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January 15
more... MicrophonesRecording TipsRecordingMarch 2009

Ribbon Microphones 101

As this issue of Premier Guitar focuses on recording, we’re going to examine a device most of us rarely encounter (except on recordings and in movies), known as the ribbon microphone. Rarely used at guitar gigs, the ribbon mic is de rigueur in recording studios all over the world. We’ll examine the reasons why in a moment, but first…

Some History
Microphones were common enough since the advent of the telephone. These mics were carbon-granule type: sound vibrations would cause the granules to vary their conductivity, resulting in an electrical signal form of the sound. They were quite rugged, and even the first men in space and on the moon spoke though antiquated, but dependable, carbon mics. Unfortunately, they had poor frequency response, so they were eventually constrained to telephone use only until around 1970.

It wasn’t long before carbon mics had competition. Crystal mics actually used crystal, like a cigarette lighter. By using a mechanism to push on the crystal (or a ceramic mounted equivalent), electricity is generated. While crystal and ceramic microphones surpassed the old carbon types in performance, you and I would never use one on stage or in the studio. Their performance is still rather inadequate for quality recording (blues harp players excepted). Dynamic microphones are the de facto standard for most of us. They sound great, are relatively simple and cheap to manufacture, are rugged, and have easily controlled coverage patterns. A dynamic mic is like a loudspeaker in reverse. The key element in a dynamic mic is a diaphragm attached to a little coil, which usually fits into a magnet. When sound vibrates the diaphragm back and forth, a tiny voltage is generated, which is an analogue of the sound. Most dynamic mics sound pretty good, with the best units offering up excellent performance—especially for the money spent.

How many of you know the name Walter Schottky? Ol’ Walt is primarily known for his Schottky diodes but he, along with an unsung partner, Erwin Gerlach, also invented the ribbon microphone in the early 1920s. His designs are now used daily in the recording world, and his diode designs are even more plentiful in high-quality electronics. By the end of the twenties, RCA geared up to produce commercially viable ribbon mics. Many are still in use, sound fabulous and are quite collectible. How Do They Work? In a ribbon microphone, a very thin sheet of foil is suspended in a magnetic field—very thin, around one to two microns thick (.00005 inch). The ribbons are folded or corrugated anywhere from less than 12 to a little more than 24 corrugations per inch. While it’s possible to actually replace the ribbon element, I don’t recommend you try it—the ribbon will likely fall apart in your hands. Despite that admonition, you can actually improve the performance of a mediocre ribbon mic by replacing the diaphragm if you do it correctly.

The electrical signal generated by a ribbon is different from that of a dynamic mic, but technically speaking, a ribbon mic is a type of dynamic microphone—they both move back and forth in a magnetic field. Older ribbons differ from modern designs in significant ways. Older designs have very low output and very low impedance (0.2 Ohm in many cases). This impedance is so low that we need to bring it up to drive what we usually consider to be low impedance lines, so a transformer is used to increase both the signal and the impedance.

Transformers are tricky. In a guitar amp, one can hear the difference between an ordinary transformer and a boutique equivalent. The same applies to microphone transformers. If you lack the inclination to replace the ribbon element, a transformer upgrade can yield equally impressive results, but if distortion of a signal is to be kept as low as possible, a good transformer can be quite expensive. Fortunately, many of today’s designs have plenty of output—even more than moving-coil dynamic mics in some examples. In either case, a high-quality, low-noise microphone preamp can sound great. With the right preamp, you might be able to forgo the supplied transformer. Note that you might need 60dB of gain, so audition any preamp you buy not only for sound quality, but also for noise.

Condenser mics need a power supply. Ribbons do not. Older ribbon mics could be damaged by hooking one up through a preamp designed for condensers, so be careful. Many preamps allow a choice. Most ribbon mics have on-board transformers or preamps. The latter may need a phantom 48V power supply from your mixer, just like a condenser. Use care here: many ribbons will be damaged if hooked to a phantom power supply. Never hook up a ribbon mic to your mixer without checking. Newer designs are more likely to shrug off the mistake, but read the manual.

Speaking of shrugging off mistakes, another difference is durability. Some of the older—but really great-sounding—ribbon mics would fail from physical shock, but often stood up well to high SPLs. This can be tricky, too. A full symphony orchestra can play louder than my blues band usually does. A ribbon can certainly be used in high SPL environments, but be careful about a loud singer blasting away right into the mic without a pop shield. In fact, it’s good idea to always leave the pop shield on, especially when transporting or setting up the mics. Once set up, you might be able remove it, depending on the design, unless you’re recording a singer. Today’s ribbons are far more rugged. Even so, never scream or blow directly into any ribbon without a pro-quality shield. In fact, don’t blow into a ribbon mic, period.