Mixers Knowledge

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DM Music > Knowledge Base > Sound / PA Knowledge > Mixing Desks

Mixing Desks

Why have balanced mic inputs?
Mixer channel configurations explained
How solo/pfl help when setting up a mixer
What’s the difference between a pre-fade & a post-fade auxiliary?
Instrument EQ Guide
How to reduce feedback

Why have balanced mic inputs?

Balanced inputs help to reduce RFI interference and hence deliver lower noise and better audio performance.
An XLR balanced input contains two signals (as opposed to the usual one), with the signals being out of phase with each other.
When these signals reach the mixer the ‘out of phase’ situation is resolved, cancelling out unwanted noise and giving an overall clearer and more professional sound.

Mixer channel configurations explained

The world of mixing consoles can be a minefield with manufacturers’ specifications causing confusion, particularly regarding the number of channels/inputs.
For example, some manufactures will count all the available mic, line and auxiliary inputs when quoting a mixers configuration eg 12:4.
Here at DM Music, we prefer to categorise mixers by the number of separate mic inputs.
If it has 12 mic inputs we call it a 12 channel mixer. - regardless of the number of extra line and auxiliary inputs.

How solo/pfl help when setting up a mixer

To get the best audio quality and the lowest possible noise levels you need to ‘match the levels’ of the incoming signals to the optimum operating level of the mixing console.
This task is best undertaken by using a combination of the input gain (usually the rotary control at the top of the channel strip), the SOLO switch and the PFL metering.
Basically a SOLO button isolates a given channel so that only that channel is heard through the headphones and only its level is shown on the meter. The gain should be adjusted so that the average signal peaks indicate somewhere in between 0dB and +3dB on the meter. Any more than this and you may run the risk of distortion, any lower than this and unwanted noise could be introduced.

What’s the difference between a pre-fade & a post-fade auxiliary?

A pre-fade auxiliary occurs in the signal path before the channel fader, ie, the level of aux is only affected by the aux pot and independent of the channel fader.
Often used for a monitor mix when aux levels need to be independent of the main mix.
A post-fade auxiliary occurs in the signal path after the main channel fader, with the aux level always in proportion to the channel fader.
Often used to send to an FX unit, eg reverb, where the amount of FX needs to follow the level of the channel fader.

Instrument EQ Guide

The table below is a guide to using the EQ on your mixer. Note every system and every building is different so the below is just a guide for reference.

Instrument	Boosting	Cutting	Comments
Human Voice	Hot at 8 or 12KHz Clarity above 3KHz Body at 200-400Hz	Scratchy at 2KHz Nasal at 1KHz Popping p’s below 80Hz	Tends toward thin when blending many voices
Piano	Presence at 5KHz Bass at 125Hz	Tinny at 1-2KHz Boomy at 320Hz	Not too much bass when mixing with rhythm section
Electric Guitar	Clarity at 3.2KHz Bass at 125Hz	Muddy below 80Hz
Acoustic Guitar	Sparkle above 5KHz Full at 125Hz	Tinny at 2-3.2KHz Boomy at 200Hz	Try to use a microphone with small diameter
Electric Bass	Growl at 620Hz Bass below 80Hz	Tinny at 1KHz Boomy at 125Hz	Sound varies greatly with the strings used
String Bass	Slap at 3.2-5KHz Bass below 125Hz	Hollow at 620Hz Boomy at 200Hz
Snare Drum	Crisp above 2Kz Full at 125Hz Deep at 80Hz	Annoying at 1KHz	Also try adjusting the tension of the snare wires
Bass Drum	Slap at 3.2-5KHz Bass at 80-125Hz	Floppy at 620Hz Boomy below 80Hz	Usually record with head off. Put blanket inside
Tom Toms	Slap at 3.2-5KHz Bass at 80-200Hz	Boomy at 320Hz	Tuning head tension is very important
Cymbals, bells & tambourines	Sparkle above 5KHz	Annoying at 1KHz	Record these at a conservative level
Horns & Strings	Hot at 8 or 12Khz Clarity above 2KHz Lush at 320-400Hz	Scratchy at 3.2KHz Honky at 1KHz Muddy below 125Hz

How to reduce feedback

The squeal, ringing or rumble caused by the sound from the loudspeakers reaching the microphones and travelling back through the system over an over at a resonant frequency. The frequency of any feedback is determined by the characteristics of the room and the equipment.

To reduce feedback:

1. Turn down the volume on the offending mic

2. Use as few mics as poss. Gain before feedback decreases 3dB each time the no. of mics doubles

3. Directional mics pick up less feedback

4. Place the mic close to the sound source for higher gain before feedback

5. Equalise the room with a 3rd octave graphic equaliser

6. Keep mics as far behind main speakers as possible

7. Place monitor speakers directly behind mics

8. Use directional mics. Hypercardioids and supercardiods are best at rejecting feedback. Omnidirectionals are best avoided live but are good for recording

9. Use same mics where poss. A cardioid mic provides x4.8dB more gain before feedback than an omni at same distance from sound source

10. Turn off mics not in use. Use the mixers’ channel on/off buttons.

DM Music > Knowledge Base > Sound / PA Knowledge > Mixing Desks

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