Preamp considerations

What types of preamplifiers work best with ribbon microphones?

All ribbon microphones, regardless of brand, share certain fundamental characteristics and have similar preamplifier requirements. The only exceptions are active ribbon microphones, which are similar in operation (but not in sound) to modern phantom powered condenser microphones.

The two most important preamplifier issues to consider with ribbon mics are gain and input impedance.

Preamplifier gain

Ribbon microphones produce low level electrical signals and need preamplifiers with enough gain to amplify that signal without degrading it.

The preamplifier you use needs to have enough gain to amplify the microphone to a suitable level to efficiently drive the recording equipment (equalizers, compressors, A/D converters, recorders, etc.). If the gain is too low, the signal will not be strong enough to operate the following stages at their optimum level. The resultant performance will be lackluster, devoid of punch and possibly noisy. Obviously, for recording loud sound sources like guitar amplifiers, horns or drums, more signal is produced by the microphone and less gain is required from the preamplifier. But if the source signals are soft, as is often the case with acoustic instruments or voices, much more gain is needed. For maximum versatility we recommend that a preamplifier have at least 60 dB (or more) of usable gain capability. For very quiet sound sources, 70+ dB of gain may be required.

If your preamp or interface falls short on gain, an in-line booster placed between the microphone and the preamp makes up the difference. Our dBooster adds 12 or 20 dB of clean gain and runs on phantom power.

Preamplifier input impedance

Ribbon microphones need to be mated to preamplifiers with suitable input impedance. An impedance mismatch will degrade the performance of any ribbon mic.

A ribbon microphone needs to be paired with a preamplifier that has high enough input impedance that it won’t load down the microphone. The lower the input impedance, the more the ribbon becomes damped, meaning that its movement is restricted. An overly damped ribbon tends to sound dull, with rolled off bass and slightly diminished highs. The output of the mic will also be reduced, requiring more preamp gain, which in turn adds more noise to the recording.

In order to deliver its full frequency response, a ribbon microphone needs to see an input impedance of at least 4 to 5 times its output impedance. For example, if a microphone has an impedance of 300 ohms, the preamplifier should have an input impedance of 1200 to 1500 ohms, or greater.

Don’t confuse preamplifier input impedance with rated microphone impedance. Many preamps will specify what microphone impedance they are designed for, but this does not necessarily translate into the preamp’s actual input impedance. If the input impedance is not specified, contact the manufacturer to find out what it is.

Preamplifier noise

The more gain a preamplifier is called on to produce, the harder it is to keep noise down. Even with modern electronic advancements, designing ultra-low noise preamps is no easy trick. Manufacturers need to invest a considerable effort in the R&D department, and ultra-low noise components don’t come cheap either, so expect to pay more for higher quality preamplifiers. Pay particular attention to the preamplifier’s noise characteristics: the lower the better!

Headroom

A good preamplifier will have plenty of headroom. In other words, it won’t distort or change its tonal character if it is suddenly required to handle an instantaneously loud signal. The preamp should sound clean and unencumbered, regardless of the intensity of the source signal. Fortunately, most modern preamplifiers fare very well in this area.

Tubes or transistors?

There are two basic types of electronic circuit topologies utilized for analog signal processing: solid-state (transistorized) and vacuum tube. The debate over which style sounds better will go on forever, but what it boils down to is taste and practicality. Each design offers something special or unique.

Vacuum tube preamplifiers sound punchy, warm, yet wonderfully airy and transparent. Vacuum tube circuitry is also renowned for its ability to bring digital recordings to life. On the other hand, vacuum tube preamplifiers are generally not as quiet as solid-state preamps, because electron emissions from tubes tend to convey more “thermal” noise than transistors do. Vacuum tube preamplifiers require a bit more maintenance and are less practical for remote recording. In addition they produce heat that must be adequately vented.

Solid-state preamplifiers tend to sound very clean (some say clinical) and offer the least coloration to the signal. They usually run cooler, travel better, and require less maintenance. You can expect less “personality” from a solid-state pre, which may be good or bad, depending on what you’re looking for. It comes down to your preference, taste, budget, and need for convenience.

To transformer or not to transformer?

Many modern solid-state preamplifiers have eliminated the traditional input-coupling transformer altogether, and in many cases the output coupling transformer as well. This not only saves space, weight and money, but also offers a more direct route for the signal to travel as it makes its way through the signal path.

So, do transformerless designs sound better? Not necessarily! Transformers add their own special alchemy to the mix. Call it coloration, but transformer coupled designs tend to sound punchy and full-bodied. Transformers also offer the benefit of true electronic isolation. This greatly enhances a preamp’s ability to interface with other equipment with minimal noise, RF, hum or induced interference. Once again, it’s a matter of taste and, in some cases, budget. Transformer coupled amplifiers usually cost more than their ironless cousins.

The bottom line

The main points to consider when choosing a microphone preamplifier for use with a ribbon microphone are:

1. A suitable preamplifier should have lots of clean gain. Choose a preamp that has enough gain to suit your recording needs. For example, loud rock recording requires less gain capability than classical recording. We recommend 60 dB (or greater) total gain capability.
2. Make sure that the input impedance of the preamplifier is high enough to impose a minimal load on the microphone. The rule of thumb is that the input impedance should be at least 4 to 5 times higher than the output impedance of a given microphone.
3. Choose a preamplifier that offers very low noise performance, especially at the higher gain settings expected with ribbon microphones.
4. Coloration. This is strictly optional and at the user’s discretion.

There are many excellent preamplifiers on the market today. Choose one that fits your budget and offers good performance, but remember that you get what you pay for. If you have an opportunity to audition one or more preamplifiers before you buy one, do it. Microphones and preamplifiers work together as a team and some matches are just better than others.

Our recommendations

If you’re choosing a preamp for Royer ribbons, these are the two we point people to first.

1. Undertone Audio MPDI-4

Our first recommendation. The MPDI-4 is a four-channel, Class A mic preamp and DI in a single rack space, built by our sister company Undertone Audio. It has up to 60 dB of gain in 5 dB steps, a switchable impedance mode designed for passive ribbon microphones, and input and output transformers you can bypass. Engage the transformers for warm, vintage Class A character, or bypass them for a pristine, transformerless signal path.

2. Grace Design, with the Ribbon button

Grace Design preamps with a dedicated Ribbon mode, such as the m101, are an excellent match. Press the Ribbon button and the preamp raises its input impedance to 20 kΩ, bypasses the input decoupling capacitors and locks out 48V phantom power, so a passive ribbon mic sees an ideal load and stays protected.