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Gain structure, EQ, and fader positions
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alanchu
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Joined: 16/02/2014 23:40:56
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Hello,

This is probably an old topic for many people, so I apologize in advance for trying to beat a dead horse again. I am having problem finding something comprehensive that would help me to understand the whole story regarding to the relationship between gain structure, EQ, fader position, and gain setting on the power speaker.

Here are some background. I helped out at a local church with the sound mixing. I only had about two years of experiences. Everything I know about sound mixing, I learned either from people from church, youtube video, and Internet articles. There are lots of confusing and conflicting thoughts on proper mixing methods. I would still consider myself a newbie since I am having problems sorting out the facts and errors.

My church uses a Presonus 16.4.2. The speakers we used are the QSC K12 and the K series subwoofer.

My question is regarding to the relationship between gain structure, EQ and the fader positions, and the gain knobs position on the QSC K12 power speaker and K series subwoofer, and their effect to SNR. For this discussion SNR is referenced at the output of the powered speakers.

We follow the youtube video example from Presonus on how to set the gain knobs on the mixer. At the very end to this video, the fader on the selected channel was moved to unity. I understand that there are difference of thoughts/approach regarding to the fader position at this point of set up, but lets leave the fader position discussion alone for the time being. After setting up the gain structure we move to setting the EQ. Right now the worship team is insisting using a pure boosting EQ method instead of the cut before boost technique. For vocal and instruments, there are 3 boosting lobes each with a magnitude of + 8 dB. The effect of this on the fader is that the fader position is anywhere from -5 dB to -10 dB down from unity (not counting the drum, drum is more than 10 dB below unity). The main fader is about 10 dB below unity.

Here is my first question. Without regard to the correct fader position at the end of the gain structure setting for the time being, when setting up EQ, should we adopt an EQ approach that has a minimum impact to the fader position? If a pure boosting EQ method is used with heavy boosting (+8 dB at 3 frequencies), this would result in lowering the fader knob to maintain the same volume. Does the lowing of faders do anything to the SNR when the SNR is referenced at the output of power speakers?

A second related question, are EQ and mixing two distinct phases/activities or is EQ is part of "mixing" phase? In my mind, I think of EQ in terms of getting the the kind of sound the worship team wants and mixing is the correct relative loudness between each vocal and instruments channels, and the master channel fader is used to bring the overall sound to the appropriate level.

My last question is related to the gain knob on the QSC K12 power speaker. Right now the main channel fader is at about -10 dB position and the output LED is between the highest green and maybe 2 notches into yellow. The speakers gain knobs are at the 3 o'clock position (probably 7 to 8 dB range. maximum gain is 10 dB). The speaker is putting out the right amount of sound. My understanding of this gain knob is that it is a power speaker input pre-amp; it brings the input signals from the mixer to the correct operating range so that the speaker amplifier and work optimally. The worship team seems to think that this gain knob sets the speaker output amplifier gain.

Here is my last question. If I adopt a cut before boosting EQ technique so that the end result filter curves on the VSL software looks similar in shape with the one from the pure boosting EQ method but at a lower reference point (shifted down), I could potentially move the channel faders and the main channel fader closer to unity, and at the same time I can reduce the amount of gain on the power speakers so that the overall volume stay pretty much the same. Would this technique result a better SNR at the output of speakers?

Thanks in advance and thanks for your patience.

Al





mwright137
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Joined: 28/10/2011 17:57:00
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You are not going to get consensus on a question like this. Here's my take on it:

First, I set the gain on all inputs so that I'm lighting about half of the signal LEDs for that channel. I'm almost always recording and I want to avoid digital clipping at all costs. That is my "unity" level. From that point on, my signal is at unity gain - by definition - if the signal leaving a part of the path is the same level as the signal coming in. That means that your 3 boosts of 8dB cannot possibly be following unity gain principles. This is the reason a compressor has a makeup gain. A compressor will lower the output level, so you need some gain to get back to your goal - input level = output level. In a DAW plugin, you'll notice makeup gain on EQ as well. This is because EQ changes will make your output higher or lower than your input.

Second, I load a preset that gets me closest to where I want to be. Sometimes I can't find one and have to start from scratch but it's rare. Some have boosts. Some have cuts. I personally don't like large boosts. I'll put a boost in a signal only until I find a frequency I don't like. Then I'll change the boost to a cut to get rid of the bad. I think of it like sculpture - remove the bad parts and leave the good behind.

Last, I start with the fader at 0. This is unity. This means you are neither cutting nor boosting the signal with the fader. For each channel I add, I'll take it below unity (usually) to get the proper mix. So you maintain unity until the fader. Then the fader mixes the individual tracks into one stereo signal. Then you follow proper gain staging for that signal. If it goes into a compressor, the input and output levels should match. Follow this right up to the power amplifier. It's the amp's job to take your line level signal and make it strong enough to move the voice coils in your speakers.

I might have rambled a bit but it's been a long day. As I said, your sure to get 8 different answers from 8 different people. Speaking of 8 - three 8dB boosts just doesn't sit well with me. But if it works for what you're doing, then go for it...
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Presonoid
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Joined: 17/02/2011 01:10:03
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To add to that:

SNR isn't your main worry with digital boards. That position is for clipping. So the best advice I can give to you is to use the SL meters. Yes, half way up the input meters is a good input level for the 16.4.2. Touching, but not constantly in the yellow is a good output level. Once inside the mixer the SNR is more or less fixed, unless you use EQ and dynamics to boost the noise frequencies. Fader positions have no effect. The channel gain settings do. Avoid having to turn the trims fully clockwise for a good input meter level. If you have to then provide a better input level in the first place (mic positioning, different mic, different DI box, ...).

Level knobs on amps are in most cases input attenuators on fixed gain amplifiers. No different with the QSCs. So SNR will benefit from lower settings simply because that needs a higer input signal level, so cable noise will play a smaller role. For the SL this means that most of us use it with the output trim (on the back of the mixer) set to max for highest level to the amps, and the amp level set to whatever is needed for the venue. The only possible drawback of that strategy is the possibility of talkover in low quality snakes. Rarely heard of, but should it happen then trim the SL output level 'to taste'. Oh, and watch the clipping lights on your speakers. You never want to see those light up!

As for the subs I assume that you feed those straight from the tops. They are designed to work well that way. When you do a search for 'aux fed subs' you'll find alternatives for that setup which may help you get rid of LF noise if that's a problem in your church. However it will complicate operation a little.
lowdbrent
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Joined: 13/11/2010 22:17:51
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When you have solo'd your channels and set the input gain and want to make EQ adjustments, you must adjust your gain when adding or decreasing gain with EQ. So, you can't get the channel set for -15 on the meters and then boost 60Hz 10dB with a shelf without increasing gain.

I do not subscribe to having all faders at "0" all of the time. You need to have gain for monitors, effects, etc. If you are mixing with gain controls for front of house, you will be screwing people on stage.
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mwright137
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lowdbrent wrote:I do not subscribe to having all faders at "0" all of the time. You need to have gain for monitors, effects, etc. If you are mixing with gain controls for front of house, you will be screwing people on stage.


Not sure if you were responding to me or the OP, but just to clarify: I start all my faders at "0" and then adjust the mix - usually by lowering some faders. I agree - the band would be pissed if I kept tweaking the channel gain. Not to mention the fact that the captured tracks would not be consistent when I go to mix them down in Studio One.

So "0" is my starting point, but very few channels remain there, if any.
I am Sir Melvis Bacon, Knight of BaconHam Palace.

MacBook Pro 13. OS X Mavericks - version 10.9.3
StudioLive 16.4.2; StudioLive 328AI (x2)
AudioBox 22 VSL; BlueTube DP V2; FaderPort; Monitor Station; FireStudio Project (x2)
Studio One 2 Pro 2.6.2 (64 bit)
I use Gobbler to back up my projects.
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alanchu
Prenoob

Joined: 16/02/2014 23:40:56
Messages: 7
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Thank you all for your useful comments.

Al
mrpicholas
Presonic
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Joined: 27/04/2011 03:06:14
Messages: 284
Location: California
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This is a really good book for dealing with all you're asking about and more:

Yamaha Guide to Sound Systems for Worship

This message was edited 1 time. Last update was at 28/04/2014 03:16:43

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