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About Ribbon Mics

Page history last edited by Randy Coppinger 10 years, 11 months ago

Randy Coppinger's notes from the Ribbon Microphone Workshop

March 27, 2010

Hosted by the Los Angeles section of the Audio Engineering Society

http://www.aes.org/sections/la/page13/page13.html

 

 

Theory and practice - Ron Streicher

Ribbon elements are usually aluminum, sometimes gold.

A good ribbon will be 90dB level down in the null plane (+/-90 degrees from font).  This follows the cosine law.

Response is most linear above the resonant frequency, typically 15 – 65 Hz.

The RCA 77a was cardioid! Half of rear baffled with a physical tube.  Later versions, 77d and dx, had a rotating shutter that was placed behind the single ribbon element.

Working both sides of the ribbon (assumes bi-directional) mic goes all the way back to radio days.

Better made ribbon mics remain consistent across freq spectrum as you move off axis in the horizontal plane.

It is air turbulence (pulses of high SPL), not static SPL, that harms ribbon elements.

 

Historical Implementation of Ribbon mics in Broadcasting and Recording - Don King

There were 3 versions of the 77a.

A popular radio mic was the Western Electric 639.

No two RCA 77s sound the same. No one could ever figure out why.

These mics were designed in an era when EQ was uncommon and not well implemented.

 

Interfacing Ribbon Mics in the “Real World” - Julian David

Ribbons have a relatively low output level.  This REQUIRES an output transformer.

 

Issues of Impedance:

Very low DC resistance (0.2 ohms)

Impedance at resonance will be much higher

Complex source impedance – varies by mic model (motor)

Even different polar pattern selections of a 77dx have different impedance curves.

 

Transformers:

Transforms voltages, current and impedances.

Increases output level (1:28 and 1:40 ratios are typical for ribbon mics)

Also increases impedance, by square law

Inherent parasitic elements

Transformers have no inherent impedance of their own, just a ratio of the motor itself.

Low frequency behavior changes dramatically, below 200 Hz.

 

Load impedance:

Input impedance of preamp, cable (quality, length)

Low impedance “loads” the ribbon – you lose bass, sensitivity

Conversely, when output goes up from raising impedance, bass goes up too.

You would typically like to see an input impedance of 10k ohms or more.

Excessive cable length can cause ringing on the high end.

 

Ribbons are VERY sensitive to structure born vibration because of the low resonant frequency. So always use a shock mount.

Ribbons are prone to stray magnetic fields.  And they CREATE a field, so don't put them near anything sensitive to magnetic flux.

Because ribbons require much more gain.  So any noise created “before” the preamp (cable, connectors) is significant because of the significant gain that will be applied.

 

Royer - David Royer

Bang &Olufsen BM-2 and BM-3 ribbon mics were inspiration for the Royer R-121.

Mojave Audio actually pre-dates Royer as a company.

Spedan SF-12 stereo mic was influence for Royer stereo mic.  In fact, Royer took over production of the SF-12 when Spedan sold.

RCA used cobalt-steel magnets – which were huge.  R-121 used tiny little bar magnets.

R-122 is same motor as 121.

R-122V is vacuum tube version of 122.

 

When the ribbon is not perfectly aligned front/back to the magnets, an air cavity is created with produces an asymmetrical pattern. Also differentiates frequency response of front relative to back.

 

AEA - Wes Dooley

In the old days one mic had to do it all. The 44 was designed to do the entire orchestra.

Everything at AEA is based on the 44 motor, the first “high-fidelity” microphone.

16.5 Hz tuning.

Path length difference front/back is not only around the sides but also top/bottom.

Ribbon is always on center front/back (symmetrical) for AEA mics.

Any variation of pattern from front to back is acoustic, not motor config.

 

 

Panel Discussion

Ribbon element is often suspended between silver bars, which will corrode, messing up high freqs and output level.

Storing a ribbon mic so that the element "sags" has a damaging effect over a LONG time... years.

Likewise, a ribbon mic in good repair does not sound any worse when operated on axis with the floor ("sag").

Operating a stretched (damaged) ribbon parallel to the floor may result in scraping sounds and/or bad IM distortion.

At 10k Hz the wavelength is approx an inch. As you move off axis on the horizontal plane the highs fall off evenly, but not in the vertical plane.  A circular diaphragm mic will have funky off axis highs on any plane.

The white stripe on the back of a, RCA KU-2 “skunk” mic helped the boom mic operator see and aim the mic in a dark studio environment.

Wes: “An RCA 44 is a ‘bass magnet’”

Wally Heider: “When you reach for EQ, you’re in an emergency situation.”

The original RCA mounting bracket used clothesline, which is not very good at transmitting bass.  This remains a very useful technique.

 

Recording Session Notes

Coles mics used inside the piano, A/B, with nulls pointed at the lid.

Shure bought Crowly and Tripp!  That “indestructible” ribbon element seems like a perfect fit for Shure.

Blown away by how good the Blue Woodpecker sounds on vocal.  Might just be her voice.

 

Mix Session Notes

“Dead-siding” with fig-8 mics and being aware of what’s in the back side.

Avoid pointing mics into the corners of the room.

Seating and arranging musicians is the key to mic placement.

 

Audio Wiki front page

Randy Coppinger - who I am and other stuff I'm doing.

 

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