Impedance—How Low Can You Go?

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