One of the most important things you must get right In a live Sound system environment are the mixing desks input channel control adjustments.
The “Gain Control”. With out thi
s being set up correctly, no matter how good the rest of the system is, it wont sound its best and at worst this error could damage some of the parts of the system, in particular the high frequency drivers. Distortion of the signal particularly clipping causes massive amounts of high frequency energy far beyond what the HF drivers should have to tolerate.
Use of mixing desk Gain Control
This control is the most important control on a mixing desk, if it is set correctly everything else will be ok, if it is incorrectly set the result can range from increased noise to sever distortion. It is important to understand the function of this control, what it does is control how much amplification is applied to the incoming signal in the first stag of the electronics that the signal is subjected to when it enters the desk, this usually ranges from a value of one to hundreds of times .The point of the control is to make the resulting output of the first stage as big as it can possibly be without being so large as to exceed the point when the first stage output signal is clipped (top of the signal is squared off) thus keeping the signal to noise ratio as high as possible without any clipping occurring. It should be born in mined that if you change the eq of the channel , in particular increase the gain in any band the input gain will have to be readjusted to reach the optimal operating level in the channel.
The method to do this adjustment that I use is: –
Set all the tone controls level controls to 0 dB so the tone controls are not boosting or cutting the signal at all.
Set all the aux send controls to – infinity i.e. send set off.
Set the Hi pass filter on if the channel is being used for a vocal or other non bass instrument, if the signal is a bass guitar or bass drum or music play back leave the Hi pass filter off.
Set the channel fader to – infinity i.e. set off.
If the source requires phantom power i.e. an active Di. box or a condenser mike turn the channel phantom power on.
Turn the pad switch off, so it is not attenuating the signal
Rout signal to main output.
Apply the maximum input signal to the channel. If the source is a CD, tape or other recorded signal ensure the loudest passage is used. If the source is a singer guitarist, or performer, ensure they are providing a representative signal of there loudest output.
Engage the channel’s Pre Fade Listen (PFL) switch so that the level of the incoming signal is displayed on the meter and can be listened to on the monitor headphones.
Adjust the Gain control so the signal causes the meter to display a level that just reaches +3 dB during the loudest passage.
If the level displayed is too large even when the gain control is turned fully anticlockwise engage the pad switch to attenuate the incoming signal, then adjust the gain control as above.
If the level displayed is still too large even when the gain control is turned fully anticlockwise and the pad switch is engaged, the incoming signal may be connected to an input that is too sensitive i.e. a line level signal is connected to a mike input! Re connect the incoming signal to a line level input and re adjust the gain control
Check that the performer really was at the loudest they will be when they come to perform later with the Excitement And Adrenalin of performance. If they were not re adjust the gain control correctly.
Now the gain control is set up for the best performance it can be. Now you can go on to set up the channel tone control.
Note. There are occasions when the gain might be set to a lower level so the channel fader can be used around 0 dB at a normal operating level this is to enable the operator to use more of the resolution of the faders travel. However this is done at the expense of the noise performance of the preamp.
