There’s a lot of pseudo-mythical adjectives thrown around when discussing the merits of mic pre-amps. The get called ‘warm’, ‘full’, ‘fat’, ‘present’, ‘silky’ and ‘creamy’, and whole lot of other words that are better suited to describing food. So what’s really contributing to the qualities of those ‘holy grail’ preamps that everyone fawns over? One of them is the pre-amp’s CMRR – common mode rejection ratio.
Common mode rejection is what balanced cables are all about. All of our audio cabling and componentry is subject to electromagnetic radiation, which is all around us both naturally and unnaturally. The man-made stuff is stronger, of course – you’re more likely to have problems in your PA caused by someone using a CB radio next door than the background radiation of the universe. Our solution? TRS and XLR balanced connections, and audio transformers.
Your three-core cable balanced cable carries one earth and two signal lines. When going into a pre-amp, the first thing they usually hit is an audio transformer, which has an input coil and an output coil (not all mics pre-amps use transformers, some use other methods, but they still have input and output coils). When the two signal lines hit the two sides of the input coil, the varying levels of the input signal from your mic on each line creates a difference in voltage between the two ends of the coil, and that constantly changing voltage difference is our signal, which we send into the output coil.
Electromagnetic radiation seeps into both signal cores in your cables or components more or less equally. So what happens when THAT hits the two sides of the input coil is that there is NO DIFFERENCE between the voltage on either side – no voltage difference, no output. That’s the theory, anyway. In practice, nothing we humans make is perfect. The amount that a piece of audio circuitry can actually reduce common mode noise is referred to as its common mode rejection ratio.
And this is when we get to the differing levels of CMR that components can achieve. Many audio engineers get misty-eyed when describing the sweet tones of a Jensen transformer, or lust-after the transformers in Neve pre-amps, the best of which were hand-wound by Rupert himself. Others speak of the clean English tones of Soundcraft, or the German efficiency of Studer. Generally speaking, the cheaper the components, the lower the common mode rejection ratio.
It’s not just CMR that the audio transformer is providing. Due to the inherent impossibility of perfectly replicating the phenomenon of moving air as an electrical signal, all transformer (and non-transformer based) pre-amp designs will introduce errors and inefficiencies. Some of these are considered a plus in the audio world – increased level in the bass and low mids, ‘smoothing’ of the top end, or just a touch of that sweet, sweet third harmonic distortion.
Whatever the sonic compromise, it’s almost certain that whenever you’re hearing a pre-amp that you’re less than impressed by, it will almost certainly have bad CMR stats. This is the leading cause of your signal not sounding quite right or natural, the very opposite of what some consider the apex of pre-amp performance, which is ‘your get out what you put in’.