The basic purpose of the mixer is simply to mix two or more audio signals. Why the signal needs to be mixed? The situation where a mixer is needed is because the number of signal source is so many (vocal mic, cab mic, drums mics, stereo keyboards, CD player, samplers, etc) and the output media where the signals are sent to their final destination is limited or fewer than the number of source (it can be stereo L/R speaker, L/C/R speaker, multitrack recorder, mixdown stereo recorder etc). You will also need to balance the level of each signal inputs to create pleasant mix to audience ears. That the job for a mixer.

Before the signal is mixed, it can be shaped and manipulated (pad, gain, EQ, reverse polarity), or sent to other destination (aux send). We’ll explain that terms later. The signals are mixed by controlling each level using fader. The output of mix can be sent to various destination depends on mixer capabilities (stereo bus, matrix bus, etc).

Channel Section

No matter how big the console, it always contains two sections: the channel and the master section. A channel strip or simply a channel is a tool to:
• shape your signal input (your source can be microphones or audio devices) by EQ, trim gain, reverse polarity, etc.
• copy the signal to other destination (can be monitor wedges or reverb/delay effects)
• control the level of shaped signal by dedicated fader to balance the mix
• send the signal to a group or final mix bus.

Channel type mostly mono and some offer a few stereo channels to accommodate stereo devices input). The picture below shows example of one channel strip in an analog professional console. By mastering one channel you can master all the channel in the mixer (they can be 4, 8, 16, 24, 48 channels and so on) because they’re identical.

We’ll explain in details what the shaping tools in the channel strip pictured above. Note that not all the mixer has the same shaping features. Some lower entry mixers offer fewer shaping tool and some higher ends have broaden shaping capabilities (such as powerful and musical EQ, ultimate dynamics processing: compressors & gates, delay) than the example.

+48V
This button serves to activate +48 V DC voltage sent through microphone cables connected to input channel. The DC voltage, called phantom power,  required only to operate condenser microphones or active DI Box. Dynamic microphones don’t need this so know what your microphone types by its box label or manual.

Polarity
This is switch to reverse input polarity (plus and minus sign). Useful to rectify cable connection errors and mic position with phasing issue (example: miking snare from top and bottom, reverse polarity of bottom mic to avoid phase cancellation).

Any miking situation where two microphones picking up the same instrument source can lead to phasing issue which you can detect by swirling , hollow, thin, or lack of bass sound. Depend on the severity of the problem, a mirror image miking like dual snare miking above causes 180 deg out of phase between upper mic and bottom mic. This can be corrected by simply flipping one channel’s polarity out against the other to make 0 deg phase hence both signals adding up.

For more complex phase shifting issue caused by multiple mics picking up the same source such as close and ambient miking for guitar cabinet (dual miking for cabinet less likely employed in live sound, only in recording studio) where the mics position not in mirror image between each other, you need a tool called all-pass filter. One of tool to do that is Little Labs http://www.littlelabs.com IBP Analog Phase Alignment Tool.

Pad
Too hot signal (maybe you’re miking Nashgul’s ride) shall be reduced to prevent equipment and audience ears damage. Pad will tame loud signal source down to 20 dB or 30 dB depends on mixer spec. This allows mixer’s preamp to breath longer and provide massive headroom to prevent signal clipping. Experiment with this button and Gain knob (see below) so you only see green LED forever on each channel (very rare intermittent red light is allowed though).

Gain
The knob provides gain and attenuation to adjust input sensitivity. Some mixer provides +6 to +60 dB gain for mic or -14 to 40 dB for line or padded mic input. Set this gain so that the signal enters the mixer with its optimum signal to noise ratio. Reduce gain if red meter lights. I’ve seen some people leave all faders in unity mark and adjust gain to balance the channel level. You shouldn’t mix from the gain knob, that’s what the faders are for.

EQ
Probably the most famous and abused shaping tool in channel section. Rule for EQ: less is more. Probably leave it off will sound much better in overall mix. You should resist the temptation to EQ the signal beyond its frequency band otherwise it gets muddy and adds noise. Good mix comes from the good source. Tune your instrument well, position the mic properly then shape it only if needed extra sparkling (thumping kick drum, sizzling cymbals, cut though vocal, fat bass, etc). Too much spice will ruin your food.

The EQ capabilities along with preamp quality separates cheap mixer from expensive esoteric ones. Powerful EQ can be 4-band independent parametric type capable of surgical 20 Hz – 20 kHz range operation for precise tone shaping along with HF and LF shelving.

The channel picture here depicts example of 4-band semi-parametric EQ. HF (hi-freq) is shelving filter for frequency above 12 kHz, LF (lo-freq) for 60 Hz and below, HM (hi-mid) is bell shaped peak or dip filter around centered frequency, which can be swept from 500 Hz – 5 kHz, and LM (lo-mid) is from 35 Hz to 1 kHz. The bell width is fixed at Q (bandwith) of 1.8 for optimum tone shaping in live sound. All four band can be boosted up to or cut down to 15 dB. For optimum headroom, it is advisable to cut instead of boost, and do as little as you can. Focus on instrument, microphone type and position first.

Aux Sends
These knobs serve to copy channel and send it to aux jack output. In this example as much as 6 copies of channel can be sent to Aux 1 to 6. You want to copy the channel for parallel effect feeds such as reverb or delay or send to stage monitor for dual FOH-monitor console function.

The knob provides fully off to +6 dB gain control. The PRE/POST button near the knobs control how the channel is copied, is it before (PRE) or after fader (POST). For effects, button should be POST since the effect mix (wet signal) level should be in-sync with dry signal level controlled by fader. For wedge monitor, button is PRE so your mix by faders doesn’t affect stage monitor mix.

For console having excess channels, common practice is, an effect unit is set to 100% wet and its output is routed to another channel so you have control of how the dry and wet signal are mixed. You can also EQ the effect channel since some of reverb needs to cut in low freq region. But you shall turn the Aux Send knob off in effect-return channel otherwise closed loop circuit is occurred and electric feedback guaranteed to scream.

Several low end mixer has Aux returns. These can be treated as spare channels. Aux return usually offers simpler control, maybe just level and pan, where you want to connect to effect unit output, or use it as another channel for additional instrument. While Aux sends and Returns are numbered, they don’t correlate each other. For example you can tap/copy a channel by Aux send 1, send it to a delay unit and return output delay to Aux return 2.

PAN
Set PAN hard left or right to create stereo effect. It’s simple. Pan left directs the channel signal to left / L bus. Same thing with right pan goes to R bus. But in live sound, you should wisely choose uniform audio coverage to audience instead of insist mixing on fancy stereo. If your PA is L/R stack and audience spread widely, mix in stereo will be enjoyed only by few people in sweet spot. Best to apply stereo mix is in L/C/R stack where left, center and right speaker stack are provided.

MUTE
This button shuts-up the channel and affects L/R bus mix, aux send, and direct out, but not insert send. Mute channel when plugging cable and apply phantom power to avoid loud clicks.

PFL
Pre-Fader Listen. Listen means local monitor/headphone output. To monitor channel before Fader in local monitor (not wedge monitor in aux send) or headphone press this button. You can select more than one channel to PFL. All mix sent to local monitor/headphone outs.

METER
This is (one of) important indicator to separate soundman from deaf clown. SIG detects a signal presence above -12 dB. ‘0’ is nominal level 0 dBu hence ‘6’ = +6 dBu. ‘PK’ means channel pre-fade is  +5 dB before clipping. Never let ‘PK’ lights.

ROUTING
L-R: route the channel to L/R bus. This is the main output to amp rack and main speaker.
1-4: route the channel to group. You can group all drums channel as one group, all vocal and backing vocals as one group etc to control easier. Although you can, best practice is, don’t route L/R and group at the same time. Route to group bus first, then route group to L/R bus. This logical approach will ease troubleshooting.

Bus in here is like bus as transportation vehicle. Many people met in the bus and travel to certain destination. Many signals carried by a bus and directed to certain destination, it could be another bus or output jack.

FADER
This long slider (usually 100 mm logarithmic) controls channel level that feeds L/R and group bus, and post-aux sends. Able to boost +10 dB above unity gain. If poor guitarist is posing like guitar god and gesture to take solo and he doesn’t have booster pedal please turn his channel up will you.
 
So that’s the channel section. Next part you will learn master section, where all the channels are controlled and regulated.

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