The various features of an audio mixer can be broken down into several sections in order to simplify explanations. The sections break down as follows:
Inputs | Outputs | Controls
THE MIXER - INPUTS "MIC" The microphone input is usually low-impedance, i.e. 5000 ohms or less in mixers with active input circuitry or approximately 600 ohms in mixers with input transformers (Note: there is no problem with plugging a 600 ohm or lower impedance microphone into a 5,000 ohm imput). It will usually have a 3-pin "XLR" connector and it will always be a female XLR (male XLR's are always outputs). Mic inputs may also be 1/4-inch "phone" jacks in older or smaller mixers, however they may be high-impedance (over 5000 ohms) so check the owner's manual. A high impedance ("HI Z") mic may produce a low level or distorted sound when connected to a low impedance ("LOW Z") input. A low Z mic in a high Z input can produce similar results. If you have a low Z mic and only high Z inputs are available, a "low-to-high Z" adapter can be used to overcome the problem (eg.Yorkville model LHNT-1).
The channel input gain control or input attenuator on mixers with such features, will always regulate the "mic" input. And you may find a phantom switch near the Mic input or somehere in the master section. This feature applies a small amount of DC voltage to one of the Mic input connector leads - usually 24 - 48 Volts. This travels backwards up the cable to the mic (the only time electrons flow "upstream") for the purpose of powering a condenser mic.
"LINE" Line inputs are always high-impedance (10,000 ohms or more) and employ 1/4" jacks or sometimes RCA (phono) jacks. This is where you would connect a high-impedance mic or the line-level output of a signal source. The term "line-level" is used to cover signals referenced to 0dBm or 0.773 Volts rms. These are much greater than those produced by mics or instrument pickups which produce signals down around 0.1 Vrms or less. The range of things which produce line-level output signals is very wide. Almost, anything which runs on AC or DC power produces this level of signal. The exception would be the output of a "phantom" power supply which is mic-level. Beware of "speaker" outputs or "extension" speaker outputs as even the smallest amplifier produces too much power for a "line" input and will overload the circuitry.
Also, remember to use shielded cables with a centre lead wrapped in the shield wire (as opposed to speaker cables which only have two regular leads). They help minimize hum and noise when connecting line outputs to line inputs. The channel input gain control or input attenuator on most mixers will regulate the "line" inputs and the input circuits have sufficient gain for high-impedance microphones. You may need to set the Input Gain control higher for high Z mics than line-level sources but some mixers have Mic/Line selector switches so that the Gain settings can be roughly the same for mics and line-level sources.
"BAL. LINE" Balanced line inputs, when connected to balanced outputs, offer greater degrees of protection from hum and noise. Some of them have three-pin XLR connectors, but the majority these days tend to have stereo 1/4-inch jack sockets. In this case you use balanced shielded cables which are outwardly similar to regular shielded patch cords, but the wire contains two centre leads plus shielding and the 1/4-inch plugs will be stereo, i.e. with two insulator bands near the tip rather than one.
The standard lead designation is tip=in phase, ring=reverse phase, sheild=ground (although there may be a few exceptions). XLR-type balanced patch cords - mic cables can be used for this - are usually designated pin 1=ground, pin 2=in phase, pin 3=reverse phase. It's worth noting that, in a situation where the output is unbalanced but the input is balanced, a regular patch cord will usually work (ditto the reverse situation). Additionally, Audiopro AP-series mixers feature special balancing circuitry which provides the noise rejection of a totally balanced circuit even when the signal source's line output is unbalanced. Simply use a balanced patch cable.
"INSERT" The insert jack is usually found amoung the channel input connectors and sometimes among the main and monitor outputs. It is a 1/4-inch stereo jack socket which is actually two jacks in one - a send and a return. This unusual setup is used in place of two separate jacks to save space (some mixers have separate send & return Insert jacks). Using a special "Y" cable with a stereo 1/4-inch plug branching out to two mono 1/4-inch plugs (eg. Yorkville model PC-6ISPH), you can patch a graphic EQ, compressor/limiter, aural exciter, digital delay, etc. directly into a channel or main/monitor buss. In some cases, the patch cable wiring code is tip=return, ring=send, sleeve=ground, while in others, the wiring goes tip=send, ring=return, sleeve=ground. Check your owner's manual to find out which one your mixer uses. In most mixers, the "send" part of the Insert jack is buffered - in other words, regulated by the channel Gain control.
"EFX RETURN" or "STEREO RETURN" The effects or stereo return jacks are usually employed as part of the effects "loops". They accept the output of external units - reverb, delay, etc. - and feed it to the mixer's master RETURN controls which regulate its passage to the next master mixer stage. Effects return jacks are usually unbalanced 1/4-inch jack sockets. On stereo mixers, they would be in left/right pairs in order to accept the output of stereo effects units. Altrnatively, these can be used as stereo auxilliary inputs, perhaps for connecting a keyboard mixer.
"TALKBACK" Some mixers feature a very simple channel consisting of a mic input, a level control, possibly an on/off button, and a line-level output. This is included to enable the mixing technician to address the lighting technician, a stage hand or road manager through a small amp/speaker system. Some mixers may have the ability to also send some of this signal to one or more of the Aux/ Monitor busses so that the performers may be addressed through the stage monitors. The Talkback input is normally a standard, female XLR mic connector.
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The various drum channels' Pan controls would simply be turned all the way Left, for example, and the rest turned Right. Now the Sub or Group One master would regulate all the drum mics at once and the Sub or Group Two master would regulate the rest of the band. The Main, Mono or Sum Master would now regulate the overall level through the PA. In a multi-buss mixer there would, of course, be more submix possibilities. As an example, a four-buss board could accomodate separate vocal, drum and keyboard submixes plus one for the rest of the band.}
"MON" The monitor outputs, like the Main and Sub/Group outs, are usually balanced (see INPUTS "BAL LINE" for cable wiring) and have their own masters. These would be connected to the monitor amp/speaker system.
"TAPE" The tape outputs are usually RCA (phono) type and are not balanced. A Tape Send or Tape Out control may be among the masters to regulate the level. If there is no such control, check the owner's manual; chances are the Main or Sub stereo masters control them.
(INPUT CHANNELS)
"GAIN", "TRIM", "ATTEN" or "PAD" All sources put out different amounts of signal. Some may be weak and others quite strong. It is thus important in larger mixers to be able to adjust the amount of source signal entering the channel. If there is too much, as stated earlier, distortion will result. On the other hand, if there is too little, the channel Level may have to be set much higher than the rest of the channels and the signal-to-noise ratio on that channel will be less than ideal.
A few of the more recent mixers only have a Gain or Trim control, no switch. This is possible because active circuitry replaces the input transformers and, as a result, they have such large amounts input headroom and Gain control range that a "pad" is not necessary. One way or another, the channel Clip indicator should be your guide to setting the Gain/Trim/Atten./Pad. While a signal is applied to the input, increase the setting of this feature until the Clip LED flashes, then decrease it slightly.
"CLIP" In the absence of a channel VU meter, the input clipping indicator is your best aid for setting the Gain/Trim/Pad. This LED is designed to illuminate when the input signal is approaching the upper limit of the input circuit's capacity, but still leaving around 3dB of headroom in most cases (check the manual to be sure). It is thus possible to set the Gain controls simply by watching the channel Clip indicators during a soundcheck and adjusting them for slight amounts of activity.
"MON." or "AUX." Depending on the mixer design, the Monitor and/or Auxilliary send controls may come next. On mixers with "Pre/Post" EQ selector buttons for these controls, they will come after the EQ section, otherwise they will be right after the Gain, Trim, etc. In order to avoid confusion about how "send" controls work, here is a brief explanation; each channel is capable of sending some of its signal via the internal circuitry to various locations (busses) within the master section. In order to do this there needs to be channel controls to regulate the amount of signal going to each buss. It is not always ideal to have these signals affected by the channel EQ controls since that EQ is there primarilly for regulating each channel's sound through the main PA which has different frequency response than the monitor amp/speaker system.
Some mixers offer "semi-parametric" EQ. This usually comes in the form of one or more cut/boost controls, each with a frequency control to position the cut/boost exactly where you want it along the frequency spectrum. One application of such a feature is in the fight against feedback. Here you would turn the cut/boost control counter-clockwise to produce a "dip" in the frequency response, then rotate the frequency control until the dip reaches the guilty frequency and the feedback is reduced.}
Together, MID and SWEEP controls can be used to accomplish a variety of tasks from combatting feedback to improving the way things sound through the PA or on recording. Here are some of those tasks & settings:
Generally speaking, you will probably end up with the MID in cut mode for most problem-solving uses of the SWEEP control. In any case you will learn to use this feature judiciously. The best PA EQ setting is the one with the LEAST adjustment, but when you need to solve a problem it's good to know how to use the tools.
"EFX" Unlike the MON/AUX send controls, the effects send controls are always post-EQ and post-fader, i.e. they are affected by both the channel equalization and the channel fader (in PA vernacular they are "post-post"). There may be more than one EFX send control and they may feed either an internal effects circuit (reverb) or a master Effects Send buss, check the owner's manual if there is more than one EFX control on each channel to see which one is which. In any case, the channel EFX signals are internally routed to their designated master effects summing busses where they are mixed together on their way to the EFX SEND jack or internal effect.
"MUTE" The Mute button is usually inserted just after the EQ section. We mention it at the end of the channel section simply because that is where the button most often appears - i.e. conveniently close to the channel fader and PFL/Cue button. As the name implies it silences the channel through the FOH system and possibly the monitors (check your manual ). Its prime function is to enable the user to pre-set a channel's level, EQ, Efx sends and Mon./Aux. sends then shut the channel off to be added to the music program later on. Muting is a convenient feature for infrequently-used channels such as harmonica mic, acoustic guitar, banjo, mandolin, certain wind and percussion instruments, pre-recorded music or sound effects, all of which should be left off when not in use to reduce unwanted sound pickup and the risk of feedback.
"L-R", "1-2", "3-4", etc. Multi-buss mixers often feature pushbuttons on the channels which direct the post-fader, post-pan channel output to selected "pairs" of Main mix busses. That way, you may employ the stereo submixing section of your choice and Pan between its two masters.
"MAIN" This master regulates the output of the MAIN, SUM or MONO buss where the outputs of the SUB or GROUP busses or Left & Right stereo master busses get mixed down into a single signal.
"SUB" or "GROUP" These masters regulate the SUBmix or GROUP output levels.
"MON"," AUX" or "EFX" These masters regulate the output level of their designated SEND or OUT jacks.
"RTN" These masters regulate the input levels of their designated RETURN jacks.
"EFX TO MAIN/EFX TO MON" Some mixers with effects busses feature controls which are actually effects return masters, but one sends the effects signal to the input of the main busses and the other to the input of the monitor buss. In either case, the effects signal gets mixed with the straight signals coming directly from the channels (yes, electrons actually travel fast enough for some of them to leave the mixer via the Efx Send jack, go through perhaps several cables and effects devices, come back in through the Efx Return jack and still arrive inside the mixer at the input of the main or monitor buss circuits at the same time as internal signals direct from the channels - Believe It Or Not).
"PAN" These controls pan the Stereo Return signals between pairs of SUB or GROUP masters.
"TAPE" or "2-TRACK" This regulates the level of the Tape or 2-Track outputs.
"TALKBACK" This regulates the level of the Talkback output. See under "MIXER INPUTS" for more details.
"EQ" A graphic equalizer is featured on some mixers and a few have more than one, in which case one of the graphics will usually be for the monitor buss. As mentioned earlier, equalizers work by increasing or deceasing the signal strength (a.k.a. "gain") over various narrow bands of frequencies. As a result they are equally capable of curing or causing problems and should be treated with care.
The process of "sweetening" the FOH sytem's sound with low and high-frequency EQ boosts should be done with great care in live music applications. Keep them to a maximum of 3dB (preferably less). For DJ applications this is less of a stringent limitation, but boosts of more than 6dB should be avoided. And when in doubt, LEAVE THE FADERS AT CENTRE - there's no shame in a "flat" EQ; quite often it's the sign of a good system and a wise technician. For more information, see "EQUALIZATION" under Signal Processors.}
