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We take it for granted that a microphone preamplifier should have a flat frequency response, otherwise certain frequencies would be emphasized at the expense of others.
It should have low distortion, otherwise the signal you record will be different to the signal produced by the microphone.
It should be low noise, otherwise there will be an audible background hiss.
So if all of those parameters are perfect, the mic preamp will be perfect, yes?
Well actually, not necessarily. There is another significant parameter that directly affects the sound of the preamp, and that is the common mode rejection ratio.
A microphone produces a signal that is balanced. That means that it has two output signals, one of which being exactly the opposite of the other. Where one goes positive, the other goes negative, both by exactly equal amounts.
The microphone preamplifier is designed to accept a balanced signal. In effect, it inverts one of the signals and adds it to the other, producing a signal of twice the strength.
But there might be another signal present - interference. Interference typically comes from radio waves generated by switching sparks, lighting dimmers, or simply the mains wiring of the installation. If amplified, it will sound like clicks, crackle, hum or buzz.
Fortunately, when the preamplifier converts the balanced signal from the microphone into the unbalanced signal it works on internally, it cancels out the interference.
Well, most of it.
Where the signal from the microphone is equal and opposite on the two conductors, the interference is equal and the same. This is called a common mode signal.
Yes, the preamp will reject the common mode signal, but not entirely. The preamp's ability to perform this important task is called its common mode rejection ratio, or CMRR.
For instance, a preamp such as the Crookwood Paintpot, a high-end preamp, has a CMRR specified as better than 85 dB @ 1 kHz, and better than 75 dB @ 20 kHz, all at 40dB gain.
Notice how the performance is less good at 20 kHz; this is to be expected.
The key to achieving a good common mode rejection ratio is to ensure that the components are well matched for both input signals from the mic. Also, ensure that their input impedances, or their 'hunger for current', are the same - which is something that isn't always done correctly, I have noticed.
These figures have to be good, because interference is particularly audible, even at very low levels. This is particularly so for live sound because of the electrically noisy environment and long cable runs.
So could good CMRR be the parameter that marks the difference between a good preamp and one that is not-so-good?
The debate continues...