The input stage of a mixer serves two purposes: it routes microphone and instrument outputs into the mixer and adjusts the signal to its optimum strength. Taking a look at the very first element that a signal hits in a mixer, the input stage, you’ll find an onboard microphone preamplifier. How a mic preamp is designed is a strong indicator of the sound and character of the entire board, and any loss in quality at this initial stage can never be regained, making these preamps very important. This is why it’s a common practice in multitrack studios to use a few very high-end – and very expensive – external mic preamps in place of the often run of the mill ones built into what would otherwise be a top notch quality mixer.
With that said, the job of the mic preamp is a very difficult one; it must provide a sufficient amount of signal gain, which is a lot by the way, while keeping the background noise at an absolute minimum. It must also have a high amount of headroom so that unexpected peaks don’t cause overloads. And if that wasn’t enough, it has to preserve all of those little subtle nuances of the waveform captured by the mic – from the lowest frequency to the highest as well as a wide range of volume.
Let’s say that a mic preamp on a mixing board has these initial, basic important things covered, there are still a few more practical demands that it should be able to achieve. If we take a look at the highest professional grade microphones around, they generally tend to be of the electrostatic variety which usually needs a little more juice to power their capsules and internal preamps – more than a straight connection can offer. Condenser microphones fall under this type. Mixers offer this extra boost of needed voltage in the form of “phantom power,” usually independently switchable from each channel, although budget mixers might relegate the phantom power to a fixed channel or two.
Moving on, another unwanted problem that the input stage is meant to handle stems from all directional microphones being susceptible to low-frequency, mechanical vibrations. This tends to happen when they’re trying to handle too much noise. These unwanted low-frequency signals can quickly and drastically take up a lot of the mixer’s available headroom. In order to fight this inherent problem, the input stage on a board will normally include a switchable high-pass filter which will siphon off all of that unwanted noise, preferably without taking any of the wanted sounds with it.
Higher-quality mic preamp designs usually come with a very wide gain range so that a functional signal level can be achieved no matter how loud or quiet the original sound, or how close or far for that matter (within reason!). The way the input stage is able to handle all sorts of signal sources comes from its switched coarse-gain control along with a separate, continuously variable, fine trim. Higher-quality mixers usually offer this feature with the ability to dial it in at 5dB to 10dB increments, although more economical boards cut costs by using a single variable control that covers the entire gain range. Technically speaking, being able to fine tune the exact amount of gain control is the better choice but that doesn’t mean there aren’t a number of perfectly good boards using the other technique these days. If you’re after maximum flexibility, an input stage with up to 70dB of gain is what should be aimed for, but the drawback in this is that it places a significant deal of stress on the circuits. Most home-studio boards do perfectly fine with as little as 50dB of mic gain, which eases the demands on the board considerably.
Mixing boards with a particularly sensitive mic preamp in their input stage can be extremely useful given the right situation, but what happens when you need to record something a bit too loud, such as the inside of a kick drum or even a live concert with hundreds of screaming fans? It’s not unusual for microphones to become easily overloaded in these types of situations, which then overloads the input stage on the board. To avoid this problem, mixers usually come with a switch to activate an attenuator ahead of the preamp which then reduces the signal level, usually by about 30dB.
Another feature that the input stage of a mixer usually comes equipped with is the ability to invert the polarity of the input signal through a switch called “phase reverse.” Trust me – this little switch has saved plenty of people a ton of time and headache! This switch becomes immensely useful when combining the output signals from several different microphones that are recording the same sound source at the same time. Although there does exist a set standard for microphone polarity – XLR pin 2 positive, pin 3 negative – not all manufacturers follow it, meaning that not all microphones generate the same polarity of output signal under the same circumstances. The problem occurs when mics of opposite polarity are mixed together – instead of adding on top of each other like you would think, the signals cancel each other out. The phase invert switch was made specifically in order to fix this issue, allowing us to control the polarity of each mic’s signal and in turn make sure they add up instead of cancel out.
And finally, most mixing board’s input stage also tend to offer a way of selecting either microphone or line-level inputs for each channel, normally being connected on different sockets. Higher end boards will also offer separate gain controls for the microphone and line inputs while cheaper mixers usually simply include a selection switch. One important thing to remember when you’re setting input gain on a channel is to always deselect any added equalization and put the channel fader at its normal, root position (0dB on the scale) before trying to adjust the gain. This allows you to bring the sound to an appropriate level. If you fail to do this, the input stage won’t be operating as intended and will suffer from an increased noise floor and reduced headroom. Don’t forget that the channel faders are meant to work as a convenient way of adjusting levels during a live recording or performance. If you happen to place the faders on either its max or minimum point, the input gains are wrongly set and your board won’t work at its optimum level.