When someone sets out to do a recording, whether it is popular music, classical music, audiobook narration, audio drama, sound effects, etc. they usually say “What microphone should I use?” or perhaps “what kind of recorder should I use?.” Sometimes “What mixer?” I created a post about mics for spoken voice, but which also addresses issues for music recording. The mic is the glamour “star” of recording and they are heavily advertised and promoted and I plan to put up posts about mics again in the future.
But let’s look at everything that is a part of the digital recording chain:
- Acoustic space
- Microphone
- Microphone preamplifier
- Mixer (if used)
- Analog to digital converter (AD converter)
- Recorder
- Digital to Analog converter (DA converter)
- Playback amp(s) and speaker(s)
An earlier post also talked about the kind of acoustic space was suitable for spoken word and compared it to spaces for music. Again, I plan to post more later.
Today I want to discuss the microphone preamplifier, sometimes just called a “mic pre.” Other posts will cover other aspects of the digital recording chain.
Microphones put out a very low electrical signal compared to, say, music synthesizers, CD players, the output of mixers, and other so called “line level” devices. Mics produce a much lower “microphone level” and even this can vary from a fairly robust signal to an extremely weak one.
A lot of times people don’t even consider microphone preamps as they are built into most mixing consoles and portable recorders. Sometimes those built-in preamps are fine for a particular application, but not always.
Maximum quality requires every component in the chain to be of high quality, not just one or two. I would be reluctant to feed the output of, say, a DPA 4006 microphone directly into the mic pre of a $150 hand held portable recorder. Would it work? Assuming the low cost recorder could provide needed power to the mic (so called “phantom” power) then you would get sound.
But using a low quality preamp with an ultra high quality microphone is kind of like using a Ferrari to haul trash. If your application does not require the highest in quality, probably a low cost preamp, such as those built into inexpensive recorders or mixers, is just fine. But when might you want to consider something better? Here I could go into a lot of technical jargon about “slew rate” and the like, but I will keep it focused on more simple basic concerns of the sound producer.
Low noise and high gain
All electronic gear produce some form of what is called “self noise,” usually a kind of hissing sound that you can easily hear when you turn up the gain (loudness). Microphones with a very low output signal, especially ribbon mics, need lots of gain. A low cost preamp may not be able to deliver enough gain to record a signal at normal levels, or, if it can be turned up high, could produce lots of self-noise (hiss) in the audio signal. Since even microphones, like any piece of electronic gear, do produce some self noise of their own, some people who turn up a ribbon mic with a low cost mic preamp assume the problem is in the microphone itself, even when it really is the preamp. This problem can also occur with many kinds of mics (including dynamic and capacitor), but is most severe with ribbon mics. And the concern is not just for people recording music (such as a choir) but also other subtle sounds. Nature sound recordists tend to talk about low noise mic preamps since many natural sounds are very quiet.
Some ribbon mic manufacturers, such as Audio Engineering Associates (AEA), also sell special high gain, low noise, preamps designed especially for ribbon mics. But most very high end mic preamps can also work well with ribbon mics provided they can offer enough gain and still keep the noise low. Back in the days when most recordings were done with ribbon mics (c. 1940-1960) standard radio station consoles had high gain mic preamps. But lower cost boards sold today are designed either for capacitor mics (which produce higher output) or even dynamic mics used close to a loud sound source. A dynamic mic won’t produce a terrible loud signal from a weak source, but do well recording loud sounds up close. Thus they are popular with singers who sing two inches from the mic or engineers needing to close mike the individual parts of a drum kit. In those applications the dynamic mic generally produces a reasonable output. Softer sounds are more of a problem with the dynamic mic, but the ribbon mic is always quite low and really cannot be used for extremely loud sounds as the ribbon is easily damaged.
Another solution to this problem is to use an intermediate amplifier (pre-pre amplifier?) between a low output mic and a mic pre in order to feed a stronger signal to the standard mic preamp. These are often used with dynamic mics as well as ribbon and are usually “phantom powered” by the mic preamp in a console or recorder. This is the same DC power normally used to power capacitor (also called “condenser”) microphones.
Getting the right tone “color”
Whenever an electronic signal is passed through a piece of electronic gear it can be modified, often very subtly, in certain ways. You might think that the best mic preamplifier is one that simply amplifies the signal without changing it at all. But remember, even the best microphones do not capture the sound exactly as someone would hear it in person. Thus a good mic preamp might subtly change a signal to produce a warmer sound, a darker sound, or even a harsher sound (as might be desired for Heavy Metal music). Major recording studios tend to have a large number of “outboard” mic preamps (meaning ones not built into a console) so an engineer can match a certain preamp-mic combination to the sound sought by a particular artist or producer. This means that someone seeking the benefits of a high end mic preamp should “shop around” to get the best one for a specific application. Mercedes Benz and Lexus are both high end cars, but each has a different “feel” to the driver. The same goes for mic preamps.
Shopping for the perfect high-end mic pre can take time and patience. It is best if one can try several before committing to a purchase, though that is not always possible. For many everyday applications the mic preamps built into a small console or recorder are sufficient. That is good because high end mic preamps tend to be expensive, often much more than a small console (such as those built by Mackie, Behringer, Yamaha, and others). But the serious recording engineer should be aware of the advantages of special preamps for demanding applications.
In future posts I will discus analog-to-digital converters and digital-to-analog converters. Then I plan a more in-depth series on microphones. But I don’t want The Audio Penguin to just be a “tech” site so will also put up additional posts about the creative side of audio production.
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