a shutter mechanism that was mounted in back of the ribbon element. The shutter adjusted a single ribbon in relation to the acoustic tube and damping system to achieve the variable patterns. This gave great versatility to these instruments for the audio engineer depending on the type of sound pickup problem encountered. There was another screwdriver switch on the very bottom of the mic body that selected low frequency or “bass” equalization. This allowed the mic to be used for close talking without a “muddy” or “boomy” effect for a cleaner sound transmission. All ribbon mics tend to emphasize the bass when used close up. This was a huge revolution in mic design and made it possible to overcome many microphone sound pick up problems like imperfect acoustics causing too much reverberation and noise in rooms and studios. It is also a great help in public address systems by reducing the tendency to “feed back” or “howl.”
In TV production the RCA 77 with it’s broad front side pickup pattern was perfect for overhead boom use; it rejected studio noises from the back of the mic like camera and other background noises. In radio and TV the RCA 77 series, especially the later models, 77D and 77DX, were very popular. RCA had a beautifully designed art deco style table stand for these mics. They were also used on Floor stands in radio and TV studios. If you watch the reruns of the old Lawrence Welk TV shows from the ’50s you’ll notice that ABC used the RCA 77D mics, usually two in front of the sax section and two more in back to pick up the trumpets and trombones. Later in the ’60s ABC audio engineers changed to the Electro-Voice model 666 cardioid dynamics for the Welk orchestra pickup.
The 77D and DX series units were used on TV overhead mic booms in the days when TV producers tried their best to hide the microphones whenever possible! The 77 was a popular radio control room mic too, usually hanging from a fixed or flexible boom. KYOS in Merced used a 77D in the control room. KYOS also had an RCA 77C model from the early ’40s. I visited the old KTUR, Turlock studios just after they had changed call letters to KCEY in the early ’60s and I saw more than one RCA 77D mic in their studio and control room. KBEE, Modesto was another station that used the RCA 77 as well as KRJC, the Modesto Jr. College station, as you can see in some of the photos on this web site for those stations. Other stations in the area including KSTN, Stockton used the 77 as well as TV stations KCRA and KOVR, Sacramento back in the day.
I worked in Chico most of my career in broadcasting at KHSL-AM and KHSL-TV. Both these stations used the RCA 77D mics. The TV station had one on their main studio boom as well as in the announce booth. KHSL radio used this type mic in the master control room as well as in the production studio for many years, clear up into the ’80s.
Remembering back to the “golden age” of TV…all three major networks used the RCA 77D and DX models in all phases of production. NBC was owned by RCA back in that time and they would only use RCA mics and other equipment made by RCA. The long succession of Tonight Show hosts had an RCA 77DX on the desk for years even to the early years of Johnny Carson.
If you search “RCA microphones” on Ebay you often see several 77s up for auction and they fetch up into the one thousand dollar and more range! There are several ribbon mic repair experts who will install a new ribbon in these old mics and make them like new again. One of these guy’s father used to work for RCA and has the original equipment used to repair these mics. The old RCA company ceased operations back in the 1980s; the RCA name is still around on consumer electronics but nothing is made in the USA like it was back in the glory days!
The RCA 77 series of ribbon microphones had an incredible impact on the audio industry and even today, in this digital age, are in demand by recording studios for their versatility as well as their smooth wide range sound quality.
One thought on “RCA 77 Series Microphones”
I am restoring an RCA 77A, I have restored many RCA mics over the years but have run into a problem with this one.
The transformer wires, which are in the body labyrinth, have become brittle and have broken off down inside. I am trying to disassemble the body itself and cannot figure out how to do that without forcing something and possibly breaking it.
I have seen photos of this body disassembled on the Coutant website, but cannot figure out how exactly it is done. any help would be greatly appreciated.