How to tell if your preamp is bad
First a simple truth - as long as your preamp is of professional quality and working properly, there is nothing about it that will prevent you making a good recording.
Preamps don't matter anywhere near as much as the currently popular myth suggests. Your musicians are important, their instruments are important, the acoustics of the room are important, your microphone positioning and microphone selection are important, your skills and artistry are important. The microphone preamp comes after all of that in order of importance.
But still, you might have concerns about your preamp, and one common concern is noise.
The correct way to set preamp gain is to increase it to whatever value that is necessary to achieve a good strong reading on the meters of your recording system, without clipping of course.
But you may find that when you do this, there is a lot of background noise.
This may be the acoustic background noise that is present in the room. Raising the gain of the preamp does raise this, but only in proportion to the signal you want to record. So the signal-to-noise ratio from this source of noise stays constant.
But it might also be that the noise generated by the preamp itself increases. This should not happen. In fact, many preamp manufacturers quote their noise levels measured at maximum gain, because this is where the signal-to-noise ratio is greatest.
So here is a simple test...
- Set up a microphone up in a quiet room and set the gain on your preamp to maximum. Make a recording (A) of the background noise of the room.
- Lower the gain of the preamp by 20 dB. Make another recording (B).
- Normalize both recordings in your digital audio workstation so that they are the same level.
Now, there are three possibilities...
- The noise levels are about the same. In this case you can stop worrying and get back to recording!
- Recording B is noisier than Recording A. This is normal. As above, stop worrying.
- Recording A is noisier than Recording B. This is a distinct warning sign that something is wrong with your preamp. It simply should not happen. Time to get a better one!
Of course, we have only discussed noise at high gain settings. But at least in this significant parameter it is possible to see clearly whether your preamp is up to the job.