Gain staging: Part 1
| Site: | QSC |
| Course: | House of Worship Audio Training |
| Book: | Gain staging: Part 1 |
| Printed by: | Guest user |
| Date: | Thursday, 21 November 2024, 4:03 AM |
Description
Lesson Description
Learn how to properly set gain structure throughout your sound system as well as how to establish a workflow to setup the output chain gain stages.
Video Transcript
00:12
Adjusting the gain structure or gain staging your PA correctly will dramatically improve its performance,
00:18
simplify operation and even help avoid damage to loudspeaker components.
00:22
In this video, we are going to show you the first part of this process,
00:25
which is how to set your mixer outputs, active loudspeakers and amplifiers for optimum performance
00:31
and ease of operation.
00:32
But before we get into that- What is gain structure?
00:36
Gain is the change in signal level that takes place between the input and output of an electronic device.
00:42
A gain stage is a point in a device where gain is applied or controlled.
00:47
Gain structuring is the art of properly setting the gain of all these stages relative to each other,
00:51
in order to keep your mix free of noise and distortion.
00:54
Mixers, amplifiers, active loudspeakers, even processors like compressors, EQs and effects all have gain stages.
01:01
There may even be multiple gain stages within a single device.
01:05
A mixer, for example, will have multiple gain stages
01:08
including the microphone preamps, channel faders and output faders.
01:12
When setting gain, one of the issues we want to avoid is clipping.
01:17
Clipping occurs when a signal overloads the down-stream electronics in the system.
01:21
It’s called clipping because the tops of the waveforms are literally clipped off
01:25
when a signal peak exceeds the capacity of the gain stage, which leads to distortion in the signal.
01:31
You might like this from a vintage, tube guitar amp.
01:33
But not so much from a mixer.
01:36
At the other extreme is noise.
01:38
Every device has a baseline level of noise.
01:41
We call this the Noise Floor of the device.
01:43
The range between the noise floor and the point at which a device starts clipping
01:47
is called the Dynamic Range.
01:49
The difference between the signal level and the noise level is called the signal-to-noise ratio.
01:54
If the level of your signal is too low, you will have a low Signal-to-Noise ratio
01:59
and unwanted noise is added to your signal.
02:02
With a high signal-to-noise ratio, the noise essentially disappears.
02:06
The noise floor for any specific component is fixed.
02:09
As you add pieces of gear to the system, the noise floor of each device compounds,
02:14
and the overall noise floor for the entire system increases.
02:18
Even if every stage has an excellent signal-to-noise ratio on its own,
02:22
when you chain multiple stages together, the noise of each of them is added together
02:26
and the overall signal-to-noise ratio of the system will decrease.
02:30
Which is bad.
02:31
So, the more devices you have in your system and the more inputs you have coming into the mixer,
02:35
the more important it is to set gain structure correctly.
02:38
We will talk about this more when we get further down the signal path.
02:41
The first step in setting your system gain structure is to match the input sensitivity
02:46
of your amplification or active loudspeakers to the output level of your mixer.
02:50
If you do this right, the output meters on the mixer will give you a good indication
02:54
of how much of your PA you are using.
02:57
So, mixer meters in the red will mean that your sound system is out of headroom
03:01
and doesn’t have any more to give you.
03:03
If you are using a TouchMix mixer with QSC active loudspeakers or GXD Series amps,
03:08
this is going to be easy.
03:09
Just follow these steps.
03:12
Select your output channel on the TouchMix.
03:14
Go to the Setup tab.
03:16
At the top of the screen, touch the button with the name of the loudspeaker or
03:19
amplifier series you’re going to set up.
03:22
A window will open that displays the optimum setting for the selected QSC active loudspeaker or amplifier.
03:28
Dial that setting into the loudspeaker or amplifier and you’re done.
03:32
But maybe you aren’t using QSC active loudspeakers or QSC amplifiers.
03:36
Or maybe you’d just like to understand a bit more about how and why.
03:41
Before we go any further, I want to clear up a common misconception.
03:45
There are those who believe that the Level, or Gain control on a power amplifier
03:48
or active loudspeaker adjusts power.
03:51
This is not true.
03:52
What these controls actually do is adjust how much voltage it takes at the input,
03:56
in order to drive the loudspeaker or amp to a given output level.
04:00
So your 1,000 watt amplifier can still produce 1,000 watts no matter where that knob is set.
04:05
It will just take more or less output from the mixer to get to 1,000 watts.
04:09
You also need to understand what the meters and indicators on your amps
04:12
and active loudspeakers are trying to tell you.
04:15
Does the PEAK LED mean the signal is clipping or does it mean that it's getting close to clipping?
04:21
Many active loudspeakers, including QSC models have LIMIT lights
04:25
which indicate that the limiter circuit has been engaged.
04:28
This is not the point of clipping and this is important to be aware of.
04:32
If you treat the limit light on an active loudspeaker like a clip indicator,
04:35
you will be operating your system below its potential.
04:38
If, on the other hand,
04:39
you are running your system so that the limit LED goes on with the first note of the set
04:43
and doesn’t go off until the break, you need to back off a bit.
04:46
Alright, with that out of the way, let’s start turning some knobs.
04:51
Depending on the gig and the venue, you may not have access to this stage.
04:54
If you do, here is how you’ll want to proceed with setting this up.
04:58
You can usually start with the gain knob for your amplifier or loudspeaker at the 12 o’clock position.
05:03
Play some recorded music through the mixer as a test signal.
05:06
You can do this with pre-recorded tracks on the mixer
05:09
or by connecting an external playback device to one of the mixer’s stereo inputs.
05:13
Now bring up the mixer output fader
05:15
until the peaks in the music are driving the mixer outputs well into the yellow.
05:20
Listen to the level and clarity coming from the system.
05:23
Check the indicators on your amps or active loudspeakers.
05:26
If you’re seeing Clip or Peak indication, back the gain control down.
05:30
If you’re not getting enough out of the system slowly bring the gain control up.
05:34
Your goal is to set the amplifier or active loudspeaker gain control so that the maximum,
05:39
undistorted output from the mixer will drive the amplifier or loudspeaker system
05:43
to its full, undistorted level.
05:46
There may be situations where the sound system is capable of producing more output than is needed.
05:51
If that’s the case, you can reduce the input gain of the amplifier or loudspeaker.
05:55
Repeat as needed for other mixer outputs, amplifiers or active loudspeakers.
05:59
If you are the owner of the PA, you’ll probably want to set aside some time to set this up properly on your own.
06:05
This can be time consuming and potentially very loud.
06:08
Making this a part of your pre-event workflow might not be the best idea.
06:12
Once done however, you can mark the gain positions or record them somewhere.
06:16
This way, you already know exactly where to set them when you get to the gig,
06:20
and you only need to worry about the mixer stage.
06:23
Now that you have calibrated the amps and active loudspeakers to your mixer,
06:26
you’re ready to start dialing in the gain-structure for all the channel inputs.
06:30
We’ll discuss this in Part 2.
06:33
See you later.