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There seems to be an ongoing debate as to what impedance will get the best performance out of your ribbon mic. Some argue that low impedance gives the flattest response. Others advocate for high impedance to mimic the tube preamps of the ribbon mic golden age. Some even say you must match your preamp’s impedance to your microphone’s to maximize gain. Contradictory claims like these have been made by ribbon mic users and microphone manufacturers alike, making impedance a confusing topic to navigate. What is impedance anyway? Impedance is the measure of opposition to alternating electrical current that is both resistive and reactive. In other words, an impedance is like a resistor that can react to the frequency of an AC voltage (like audio). This means that impedance might vary with frequency, and in the case of microphones, can directly affect your frequency response. So where do we encounter impedance? For the sake of today’s discussion, two places: at the output connection of your microphone, and the input connection of the first preamp in your signal chain.             It is true that classic tube mic preamps did have considerably high input impedance and performed well with ribbon mics. Vintage RCA tube mic preamps with unloaded input transformers like the BA-21A (Fig. 1) were designed for use with ribbon mics. Such preamps could have an input impedance as high as 12000 Ohms (12 kOhms) or higher. But with the advent of transistors, solid-state preamps would eventually become the new norm. While these preamps worked well with dynamic and condenser microphones, they did not perform as well with ribbon mics and often sucked their output and tone. Fig. 1 RCA BA-21A Microphone Preamplifier             So what needed to change? The preamp or the microphone? As I discussed in a previous post , the passive ribbon microphone is perhaps the most elegantly simple microphone design: A thin conductor placed in a magnetic field with a transformer...that’s it! There isn’t much you could change, and why would you? Decades of groundbreaking recordings and continued high rates of use today prove the importance of preserving the classic ribbon tone. Thus, it is the preamp that should adapt to the needs of the microphone, not the other way around!             In that previous post, I explained how ribbons are tuned to a specific frequency and why it’s important to get it right. This frequency is known as the resonant frequency of the ribbon. Remember how we said that impedance can change with frequency? Well it’s at this frequency that the ribbon microphone’s impedance is highest, as high as 1700 Ohms or more! This is because the impedance of the ribbon is greatly controlled by its mass at low frequencies, whereas its impedance at high frequencies is lower due to the steady electrical resistance of the ribbon. It is right at the standard frequency of 1 kHz that we find the nominal “250 Ohm” impedance value of the mic, usually measured between 250 and 350 Ohms. Fig. 2 — RCA 77-DX Impedance Curve             So how does your preamp’s input impedance affect your microphone’s tone? Well just as adding resistors in parallel reduces voltage, so does adding a load to your microphone reduce your output signal. But, because the ribbon’s impedance is highest at its resonant frequency, you will lose more signal around that frequency disproportionately to the rest of the spectrum. In other words, loading down your microphone will result in an ugly notch in your low frequency response. This effect may be less noticeable in microphones tuned to frequencies below 20 Hz (the limit of human hearing), but will be quite noticeable in other ribbon mics such as the RCA 77-D/DX and BK-5A/B, as well as the STC/Coles 4038 and Beyer M160.             Below you can see the effect of load impedance on the RCA 77-DX. Notice how the effect gets more severe as the impedance gets lower. Also notice how close our experimental results (Fig. 3) are to RCA's laboratory data (Fig. 4)...not too shabby! Fig. 3 — Impedance Loading Effect on RCA 77-DX, Measured by Pitman Fig. 4 — Impedance Loading Effect on RCA 77-DX, Published by RCA So what’s the solution? As you can already see, a preamp with higher input impedance will be more transparent and best preserve the ribbon’s natural tone. But how high is high enough? RCA engineers prescribed a preamp input impedance of at least 5-times the nominal impedance of the microphone. In the case of a 250 Ohm microphone, this would dictate a 1250 Ohm load. However, even a load this high would still suck close to 8 dB at the resonant frequency. In practice, RCA’s microphone preamps often had much higher impedances of 12 kOhms or more, which is closer to 5-times the peak  impedance of the microphone. This is a good place to start, but impedances as high as 20 kOhms or more have been found to yield the most transparent results. One thing to keep in mind is that not all preamps are created equal! Let’s say you want to get the highest output and truest response out of your mic, independent of load impedance, so you get yourself a preamp that’s “made for ribbon microphones”. You excitedly open it up and plug it in, only to find that you’re getting a fraction of the gain that was advertised. Why is this? Well, most inline mic preamps have gain that is output-load-dependent. This just means that the amount of gain you actually get out of the preamp may depend on the next impedance it sees in the signal chain. Although some preamps are better than others, some of the most popular inline preamps marketed toward ribbon mics still require a load impedance of 5 kOhm or more to achieve their advertised gain. This is because they employ a “cascode” amplifier that yields a high output impedance which is, again, highly sensitive to subsequent loading. If you’ve purchased a preamp to preserve your mics output independent of load, this kind of defeats the point. Ideally, your ribbon microphone preamp should have a VERY high input impedance and fairly low output impedance, in addition to high gain and low noise. Cue the shameless plug... Fig. 5 — Pitman Preamp Upgrade (Before Lower Curve, After Upper Curve) Into 1500 Ohm Load Pitman now offers an internal FET preamp upgrade that can be fitted to virtually any microphone. It runs safely on (and protects your ribbon from) 48v Phantom power, and features a high input impedance of 22 kOhms, 20 dB of clean gain (into 1500 Ohm load), and additional shielding for low noise operation. Leave the preamps behind and make your mic active today! Until next time, —Luke

Over the years, I’ve often received one question from DIYers and major microphone manufacturers alike: “how do you tune your ribbons?” While some of our methods remain trade secrets, I think it would be both interesting and important for ribbon mic lovers to understand the value of proper ribbon tensioning.   Before we get to tuning, let’s briefly go over the basic physics of a ribbon microphone. Thanks to the great Michael Faraday, we know that if you move a conductor (like a strip of aluminum foil) while it’s placed in a magnetic field (like magnets on either side), you will generate a small electrical current across the conductor. This phenomenon is known as electromagnetic induction, or Faraday’s Law. Now, if your foil is thin enough, even sound will be able to move it. And if it is moved by sound, then that motion will be translated into an audio signal across the ribbon. Voila! You have a device that converts sound into an electrical signal—a “transducer”, and more specifically in this case, a “ribbon motor” (Fig. 1). Of course, this signal is not yet ready for your console, interface, DAW, or reel-to-reel and will need an appropriate transformer and preamp...but those are topics for another time Fig. 1 — Diagram of Ribbon Motor Now if you were to use a plain old strip of household foil, or even a gum stick wrapper (yes, you wouldn’t believe how many times I’ve seen this), you would get very low output and a thin, honky tone from your ribbon mic. This is for a number of reasons, but the biggest one is that the ribbon is not corrugated. Applying lateral ridges known as corrugations along the full length of the ribbon adds several benefits to the microphone’s performance, including higher output due to increased ribbon travel, and broader, flatter frequency response due to damped resonances. Appropriate corrugations also reduce shear and rotational movement, maximizing axial directivity. Of course, there is a science behind these corrugations as well...but again this is a topic for another time.   But let’s take a step back to discuss “resonances”. Resonance is a property of objects or spaces that intensify and prolong sound at a specific frequency, and often multiples of that frequency (harmonic overtones). This property can be highly desirable in guitar strings, pianos, and violin bodies for example. But, in the case of transducers like speakers and microphones aimed for high-fidelity sound reproduction, resonance is not so desirable. It colors and limits the frequency response of your microphone. Luckily, the corrugated ribbon in a ribbon microphone is naturally effective at damping (mitigating) any resonances...but only when tuned properly!   Bonus Science I can’t help but go deeper into some physics here, so consider these next three paragraphs optional. The most effective analogy for illustrating resonance is the simple mass-spring system (Fig. 2). Put an object of mass m  on a spring with stiffness k  (fixed at one end), then displace and release the mass. What happens? In an ideal world, free of frictional losses, the mass will oscillate (or bob up and down) forever. This is a condition of pure resonance, and the frequency f  of the resonance is determined by the value of m  and k  in the following equation: Fig. 2 — Simple Mass-Spring System and Frequency Equation   Thus, your resonant frequency f  will increase with stiffer springs and/or smaller masses, and decrease with more flexible springs and/or larger masses. Ideally, a microphone’s primary resonance should be as low as possible to avoid peaks in its working frequency range. This is where ribbon mics excel beyond dynamic or condenser microphones. Because the ribbon is corrugated and only bound at two points, it maintains a very low mechanical stiffness, and thus an incredibly low resonant frequency. Dynamic and condenser microphones on the other hand are bound along the entire perimeter of their circular membranes/diaphragms, resulting in higher mechanical stiffness and noticeable mid/high frequency resonance. This is not to say that ribbon microphones do not experience any resonances throughout their frequency range, but that they are generally softer in a way that is superior to other microphone technologies. Fig. 3 — Mass-Spring System with Damper If you take that same mass-spring system and add a damper with damping constant c , the mass would still be able to move just as freely but without any resonance (Fig 3). This is the condition of “critical” damping—the perfect balance between resonance and damping—which closely approximates the behavior of a correctly tensioned ribbon. Other microphone elements with supplemental damping and resonators also sacrifice output level and leave resonances free to color the microphone’s natural tone. Tuning Methods — Experience vs. Science   So how are ribbon mics tuned? First a target tension is determined. In the case of vintage RCA microphones, this tension was specified by the legendary Harry Olson and his team of engineers after exhaustive research and testing. With this tension in mind, RCA engineers then trained technicians—like the great Clarence Kane—in ribbon installation procedures and techniques.  Because of this combination of calculation and tradition, ribbon microphone tensioning remains both a science and an art that can take years to master.   Traditional ribbon tensioning was done primarily by experience, originating with RCA’s original procedures. My mentor and RCA original, Clarence Kane, taught me how to properly tune a ribbon based on look, movement, and tone alone. With many decades of experience, he was able to re-ribbon microphones with unbelievable consistency. On the rare occasion that he  didn’t get one quite right, he would simply start over until it was. Thanks to him, I was able to learn and quantify these original techniques. Though I’m always working to get my “do-over” rate down to his numbers, I’m proud to say that Pitman honors this high standard for traditional re-ribboning. Fig. 4 — Tensioning a Ribbon So how are ribbons tuned without such training? Is there another way? Yes, in fact, most modern ribbon microphone manufacturers use an approach known as “impedance resonance testing”. This approach involves connecting a function generator and oscilloscope to the ribbon motor and sweeping through frequencies to find the impedance rise (via a Lissajous curve). The frequency at which this rise occurs is the resonant frequency of the ribbon. As stated before, this frequency is very low—commonly near or below humans’ lowest audible frequency. This still leaves much to be desired in terms of ensuring proper ribbon travel and maintaining appropriate corrugation shape, but this method provides an excellent way to check your work with precision. At Pitman, this method is used to ensure the consistency of our traditional methods so as to truly offer the best of both worlds! Fig. 5 — A Ribbon Connected for Impedance Resonance Testing Fig. 6 — Oscilloscope During Impedance Resonance Testing Some will argue that experience is less precise, but that is not necessarily the case. A master of this craft can tune a ribbon within a few Hertz of a target frequency without checking an oscilloscope, all while balancing the shape and spacing of the ribbon. It’s also no secret that this method has gotten the result preferred by musicians for decades. Still, impedance resonance testing offers a clear advantage for fool-proof tuning, something especially important for modern large-scale production.   So what happens when the ribbon is not tensioned properly? Is it really that important? If you want to maximize frequency response, output, and minimize low frequency anomalies, the answer is yes! Here’s what happens in each case...   Too loose — You’ll get a nice broad frequency range, but lower output and susceptibility to ribbon sag and magnet contact. This means that your frequency response may change unexpectedly when tilted forward or back, and that the ribbon will begin to distort at a lower SPL threshold.   Too tight — You’ll get very high output, but the high end will be dulled. You’ll also get boomier low mids with pronounced low frequency resonance anomalies.   Just right — The perfect balance! You’ll get high output and a broad frequency response including a more detailed top end. The ribbon will retain its shape and tone in any position, and the low end will be smooth—no boomy low mids or low frequency resonance anomalies.   Fig. 7 — Frequency Response of a Microphone at Various Tensions Fig. 8 — Variations in Frequency Response from Normalized Baseline   Hopefully you can see and appreciate the difference a well-tensioned ribbon can make. In many cases, ribbon tuning can change a microphone’s tonal character by more than 3 dB. Such a difference is not insignificant to the discerning ear, and is certainly problematic in “matched” pairs of ribbon mics. Ribbon tension is also not permanent, as high SPL sources and long-term use will eventually stretch the ribbon and lower its tension below what’s acceptable. The good news is that ribbon microphones are a truly durable good, something you don’t see much in our modern throwaway consumer culture. With proper use and care, they need only be re-ribboned every handful of years for multiple lifetimes of use. This is probably my favorite aspect of ribbon microphones, their definition and their underlying philosophy: a functional assembly of entirely naturally-occurring and fully-recyclable conductive and magnetic elements that will last forever with relatively inexpensive repairs done by the hands of a real humans. How many products can make that claim? Until next time, —Luke Have a ribbon mic that doesn't sound quite right? Consider a reribbon for optimized performance! Visit pitmanmic.com  or contact us for more information.

Over the past eight years, I’ve answered countless phone calls and emails with questions about ribbon mics. When to use them? How to use them? Storage position? Preamps? Input impedance? Max sound pressure level? Phantom power? Wind? Dust? Foil thickness? Corrugation method? Ribbon tension? Polar patterns? Frequency response? Transients? If you do a quick online search, I can guarantee you will find answers, but I can’t guarantee that those answers will always be correct or based on more than intuition or blind tradition.   Before his passing, I was blessed to have my mentor, Clarence Kane, to answer any questions that only he could as RCA’s last living ribbon microphone technician. Over the years, I was fortunate to work on nearly a thousand ribbon microphones under his close supervision. I could fill a book with all of his tricks-of-the-trade and stories—many pertaining to microphones but many more to age-earned wisdom. Following my study of ribbon microphones in an academic setting, Clarence and I together performed many tests and experiments to prove or disprove the prevailing claims about these mics. Thus, my research and testing of ribbon microphones has continued for over eight years, and I’m just getting started. This, combined with the knowledge that was carefully passed down to me, leaves me with a great responsibility to ribbon microphone users everywhere.   I am surprised by the widespread myths surrounding ribbon mics that often have little to no basis in scientific testing. The primary parameters of ribbon microphone design have already been meticulously tested and optimized to death by legendary acoustician and ribbon microphone inventor Harry Olson. Although reading his academic work may be challenging (even I grumble at the words “dynamical analogies”), it is misleading when musicians or repairmen make claims that contradict Olson’s tried-and-true research. So, in the tradition of the great Harry Olson, I will be doing my best to dispel these myths one by one and shed light on the timeless technology that is the ribbon microphone—all through scientific testing. Beginning with...   Frequency response—the mother of all microphone specifications. It is your microphone’s unique tonal thumbprint. It tells you everything you need to know, that is, when it is telling the truth! Let’s quickly go over the basics of frequency response graphs…   The horizontal axis represents the spectrum of sound, with lower bass notes/frequencies to the left, and higher treble notes/frequencies to the right. Ideally, this axis should range from the lowest note humans can hear (20 Hz) to the highest (20,000 Hz or 20 kHz, although as we age this slowly drops). However, it’s not uncommon for microphone manufacturers to limit the range of this spectrum for two primary reasons: the natural limitations of their testing equipment, or hiding microphone performance anomalies they aren’t proud of. If you limit the range of your graph, you could deceive the viewer by making the range of your microphone’s response look broader at first glance (Fig. 2 compared to Fig. 1). Fig. 1 – Frequency Response with Appropriate Axis Fig. 2 – Frequency Response with Limited Horizontal Axis Fig. 3 – Frequency Response with Inflated Horizontal Axis   Put simply, the vertical axis represents the sensitivity of a microphone in decibels. Decibels are an incredibly powerful yet often confusing scientific unit that come in many flavors for a variety of uses (Relative dBV, dB SPL, dB Z, dBA, dBC, dBV [re: 1V], etc.). For today, all you need to know is that higher sensitivity will sit higher on the graph and lower sensitivity will sit lower. The thing to look out for here is axis range. Some microphone manufacturers will inflate this axis to make a microphone’s response appear more flat (Fig. 3). Reasonable frequency response graphs will have a total range of 40 dB or less, say -20 to +20 dB.   The ideal, most versatile microphone should in theory have a smooth, flat response from 20 Hz all the way up to 20 kHz. So how do ribbon mics fair by this standard? Many say they are too “dark” or lack “high end sparkle/air”. This is another way of saying that the frequency response doesn’t extend high enough (to the right). But this is not necessarily true! While many ribbon mics are designed to sound “darker”, many ribbon microphone models extend well above 15 kHz and all the way up to 20 kHz at healthy signal levels. In fact, ribbon microphones exhibit an unmatched smoothness in this upper range due to their lack of pesky high frequency inharmonic resonances (which are inherent to the circular fully-bounded transducers of dynamic and condenser microphones). In other words, the physics of the ribbon element encourage smooth, harmonic sound reproduction—much like the way we hear in real life!   And what about the low end? That is where ribbon microphones really shine! The element in a ribbon mic is tuned near or even just below our lowest audible frequency, so ribbons are able to reproduce sound all the way down to these low notes with ease. Traditional figure-8 (bi-directional) ribbon microphones also exhibit a “proximity effect”, or enhanced low end as you get closer to the sound source. This is due to the logarithmic change of sound pressure level with proximity to the source, resulting in a greater pressure gradient across the microphone at closer distances. However, this is not inherent to ribbon microphones, but is exhibited by any microphone with a figure-8 polar pattern. In fact, ribbon mics of different polar patterns don’t even exhibit this proximity effect in the same way. In other words, proximity effect is solely a function of a microphone’s polar pattern. But regardless of polar pattern, ribbon mics still excel beyond other microphones in the low end due to their naturally low tuning frequencies.   Fig. 4 – RCA 44-BX Published Frequency Response...see if you can spot the error!  Fig. 5 – RCA 44-BX Measured Frequency Response   Fig. 4 shows the published frequency response of one of my favorites, the RCA 44-BX. Fig. 5 shows the real response of a 44-BX measured in my lab, including the selectable M, V1 and V2 bass roll-off modes. This 44 (freshly re-ribboned to RCA specs) is an excellent example of the impressive range of a traditional passive ribbon microphone. The high end is subdued but still plenty sensitive for sound sources louder than a whisper. And the low end is thunderous, especially at closer proximity.   There are countless factors that can affect a ribbon microphone’s frequency response, but today I just wanted to cover the basics. I will begin to explore more of these factors soon and include their effects on frequency response along the way. For the technically curious, my testing setup includes a calibrated full range monitor and software that calculates an accurate near-anechoic response down to 77 Hz by rejecting known room reflections. Measurements were taken at 94 dB SPL (1 Pa) at 12 inches into a 22 kΩ load with variable smoothing. Until next time, –Luke   Curious about your own microphone’s frequency response? We now offer microphone testing as a supplemental service. We can test your personal microphone and provide a unique specification sheet featuring measured frequency response, measured sensitivity, measured output impedance, and more! Visit pitmanmic.com or contact us for more information. Want to know more about the science, design, history, and misconceptions about ribbon microphones? Sign up for updates and tell us what you want to hear about next in the comments below!