(To read this series from the beginning, start here.)
Last week we discussed how to choose a microphone, and how you might go about using it in your recording space. After the sound is captured by the microphone, where does it go? This week, we look at the Microphone Pre-Amplifier, usually just called the preamp (or even the “pre”).
Bigger is Better
The preamp has one simple job, and that is to boost the volume of the electrical signal generated by the microphone.
But Why so Small?
A microphone uses a sensor (called a diaphragm), which physically moves in response to sound waves in the air. That movement generates electricity inside the microphone. As you can imagine, the movement of a diaphragm, in response to sound in the air, is physically an extremely small movement, and the electrical signal it generates is correspondingly very weak. In order for the signal to be stored on a recording medium, it first must be greatly amplified (made stronger) to a useful strength, referred to as “line level”.
Why should we even talk about a preamp, if all it does is “turn up the volume” on my microphone? Well, because some preamps do this well, and others do it with a lot of noise.
How Big is Big?
A bit of math: we’re talking about an increase in volume of about 60dB (or even more). Every time we boost by 10dB, we are doubling how “loud” the signal sounds. So an increase of 60dB means that we’re doubling the perceived volume from the microphone, doubling it again, and again, and again, and again, and once more. That is a LOT of volume boost. Some microphones (ribbon microphones, in particular) can require even more boosting of the signal.
It takes good quality electronics to amplify a signal this much without also adding noise, or degrading the quality of the signal.
But Wait, That’s Not All!
One other feature of a mic preamp is the ability to send power to a condenser microphone through the microphone cable. This is called “phantom power”, and most preamps have the ability to provide it. Look for a switch called “phantom”, or “48V” (since phantom power is usually provided at 48 volts).
Plus, there is an added feature that comes with many preamps, and that is a “coloration” of the sound. Yes, we’re talking about an intentional change in the sound, which takes us away from a strictly “accurate” capture of the sound, and moves us into an “improved” version of the sound. For decades, certain electronics have been used inside recording equipment to impart a subtle change to the signal. Whether it’s for a “weighty” low end, or an “airy” high end, or something we mysteriously describe as “warmth”, certain preamps are loved for the way they change the captured sound. A preamp may use transistors, transformers, or valves (tubes) in the circuitry to add colour.
A preamp may also have a line or instrument input, for capturing the sound from an instrument or musical device without using a microphone. The value of such an input is not as much to boost the volume of the sound, but to apply the “coloration” of the preamp to the sound of the instrument.
Keeping it Simple
Here’s the bottom line: when choosing between the many preamps available to you, understand that many of them will be doing the best job they can (for the price) at boosting the volume without changing the signal in any way (no extra noise, no unwanted distortion). These are called “transparent” preamps. Others will be boosting the volume AND also imparting a useful distortion, or “coloration”, to the sound. These are called “colour” preamps (yes, I’m a Canadian, so get used to seeing “ou” in my words). The most important job of the preamp is still to boost the volume without imparting unwanted distortion.
Do I Even Have a Preamp?
If you’re looking for a separate box to point at and say “that’s my preamp”, but you just can’t find it, chances are the preamp is built into your audio interface or your mixer. Follow the microphone cable from your microphone to the other jack that it’s plugged into. Anywhere you can plug in a mic cable, you have a microphone preamp. It could be a large device all its own, or it could be hidden inside a channel on your mixer or audio interface.
How to Get the Most Out of Your Preamp
Let’s start with the gain, or volume knob, on the preamp. You’ll want to increase the gain until your captured signal approaches the “line level”. There are many ways to describe this, too many to get into at this point (but I promise I’ll get there soon!), but you’ll want to see the signal meters on your preamp, audio interface, or recording software indicate lots of signal when your sound is being captured by your microphone. Avoid adding too much volume – you don’t want the signal meters showing “red”, or hitting the top of the scale, EVER! You could try increasing the volume until you see your meters showing just a bit of “red”, then lowering the volume until you see no more red indicators.
If your preamp uses tubes or transistors to add colour (saturation) to the sound, then you might see both a gain control and an output level control. Try combinations of these controls to push more or less volume into the saturation component, then compensate by adjusting the output control to get the right volume level coming out of the preamp.
Other controls on your preamp might include an impedance control, which will change the way in which your microphone and the preamp function together. Try this control in different positions and see how much you notice the effect. If in doubt, keep the impedance low.
Once you get the hang of it, you’ll be able to get the most out of your microphone preamp, and you’ll definitely hear the difference!
In the next post, we’ll look at the A-D (analogue to digital) converter.
Next Instalment: Part Five – Tearing Your Sound to Bits