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Everything you need to know about the gain control of a microphone preamplifier

The gain control is the first electronic circuit an audio signal encounters after it leaves the microphone. What does it do? What is the best setting? How can things go wrong?

The microphone preamplifier, complete with gain control, is the first electronic circuit an audio signal encounters after it leaves the microphone. That makes it the most important circuit element in the whole mixing console. No, the whole studio.

So why do we need a preamplifier in the first place? And why does it need a gain control?

The reason we need a microphone preamplifier is, surprisingly, so that manufacturers can sell them to us and make more profit. That's OK, we don't mind them making a profit because they'll put a large chunk of that back into research and development and make more interesting stuff for us to play with.

But in the twenty-first century, it would be perfectly possible to place the preamplifier within the body of the microphone, and send digital signals to control the gain. Out of the microphone would come a line level signal, ready for processing, recording and mixing.

But R&D has stopped short of that because it doesn't make good marketing sense.

So microphone design, in this respect at least, has remained pretty much where it was in around 1967. Yes, really.

So, in 1967, there were basically two types of microphone - dynamic microphones which generate electricity themselves, and capacitor microphones, which need a supply of electricity that they modify to create the signal.

Dynamic microphones generally have a fairly low output, because they generate electricity directly from sound, and there isn't very much energy available in a sound wave to do this.

So the typical output from a dynamic mic could be as little as 1 millivolt - one thousandth of a volt - even when placed quite close to the sound source.

Capacitor microphones all have an internal amplifier with a fixed gain. Generally it boosts the signal a little in comparison with the dynamic mic, and you could expect an output of around 10 to 100 millivolts.

Of course, the actual output depends on how loud the sound source is, and how close to the microphone. But these are typical levels that you could expect.

Professional mixing consoles like to work on a signal level of around a volt. The line level input of the audio interface of a professional digital audio workstation would also prefer to see around this level, to maintain compatibility with analog mixing consoles.

So the signal from the microphone needs to be boosted.

At the very least, the signal would have to be boosted by around ten decibels (that means multiply the voltage by approximately 3). More usually it would be between 20 dB and 50 dB. When a dynamic mic is used on a weak sound source, then perhaps a gain of 60 dB would be used. This means that the voltage is multiplied by 1000. 70 dB is the maximum gain that is ever useful. Going beyond this normally means that you're boosting noise to no useful effect.

The way to set gain correctly for an internal preamp in a mixing console is this...

  • Get your sound source up and running and creating sound.
  • Press the PFL (pre-fade listen) button on the channel to which the microphone is connected.
  • Set the gain so that the meter on the console shows a healthy level - not over the top, no red lights, but distinctly more than twitching.

That's it. All you have to do. Don't worry about applying too much gain. The console is designed so that if there are no red lights on the meter, then all is well.

If you have a separate microphone preamplifier, the procedure is only slightly different...

  • Connect the output of the preamplifier to a line input of the mixing console.
  • Set the line gain of that channel to 0 dB.
  • Get your sound source up and running as before.
  • Press the PFL (pre-fade listen) button on the channel to which the microphone is connected.
  • Ignore the meter on the preamp, if it has one.
  • Set the gain of the preamp so that the meter on the console shows a healthy level - not over the top, no red lights, but distinctly more than twitching.

In general, if a microphone preamplifier has a meter, it won't tell you anything useful. If you did want it to tell you something useful then you would have to measure the headroom in the preamp and the headroom in the mixing console. If they match, then you can use the meter on the preamp. If they don't, then you would have to compensate mentally. Broadcast professionals worry about this sort of thing, but for most purposes the meter on the preamp can be ignored.

If you are connecting your preamplifier to the line input of the audio interface of your digital audio workstation, then the procedure is this...

  • Connect the output of your preamp to one of the line inputs of the interface.
  • Create a new channel and set it to record-ready, so that the meter shows the level of the input signal.
  • Select the input to which the preamp is connected as the source for that channel.
  • Get your sound source up and running.
  • Set the gain of the preamp so that the meter comes close to the top of the scale, but doesn't show any 'overs' at any time.

Setting the gain of a microphone preamplifier is a simple procedure with little to go wrong.

But what can go wrong?

There are only two possibilities. One is that you set the gain too low; the other is that you set the gain too high.

If you set the gain too low, then the signal in the console or DAW will be low. It will be closer to the noise level of the console or DAW (and even DAWs generate a certain amount of noise of their own) and therefore your signal-to-noise ratio will be worse than it could be.

If you set the gain too high then you risk clipping the signal when it goes over the top. Clipping causes serious distortion and must be avoided.

If you get clipping, then the solution is always to lower the gain. Lowering the fader won't help, either in a console or in a DAW, because the signal is already clipped by that point.

If the gain is set correctly in the first place then it will not need adjustment, unless the sound source changes or the position of the microphone is changed.

There is only one time that you need consider changing the gain setting, and that is when you apply EQ boost.

If a signal is close to clipping, which is perfectly OK as long as it doesn't actually clip, then EQ boost could push it over the top.

Once again, lowering the fader won't help. The solution is to lower the gain.

To round off, there is one time when you might set a lower gain than the previous advice suggests.

That is when you have a sound source that you know will be quiet in the mix. If you set the gain normally, you will find that the fader is very low and difficult to adjust precisely. Some engineers will therefore anticipate this and deliberately set a low gain. Then the fader can be higher up in its travel and be easier to control.

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By David Mellor Monday March 12, 2007
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