Tuesday, February 28, 2012

A Recent History of Ribbon Microphones

Audio-Technica AT 4080 and 4081
Ty Ford (written sometime in 2005)

The BBC cites 1933 as the year pressure gradient ribbon microphones were introduced. It also notes that RCA's Harry Olson applied for a patent on January 31, 1941 that was granted May 9, 1944 for ribbon microphones that converted sound pressure to electrical energy using a strip of aluminum between two magnets. 

According to beyerdynamic's Bob Lowig, engineer Eugen (pronounced OY-gun) Beyer's visions of developing better sound reinforcement systems and his evolution into microphones happened at roughly the same time. The Beyer line continues to offer the m160 dual ribbon hypercardioid and the m260 cardioid. The M500 is now discontinued, but reports of a special anniversary version of the M500 keep popping up on the web.
Additional information provided by Wes Dooley at AEA indicates that the RCA 44B bi-directional studio ribbon microphone was introduced in 1936. The 44B was upgraded to the 44BX in 1938 when stronger Alnico 5 magnets were developed. The short ribbon poly-directional RCA 77D series defined high fidelity in the United States for broadcast, electrical recording and sound reinforcement until the mid 1950s when zippier large diaphragm condenser microphones became the new icon of high fidelity. 
It had been a 10kHz world until then, with the frequency response of the ribbon mics usually reaching just above that before dropping off. Although ribbon microphones prevailed as staples of microphony throughout the fifties and sixties, with the "hi-fi" 15kHz frequency response of TV and FM, they began losing their appeal. The 44 series was considered to big and bulky for television. Many AM radio broadcasting facilities shifted to the 77DX because they were smaller and lighter and could be more easily moved about in the increasing number of smaller combo air studios. The 77DX also had a slightly higher output than the 77D. 
By the late 1960s, FM radio moved from being an "experimental medium" with few listeners, to being a major driving force. The new broadcasters were looking for new microphones that were less costly, less bulky, and with a frequency response range that approached or exceeded their 15kHz bandwidth limit. Scoring stages held the line, continuing to use ribbon mics for orchestral, string and horn dates. Every well-stocked microphone locker had at least one or two ribbon mics to smooth out quirky or edgey sources.
Arguably, it was the CD and digital recording that rekindled interest in the ribbon microphone. Poorly designed A/D and D/A converters, the multiplexed D/A converters in the first CD players that worked double duty to reconstruct the digital bitstream into analog, and poorly designed analog output amps made the first CDs edgey and harsh. After improvements in design; increased bit and sample rate and better anti-aliasing filters, we found ourselves still in search of a smoother sound.
In the momentum created by the recent interest in vacuum tubes, there has been a renewed interest in ribbon microphone technology. In the US, two people on different coasts have reached mythological status as regards ribbon microphones; Charles Gant in Lakewood California, and Clarence Kane in Pitman New Jersey. It was to them that you sent your old RCA microphones for re-ribboning and other services, because they had worked for "the company" for years doing exactly that.
Charles Gant is now 83. He no longer works on microphones. In a recent conversation, Charles reminisced. "I worked at Technical Products in Hollywood from 1951, a machine shop that made all the parts. I was transferred to RCA sometime in the 50's. We made some of the tools ourselves and some parts were made by other manufacturers and we'd put them together. There were quite a few different mics involved, at least twenty or so. From the beginning we started with condenser microphones like the BK 12. We built the 10001. Mike Rettinger was the engineer. It was a ribbon velocity mic not as big as the 77D or DX. The 44B, 77D and 77DX were the most popular. They were used on the sound stages and for public appearances. I left RCA in 1981 with a deal that included some microphones and galvanometers and other RCA equipment."
"The mics don't look very complicated, but there's a lot going on inside, but if you have the books, you can tell which is which. Some had double ribbons. They liked to change the numbers for marketing, but the D and DX are quite a lot a like. The DX has a 3-4dB higher output than the D. It's the highest output of any velocity mic RCA made. On the DX, they changed the magnetic content to squeeze out a few more dB. They also changed the transformer and that helped a lot. You could strap it for the full level by leaving the strapping open. If you were too close to a band and you couldn't move back, you could strap the mic to take off some of the low end." 
Clarence Kane, was 72 in 2005 and Enak Microphone Repair (Enak is Kane spelled backwards) is Gant's younger counterpart in Pitman NJ. Kane originally worked as an engineer for the broadcast division of RCA. "We were the CRAYE (custom repair and engineering) section. We did all service work for all the broadcast studios and installed new systems. The home office was in Camden, but we were in Pennsaluken and Gibbsboro NJ. That's when RCA consolidated the entire business to Gibbsboro. They had dropped the mic repair in 1985, before the move to Gibbsboro. The last mic RCA made was the 77, and that was in 1977." 
When the mic repair section closed, Kane negotiated for the ribbon material, most of the paperwork and what parts were available. "The RCA Warehouse was in Deptford NJ. I couldn't make the deal I wanted, but I got the ribbon material. When Deptford needed ribbons I made them and they sold them to the customers. I still have a pretty good supply of the original material. We custom cut the ribbons to fit the different microphones. The ribbons are are .0001" thick aluminum foil; so thin we have to put the ribbon between two pieces of 2 mil paper and then run it through the machine. The forming tool creates 19 pitch 90 degree perpendicular corrugations." 
These days Kane has a helper who comes in three hours a day to help with the repair work. "I use a beat generator that feeds a speaker. The speaker's in a closed box with the mic and the output of the mic feeds a a GE graphic level recorder with a roll of graph paper and a moving needle. The system specs from 20hz to 15kHz automatically. It's the original all-tube test equipment used in the RCA facility back in the 40s and 50s including the RCA BA 44 monitor amp and RCA BA43 program amp. We've just gotten a new computer to test the mics, but we haven't set it up yet."
Bob Speiden is also a long-time ribbon microphone proponent. He cites RCA's Harry Olson for developing the ribbon microphone in the 1930s. "They became popular because their unique transparent sound quality was better than carbon and early condenser microphones. I ran a primitive recording studio beginning in 1970, with AKG, Neumann and Sony microphones. I gradually became unsatisfied with the condenser microphones. Horns were buzzy, sopranos (when they really let go on the top end) got hard, violins didn't fair too well. I got a pair of B&O ribbon mics. I tried a pair in Blumlein array on a location job and was blown away. "
Speiden contacted B&O to find the microphones hadn't been made for some time so he decided to build them. "I got one built in 1983. I used it on a couple of jobs and it really worked well. The first ones I built used Alnico Eight magnets because Neodymium wasn't available. I switched to Neodymium and lowered the impedance to 200 ohms. There are possibly seventy of those mics still out there."
Speiden's design is a stereo crossed figure of eight. The front and back quadrants are the identical but out of phase. The side quadrant signals are out of phase. "If you use them with one of the older Dolby encoders, Prologic I think, you get a fantastic surround environment with one stereo microphone. If you're in the sweet spot during playback, sounds that were a little to the right of the right mic will sound like they are coming from a little to the right of the right monitor. But you can't go too far."
Speiden says there's a reason ribbons sound smoother. "With large capsule condenser microphones, you get diffraction peaks that will exaggerate signals in certain frequency ranges. If you're off axis, you get off-axis phase attenuation. The ribbon is 1/16 of an inch wide. The phase difference across that is inconsequential. That means there is no off-axis coloration and a smoothness of response." 
Speiden says recent meetings with Dave Royer have resulted in Royer taking over the manufacture of Speiden's SF 12 stereo microphone, now called the SF 12 Royer. According to Royer's John Jennings, the beginnings of Royer's company goes back to DVA (Dunmore Vintage Audio), a tube audio company for which Royer was the chief engineer. Rick Perrotta (owner of Baby-O recorders in LA and co-founded the Matchless guitar amp company) took interest, seeing ribbon microphones as a dormant market, "Years ago, the tube market stopped when transistors came in, but tubes have had a resurgence and have found a competitive niche in the market. We thought, if ribbons could do what tubes could do for digital recording, that's where the market would be open."
Perrotta says Royer has addressed the three problems of ribbons microphones; fragility, output and higher frequency response. "Right now our mics have a sensitivity of -53dB. That's hotter than a Shure SM 57 and an RCA 44. We use a chemical mix of neodymium and other substances. We also use a pole piece, a piece of steel that goes over the magnet, that focuses the magnetic energy and evens out the response. We have new high performance transformers using IE core technology and toroid configurations. The toroid, when properly wound, does a better job and gives excellent frequency response and is more resistant to stray radiation."
One thing that hasn't changed yet is the aluminum ribbon, "Aluminum seems to be an almost ideal metal to use. The thinner the ribbon, the higher the output. The early RCA mics had 2.25 micron ribbon elements. We've gone to a 2.0 micron for extra sensitivity. We think it will be sturdy enough for everyday work. Our mics can be dropped and banged around, but not blown into. Perrotta says they're considering Titanium to replace aluminum if its resistive nature is found to be equal or less than that of aluminum, because it's more rugged.

Royer R-122 Active Ribbon
David Royer, at Royer Labs, points out other advantages to single pattern ribbon microphones. "Multi-pattern mics suffer with problems due to reflections inside the head grille that can make life miserable. Frequency response variations with changes in the polar pattern setting in the octave between 5 and 10Khz are very common. A two inch diameter mic body with a capsule in the middle means there's one inch to the screen. Sound will also pass by the capsule inside the head grille, bounce off the inside of the head grille and back onto the rear membrane. That sound will either be subtracted or added to the front sound, depending on whether the rear membrane is in or out of phase with the front membrane."

With his designs, Royer has addressed ribbon contamination and other ribbon-specific problems. "With modern design, an extremely fine wire mesh is used to cover the front and back pole pieces to keep metal particles out. If they get through, those particles have to be large enough to bridge the space between the pole pieces and the ribbon. The spacings are small enough (1/8") so that the screen does not affect frequency response. The ribbon is "driven" by the motion of the air pocket in the gap between the pole pieces. Because the mass of the ribbon is greater than that of the air surrounding it, the amplitude of its motion at resonance will rise sharply unless some means is provided to damp its motion, such as by interposing screens or gauze at the boundaries of the air pocket. This causes the air to move through the holes in the screen or gauze, thus causing mechanical damping due to friction. In ribbon microphones, these screens generally do double duty to provide the damping and to prevent the entrance of metal particles." 
In 1997, 
Wes Dooley AEA
Wes Dooley's Audio Engineering Associates (AEA) introduced a working replica shell of the RCA 44 at the NY AES. This complete external shell was built from replica RCA service parts manufactured by AEA. That offering led to requests for a complete working replica of the RCA 44. According to Dooley, "We realized that, as with the Coles 4038 from England, the sonic signature of the 44 is an important part of a recording engineer's audio vocabulary. Thus we decided to literally recreate the RCA 44BX as our project for this past year's San Francisco AES. All the parts from AEA's R44c will interchange with the RCA 44B and BX. We were careful to avoid "improving" the design and used an original 44BX as an absolute reference."
To insure a faithful reproduction, Dooley is using new old stock (NOS); RCA ribbon material and the RCA Microphone Laboratories crimp pattern learned from Jon Sank some 16 years ago. "AEA's R44c replica project is dedicated to our mentors: Jon Sank, Wally Heider, Richard Heyser and Dick Rosmini, all ribbon microphone advocates. Jon Sank's advice and inspiration was especially critical. Jon worked as an acoustical engineer for RCA's microphone division from 1960 until it was closed in the middle 1970s. He designed the RCA BK11, a compact, studio bi-directional ribbon microphone. Jon died suddenly this past August and will be sorely missed." 
Dooley says his goal has been not to "improve" the mic or change its sound. "The only functional differences between our microphone and the RCA are that we have our own name on them and we do not include a low cut filter. Studios have informed us that they no longer use the low cut option built into the original RCA 44, now that they have lots of console EQ. For higher stability, we use neodymium magnet material instead of Alnico."
In addition to his own designs, Dooley also carries both the Coles 4038 and 4104B XLR ribbon microphones, as does Fraser Jones of Independent Audio in Maine. According to Jones, "Coles was an engineer who had never been in a recording studio. He worked for STC (Standard Telegraph and Cable) making everything from these mics to fairground lights and all sorts of things in between. The microphones designed by the BBC in the 40s and 50s were orignally made by STC. STC closed down manufacturing in the early 70s. Coles ElectroAcoustic prevailed and continued as manufacturer of the microphones according to the original patent." 
Far from fragile, according to David Royer, the original BBC measurements showed that at 110Hz, the Coles 4038 will take 125db SPL before reaching the 1% third harmonic distortion point. Down at 55Hz it only takes 110dB SPL to reach the 1% point. "Recently we had the original designer calculate what the SPL would be at 1kHz for 1% third harmonic distortion. His answer was 165 dB SPL."
Royer advises adhering to the three ribbon microphone survival tips: Avoid phantom power. If you have a bad cable you will snap your ribbon. Avoid wind. Its very good at stretching the ribbon element past redemption. Leave a bag over the mic whenever its not in use. It protects it from both wind and the tramp iron found on many surfaces you might put the mic down on. With transformerless microphone preamps, its prudent to wait a few minutes after turning off the phantom supply before connecting a ribbon microphone to let the coupling caps bleed off their 48 volt phantom charge.

As an addendum, Audio Technica, Shure and Cloud have all brought ribbon mics to market since this article was written.

Reach Ty Ford at www.tyford.com

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