In the pic on the left, we have a simple block diagram of a four channel mixer. This shows the two main sections of a mixer: the channel strip (one for each input) and the master section. The master section combines all the channel strips and sets all output options.

This month, let's take a closer look at the channel strip, section by section. Next month, we'll look at the master section and some example setups.

The Channel Strip

 
The Inputs
A mixer has two types of input:
Line level - for instruments which have a powerful output signal, such as synthesizers, this is usually an unbalanced 1/4" phono jack socket.
Microphone Inputs - a balanced XLR (3 pin) socket for the relatively low signal level from mics. Balanced line level signals are also connected here, with the mic preamp's gain control turned down.

The Microphone Preamp
The microphone preamp is the most important component in the mixer. Mics produce quite a low voltage, so they need a lot of boosting to reach a level that the mixer can work with. This need to be done with as low noise and distortion as possible, because if the signal quality is degraded at this point in the mixer, the noise will only be compounded by everything that comes later in the signal path. So the quality of the preamps has to be very high.

Trim or gain controls on the preamp section are used to set the level of the signal entering the channel strip. This is essential to ensure that the signal to noise ratio (the amount of noise generated) by the channel is kept to a minimum.

Better mixers will often have a lo-cut switch on the mic preamp. When engaged, this will cut all frequencies below a certain point (usually 80Hz). This is used to remove the very low frequencies on the instruments that do not need them (usually everything apart from the bass guitar and kick drum mic), preventing a bass buildup in the system. A bass buildup will use a lot of the power of your amplification to no end as well as muddy the sound.

Your mic preamps may also have a switch for phantom power, which is used to send 48V of power down the mic cable to drive condenser mics. Often, while this feature will be available on every mic channel of the mixer, it will be turned on for all mic channels via a single switch. Don't worry if you have a mix of condensor and dynamic mics - any modern dynamic mic will "ignore" phantom power. However, be aware that certain badly designed equipment can object to or be destroyed by phantom power.

Inserts
Inserts allow you to insert effects such as equalisers and compressors into the signal path of the channel strip. Inserts can also be used to send the signal to another mixing desk (for advanced monitoring applications) or multitrack recorder (for multitrack recording of live performances without any EQ or effects).

Equalisation
The tonal adjustment section of the strip. Will usually be in the form of a two or three band EQ (lo, mid, hi frequencies). Better mixers often have a parametric EQ for the midrange (or even two!). A parametric EQ has an extra control - usually called 'frequency' - which allows you to 'tune' the midrange to the desired frequency. This ability to tune the EQ allows you to do wonderful things like find the exact frequency which is causing a problem (like feedback or an odd tone) and fix it. I'll get into EQ in depth in another article soon.

Fader
The fader is where you set the level of the channel - the volume in the mix of the instrument you want. Try to have the fader set somewhere around the 0dB mark or lower (usually marked clearly on the mixer). This gives you some 'headroom' so that you can push the channelhigher when needed (solos, etc.). To have it set higher all the time can introduce unneccesary noise to the signal. If you find you don't have enough level to make the instrument loud enough without pushing it up higher than this point, check your input gain/trim control.

Auxillary (Aux) Sends
This is where things start getting interesting... An Aux send allows you to send a copy of the channel signal somewhere else, often combining it with other channels to a separate output. Why? Well, this allows you to do things like send a different mix to a monitor system, or add effects to some of the channels but not to others. Tasty, and comes in two flavors: pre- and post-fader. Pre fader is what you will usually use for monitoring - the signal is taken before the fader changes the level (often before the EQ too - check your mixer's block diagram). The reason for using pre-fade aux sends for monitoring is so that as you mix - changing the level of an instrument in the main mix - using the fader, the volume of the instrument stays constant in the monitors, keeping the musician happy. For effect send we use post-fade so that as you bring down the level of an instrument, the effect level also diminishes. It's nice if a mixer gives you the option to change an aux send from pre to post, but not essential. Some mixers have the sends preconfigured as pre or post and call them 'monitor' and 'effects' respectively.

Panpot
Positions the instrument in a stereo mix. Usually for live sound, you will keep everything smack bang in the middle (mono), so that people in different spots in the room will hear the same mix. Even if you are doing a stereo mix, try to keep your bassier instruments in the middle. This shares the load of the bass end between the two channels of your power amps, and human hearing tends to percieve any frequencies below +/-250Hz in the centre, regardless of the source direction.

Other Controls/Switches
Your mixer may have some controls or switches I haven't covered above. In short:

• Buss Routing - (no, nothing to do with Golden Arrow) An understanding of busses is imperative for that, next month...
• Mute Switches - Allows you to turn a channel off completely. Useful to mute mics not being used for a particular song
• Solo Switches - The opposite of a mute switch. Switches off all the channels not solo'd. For setting trim levels and hearing one track in isolation
• Pad - Cuts down the level of excessively loud input signals
• Phase reverse - Swops the polarity of the input to cure some multiple micing problems

Next Month
The master section of a mixer and some sample setups.