A few basic mixer questions

[dancrocker]

I have been drafted to do sound for my church when I'm not playing. Ive done a bunch of reading but am confused on a few points. Bear with my basic questions.

1. Our mixing board has a PFL button that we use to set gain. I have seen other instructions that say to set the channel fader to 0 and set the gain by watching the clip led. Are both of these approaches equivalent? Or is one better than the other.
Related to this, I would have thought that the gain setting was not affected by the fader but this appears not to be the case (otherwise this latter approach wouldn't work). Am I confused? Does this latter approach not require a particular fader setting? If not, then why is the 0 setting suggested?
2. If I set the gain of two channels such that their levels are the same (not the same gain setting), I would have thought that this would mean that identical fader settings would produce identical output volumes in the mix. But via some experimentation this doesn't seem to be the case. Did I do something wrong or are my expectations just incorrect?
3. The way I was taught to use the mixer (by the music director) was to control the final output volume with the master faders. However, instructions I've read say to set the master fader to 0 and then adjust the volume at the power amp. I don't understand why this is the case.
As it turns out, we usually end up with the master faders at about -15db. If I want them at 0, I guess I'll need to turn down the power amp. The funny thing is that no one showed me where this control is and I've never seen anyone who's done sound actually adjust this

Thanks in advance for the help.

 

[tysonh]

I am confused as to why you've been assigned in depth task with specific instructions yet no one around is qualified to show you how to do this.

anyway, it sounds like you already have an adequate basic knowledge of what to do. Set your gain, then eq. Set up the vocal mics all the same (assuming they are all idential mics) gain, then eq. Its not that hard, just get something going adjust some at sound check then during performances.

Maybe you were taught to set amp volume with faders at zero for headroom ? it sounds to me like that is the case.

The question on different gain settings, same fader ?? I dont really understand what you mean? IF, you had identical inputs (instruments, vocals)
its unlikely they would be the same volume as singers sing different, people play their instruments differently.
I hope it helped ?
Good luck, its not that complex unless this is your career. Experience is the greatest teacher.

 

[projectMalamute]

1. With the fader at 0 dB you should have the same level pre fader that you do post fader. That's what 0 dB means. Whether the two approaches are equivalent depends on where you are setting things using the PFL and where the clip light is calibrated to go on. I set input levels with the PFL engaged and a set of headphones handy.

2. Your expectations are incorrect. Level on a meter and perceived volume are two totally different things. Say you set the micamps such that a banjo and a flute are peaking at the same levels. If you put the faders in the same places you should still see the same levels at the bus but the banjo will be perceived as much louder than the flute. Can't mix with your eyes.

3. If you consistently have the master fader at -15 dB you could maybe stand to turn the amps down some. This could give you finer control at the fader and a better S/N ratio. Still, mix at the board. Making volume adjustments during the show on the power amps' input attenuators is a bad idea, and impossible once you get beyond a non-trivial system.

 

[M0ses]

It doesn't matter much where you turn up the volume as long as things aren't clipping badly, IME. Which isn't all that extensive (my experience, I mean). Also, your channels won't be equal at gain 0 because the instruments have different outputs. Microphones are obviously going to have different output levels than a bass or drums etc. That's why each channel has it's own fader.

 

[Zooberwerx]

*Yes, I set the channel and master faders at 0 and adjust individual channel gains with the PFL engaged. You can actually leave your power amps off and use headphones during the process.

*I establish headroom and limiter levels using a more complex procedure detailed in the dbx DriveRack manual. Once you've adjusted the power amp attenuators, you leave them alone. Volume is regulated by the channel and master faders.

*Nobody showed me how to run this stupid thing, either. I was the only one motivated enough to read the manuals, do the research, and ask questions....much as you're doing now.

Riis

 

[dancrocker]

Thanks for all the replies. Some followup comments and questions:

1. I understand that a fader setting of 0 means that the output is the same level as the input. But, it seems to me that the gain setting is not a function of the output level but only of the input. Will the clip led light at lower input levels if the fader is set higher? If not, then is seems like I could set my gain with the fader all the way off, assuming my goal is to set then gain just below the clipping level and I'm going to use the channel fader to adjust the final output volume of the channel.

2. On our board (soundcraft epx), the master fader's 0db setting is all the way at the top ofnthe fader travel. So it seems to me that I'd want to target a nominal setting of around -10 so that I have headroom in case I need to go louder. It is puzzling why the channel faders have a +10 setting but the master does not.

3. My comment about equal volume from equal fader settings was related to vocals, not different instruments; sorry I should have been more specific. It seems like the higher the gain setting, the lower the fader setting to get the same volume. I haven't done a controlled experiment but things seem to end up this way. It still doesn't seem like it should work out this way. Comments?

 

[projectMalamute]

1. The clip LED should be pre-fader. There will be a system block diagram in the manual, Soundcraft is pretty good about providing documentation on line if you don't have a physical copy.

You can absolutely set the input gain with the fader down. I always do, using the pre-fader-listen to meter and sometimes get a rough EQ with headphones.

2. The channel strips have a 10dB gain stage after the fader. So wide open is actually 10 dB hotter than the pre-fader level and the 0 dB point is marked where the fader is actually attenuating the signal 10 dB. The summing amp for the bus is unity gain so wide open at the master fader is actually 0 dB.

3. I don't get the question. Yes, if you turn up the gain at the mic pre you will need to pull the fader back to get the same volume. Am I missing something?

 

[Stanley Design]

Yeah you have the right idea.

Mixer on. Channel input gains all the way down. Output volumes all the way down. Master output at 0db or optimum location. All mutes engaged. Power amp running 3/4 wide open.

Setting channel one:

Unmute the channel and test your singing/instrument at maximum expected volume (no "check 123" when the singer will be belting his lungs) through the channel and increase your input gain slowly until clipping, then back off slightly. Increase your output slider until desired house volume is achieved. Re-mute the channel and repeat for each channel. (Tip: masking tape on the top of the channel numbers and label them for what they are)

Do your EQ and final adjustments after this.

That's how I do it anyway. After years you'll be able to EQ and level pretty much right away, and wont need a clip light because you can hear it more accurately, etc.

 

[dancrocker]

2. The channel strips have a 10dB gain stage after the fader. So wide open is actually 10 dB hotter than the pre-fader level and the 0 dB point is marked where the fader is actually attenuating the signal 10 dB. The summing amp for the bus is unity gain so wide open at the master fader is actually 0 dB.
3. I don't get the question. Yes, if you turn up the gain at the mic pre you will need to pull the fader back to get the same volume. Am I missing something?

#2 I don't quite follow this. I guess I can see that the channel fader has a 10db gain since it's marked 10db. But, wouldn't a setting of zero on the fader be unity gain? It seems like you're saying it would be attenuating the input, but maybe I misunderstood.
Also it seems like I still want to target something less than zero for the master fader so that I don't need to mess with then power amp setting if I need the mix to be louder.



#3 What I mean is that I set the gain of two channels such that they are both just shy of clipping. Because the singers sing at different levels, the gain settings end up being different. But, it seems to me that once the gains are set, the levels of the signals going into the channels should be roughly the same and the same fader setting should produce very similar output volume. Is it possible that two people's voices are different enough to affect the perceived loudness in the mix?

 

[dancrocker]

Stanley,
Thanks. This is basically what I've been doing except I've been using the PFL to set the gain. And, as I said, it seems like the power amp is set a little high.
One more question...
I use the vu meter to set the gain with PFL. What can I use it for under normal operation? Right now it does not jump up too high, I think because the master fader is set so low. If I get it into a more "normal" range, what info can it give me?

 

[WayneP]

1. Our mixing board has a PFL button that we use to set gain. I have seen other instructions that say to set the channel fader to 0 and set the gain by watching the clip led. Are both of these approaches equivalent? Or is one better than the other.

The purpose of the gain control is to achieve the best signal-to-noise ratio for that channel. I assume your mixer activates an LED meter bridge when you activate the PFL button? If so, the meter is showing you the level of the signal. The idea is to get a signal from the stage for that channel that’s as hot as you’d ever expect to get from that instrument or vocalist, and adjust the gain until it’s a bit below clipping.

This has nothing to do with the channel fader, however. The fader is for adjusting that channel’s appropriate place in the mix (as I’m sure you know), but the proper input gain for that channel should be set first. During the gain-setting exercise, the only reason to adjust the fader would to make sure you’re not blowing out the speakers or something.

Related to this, I would have thought that the gain setting was not affected by the fader but this appears not to be the case (otherwise this latter approach wouldn't work). Am I confused? Does this latter approach not require a particular fader setting? If not, then why is the 0 setting suggested?

No, the fader setting has no effect on the gain setting. It does not especially need to be set at zero, during gain-setting or mixing.

2. If I set the gain of two channels such that their levels are the same (not the same gain setting), I would have thought that this would mean that identical fader settings would produce identical output volumes in the mix. But via some experimentation this doesn't seem to be the case. Did I do something wrong or are my expectations just incorrect?

Perceived volume often is related to the frequency content of the signal. For instance, a guitar using a distortion effect might have a lot of energy in the 2-4 kHz range, where the ear is particularly sensitive. It might seem to be louder than a something like a keyboard using a mellow Fender Rhodes sound.

Also, a signal with a lot of bass content will probably push the PFL meter harder than a signal with little bass content. Yet, when you hear those two in the room the “bassier” instrument will most likely sound like it’s the quieter of the two.

3. The way I was taught to use the mixer (by the music director) was to control the final output volume with the master faders. However, instructions I've read say to set the master fader to 0 and then adjust the volume at the power amp. I don't understand why this is the case.

That’s only for a full-system gain structure adjustment. That’s not the norm for regular mixing.

I use the vu meter to set the gain with PFL. What can I use it for under normal operation? Right now it does not jump up too high, I think because the master fader is set so low. If I get it into a more "normal" range, what info can it give me?

During normal operation the meter is showing the mixer’s output. If the mixer is showing a low meter reading during normal system use and you want it to read higher, you’d turn the gain controls of the amplifiers to a lower setting (or perhaps the speaker processor, if there is one).

Regards,
Wayne A. Pflughaupt




Pedulla Club #45

 

[projectMalamute]

#2 I don't quite follow this. I guess I can see that the channel fader has a 10db gain since it's marked 10db. But, wouldn't a setting of zero on the fader be unity gain? It seems like you're saying it would be attenuating the input, but maybe I misunderstood.
Also it seems like I still want to target something less than zero for the master fader so that I don't need to mess with then power amp setting if I need the mix to be louder.

A fader is a passive device. It has no gain, all it does is attenuate. After the fader is a line amplifier supplying 10 dB of gain. So if you push the fader all the way up the signal is 10dB hotter than what you are seeing pre-fader. The faders are labeled to reflect this.

I've attached an example block diagram, the triangles are amplifiers. Your Soundcraft is probably laid out pretty close to this.

 

[dancrocker]

A fader is a passive device. It has no gain, all it does is attenuate. After the fader is a line amplifier supplying 10 dB of gain. So if you push the fader all the way up the signal is 10dB hotter than what you are seeing pre-fader. The faders are labeled to reflect this.

I've attached an example block diagram, the triangles are amplifiers. Your Soundcraft is probably laid out pretty close to this.

Thanks for the details. Being an engineer, I like to know what's really going on. However, for practical purposes, it seems like one could still think of the fader as supplying gain even though it's a fader and an amp.

 

[dancrocker]

Wayne, thanks for your response. I wish I could quote it so it was clear what I'm referring to but the browser on my iPad doesn't seem to behave with this forum and it's the only device I have on hand right now.