Why Are Knobs on Power Amps Turned all the Way Up?

[boomerweps]

The knobs are to set up your gain structure. They can be used for other things, too. (matching sides for stereo? You CAN, but isn't that what the stereo sliders are for on the mixer. You can use the crossover/processor levels to do the same.) SOME amps, like Peavey, have protection/limiting circuits that don't operate well without the amp gains set to full.

 

Most often I run my amp full on the gains but in smaller venues I notch them back 9-12dB so I can get the proper gain on the mixer and other front end gear for cleeanest sound. Just a swag version of setting gain structure.

 

Boomerweps

[twostone]

 

Let me ask this: Do you keep a stack of cards under the gas pedal of your automobile, and then add or subtract cards from the stack based on how fast you think you'll need to go somewhere?

 

 

I suppose this was directed at me. No cards on the gas pedal lol.

This method was shown to me by a guy that claims to be a experienced Sound person let me if it's the wrong way of setting up because this guys has mixed lot of our local bands and has been highly recommended to me by the same bands that he mixes.

1. Mixer all channel strip eq's set at unity master fader set at unity and channel strip fader set at unity.

2 Rack gear all eq's and x-over set at unity gain level this includes monitor eq's

3 Amp rack amps set at 0 (I explain more later)

4 Setting the gain with the PFL and making sure that you have the hottest signal without clipping on each individual channel strip

5. Amp attenuator slowly is raised until the desire volume has fill the room with my QSC PLX usually that 2/3Rd's on the attenuator knob.

 

I used to run my amp wide open but this guy told with a amp wide open your raising the floor noise up and most modern mixer can dish out more signal then the amp needs to use.

If this procedure is wrong please inform me on the correct method of gain structure. Because I'm always open minded to better techniques.

[dboomer]

There's a lot of confusing info here.

 

First using Mark's analogy ... Do you keep a stack of cards under the gas pedal of your automobile, and then add or subtract cards from the stack based on how fast you think you'll need to go somewhere?

 

Bad analogy because maybe you do if you consider how you set up the linkage to the throttle. What you are actually doing by turning down the attenuators is preventing that linkage from pushing 6 inches past full throttle (or maybe 2 feet!).

 

Mixers are capable of overdriving the requirements of power amps by 15 dB - 25 dB while remaining clean and free from clipping. So if you drive your amp to full and you still have 15 dB left over that 15 dB is called headroom ... USELESS HEADROOM. That's not what you are after ... we're after system headroom (the same amount of headroom in every piece) but since there's NO headroom left in the power amp what you've really ended up doing is wasting 15 dB of SN ratio.

 

I can state the categorically (well almost always) your optimized SN ratio (gain structure) will be achieved by turning down the amp by 10-26 dB. As Kevinnem stated you could end up with a surprise if someone screws with the knob and turns it up. Adjustable amps (Crowns, Crests) should be set for 26 dB gain (aka X20).

 

Someone stated that leaving all the ups up sets them the same. It does not unless they are all the same exact model or are set to fixed gain. That pretty much means it does happen for Peavey amps and it does NOT happen for most QSC amps

[Not Serial]

don i have an honest question about that;

 

my current small rig has the amps all the way up and the crossover inputs and outputs at "0". mixer is a mixwiz and it clips the amps at a solid +9 on the master meters. (a brief +9 dosn't clip the amps). the mixwiz clips itself above +9

 

not that i run that way; i run at +3 on the master meters.

 

if the crossover and amps are set at "0" and clip with a mixer output of +9, it seems like i cant overdrive the amps at all this way without really hitting the board to hard in the first place. not that i am disagreeing with you, but am i missing something?

[dboomer]

OK ... let's talk specifics. What model amps? What crossover? Is everything wired balanced? Which Mixwiz?

[Not Serial]

(2) crown xs900

dbx 223xl xover

mixwiz3

 

everything is balanced; signal chain is mixer - snake - crossover - amps

 

mixer has eqs on the main inserts.

 

not that i mind the way it runs now, and i could turn down the att's on the amps by 6db so i could run the mixwiz at +9 instead of +3 but that does not seem like a smart idea.

 

i'm beginning to wonder if the clip point would change if i bypased the crossover, also i wonder if i am may be mixing up a dbu with a dbv somewhere

[dboomer]

Oops ... forgot. Speaker models and impedances?

[Steve Edwards]

On most large frame mixers the main faders go to unity-zero and no higher. I always suggest setting the mains at unity and mixing channels "up" to the main faders and using PFL to set the trims. When I see a tech set the faders at unity and mix with the attenuators I head for the door. There are different ideas about setting amp levels at 50% and issues of lose of headroom. When doing clubs I set them at 100% because if anybody messes with anything (and they do!) they can only turn it down.

[Not Serial]

(2) peavey 118sub 8 ohm 18" BW (old style - 250watts rms)

(2) yamaha club IV 8 ohm but with delta 15LFA instead of stock 15"

[congaron]

My only question on topics like this is:

 

Is there any reason NOT to follow the manufacturers instructions on each piece of equipment you have? I have done this from day one, setting my mixer gain as directed, setting my eq gain as directed and setting my amp "knobs" (call them whatever you want) as directed in the respective manuals. They do not conflict in their recommendations BTW. I end up with no hiss, no clipping, lots of headroom, and clean sound. I also run "junk" according to the party line. Nobody who hears my system thinks so. Many compliments, even from other sound guys. Some even ask for tips. Granted these are mere amateurs like me.

 

Is there any good reason not to do this according to the manuals?

[dboomer]

If you end up with no problems ... then no there is no need to worry ... BUT ...

 

You do not have "no hiss". What you have is a condition that is not bothering you (not the same thing). You may be able to have additional headroom

 

Manufacturer's instructions will slant things to favor their gear ... not necessarily your system. Maybe you could do better, maybe you don't need it.

[stevie j]

Yes, the manufacturers err on the side of caution and are probably aimed at DJ's.

 

You can run any modern amp full open, the red lights are just indicating you are approaching clipping and if you just hit them briefly on every snare hit etc. you will do no harm as long as you are not clipping the board and your speakers are matched to the amp.

 

This question pops up every month or so and it was Agedhorse who corrected me when I asked it about 6 months ago.

 

It should be made a sticky.

[lifeloverwg]

 

There's a lot of confusing info here.

 

 

There sure is!

 

While I understand the rationale for running amps wide open as a safety measure, as dboomer has stated, it isn't the best approach if you want to keep the S/N ratio down.

 

I prefer to lower the noise level as I hate speaker hiss, so I'm usually operating with amps attenuated.

 

Winston

[dboomer]

Coaster .. here's what I'd do with your system.

 

Unplug the speakers, turn the power amps all the way down. Turn on all the gear.

 

Run the tone generator with pink noise and turn up the mixer 'til you just fire the +16 LED. Adjust the input control on the crossover til you just fire the clip LED. Turn up the amps til you fire the limiting LED on the Low amps. Then the High amps.

 

I'll bet you'll end up with the amp knobs around 10 o'clock.

 

I'll also guess that you'll have a ton of Highs when you plug the speakers in. Back down the outputs on the crossovers till it balances to your ear. I'd also suggest that you use one amp for highs and lows for one side and the other for highs and lows for the other as opposed to a Low amp and a High amp.

 

The above will give you a "good balance" between optimum SN ratio and headroom while still leaving you some fudge factor. That fudge factor could be traded later for even better SN ratio but it probably won't make much real difference as it does in congaron's system (post above)

 

Now you may need to rethink your mixing style (maybe not). Think of the top red mixer LED as flat out max. What ever you choose to run below that is essentially your headroom factor. If you are running at around zero then you have 16 dB of headroom (there's actually more ... but just put that away and forget about it. Now the big question. Do you still have adequate output? I'll guess not (but how could I know from here;)). I'd guess the system will still sound very good running with about 10 dB of headroom considering the setup. If you don't have enough at this point you'll have to settle for less quality and headroom for more volume.

 

Oh yeah ... don't clip your mixer inputs or outputs ... PERIOD!

[twostone]

So Dboomer your saying the sound guy that was highly recommended to me gave me bad advice and I need to set my QSC PLX attenuator wide open. But the pros and cons is that wide open gives you the headroom but then you raise the floor noise up. I can PM you for the exact signal chain path because I have a mixture of passive cabs and active cabs.

[Not Serial]

thanks don - i'll try that during down time in september.

[congaron]

Attenuators at or around 10 o'clock is very familiar territory for my system. I guess I am doing it basically the same way, except for using a cd instead of the tone generator. I do have to fiddle with the trim a little at first to get ocassional...actually rare...clip lights on each channel when I go to actual sound check. Then I leave the master fader at unity while I mix the band with channel faders, on headset. Then I turn up the eq to unity gain, watching the meters on it. It doesn't sound any different to me until it is nearly clipping, so I just leave it there at unity usually. The amps always end up between 9:30 and 11:30....so far with the size rooms we play. The mains are getting up to 400 watts each. The subs 600 each. I never see a clip light on any of the amps run this way. They sit right next to me with all the rest of my stuff as I play. Of course it would be extremely rare for me to turn my master fader up during a performance. Likewise, our vocalists use mic technique so I don't have to ride vocal faders and lead guitar is raised via volume pedal during solos. I do the monitor mix (only one) basically the same way. Amp settings end up very similar. Thanks for the response Don.

[Dogoth]

A power amp has a set amount of gain. As has been stated, the volume control an a power amp is just to attenuate the front end of the amp to match the output of your mixer. If you find your amps clipping (or better - limiting (always use output limiters when available)) when your mixer output meters show -20db then you should probably turn down the amp by a good 12 to 16db. You always want your amps output limiters (or your systems X-over limiters) to work well before your board overloads (otherwise you've just defeated their purpose). I understand the idea of not allowing anyone to inadvertantly or maliciously turn you up (never a pleasent suprise) and if you're not getting too much excess noise then it seems like a reasonable safety tradeoff. I don't personaly usualy "dime" (or would that be .11) my amps (but I'm not in a situation where anyone would do such a thing), but if I see it done, unless there's a problem or a gross gain staging mismatch, I don't really have a problem with it. Technicly, No, it's not ideal but live sound often is about tradeoffs.

[dboomer]

But the pros and cons is that wide open gives you the headroom but then you raise the floor noise up.

 

Wide open does not give you "system" headroom and it does raise the noise floor.

 

On the other side it gives you more limiting headroom in your amp (which you ideally should be not using because you are doing other things correctly). It also allows you to be a jerk with your mixer and get away with it. Better to actually learn how to mix within the limitations of your gear.

 

There is not one way to do this ... there are tradeoffs. I'm just trying to teach what the tradeoffs actually are and then let each user decide for themselves how to give and take. It looks like everyone is a lot more worried than they need to be. The only real penalty to doing this wrong is to add hiss to the system that doesn't need to be there. If it doesn't bother you don't worry. OTOH there's nothing wrong with doing it (more) correctly either:thu:

[Craig Vecchione]

 

My only question on topics like this is: Is there any reason NOT to follow the manufacturers instructions on each piece of equipment you have? I have done this from day one, setting my mixer gain as directed, setting my eq gain as directed and setting my amp "knobs" (call them whatever you want) as directed in the respective manuals. They do not conflict in their recommendations BTW. I end up with no hiss, no clipping, lots of headroom, and clean sound. I also run "junk" according to the party line. Nobody who hears my system thinks so. Many compliments, even from other sound guys. Some even ask for tips. Granted these are mere amateurs like me. Is there any good reason not to do this according to the manuals?

 

 

Sure is. And this'll get a smile from Don, and Andy.

 

The dbx Driverack PA has a neat little back page discussion about setting up a system. It advocates setting up the entire system so everything clips at the same time. Andy and I don't agree with this, Don does. Andy and my argument is that if everything clips simultaneously, you have no gain left for the limiters to work in. Don's argument is that it maximizes s/n. I counter that with most systems, this isn't a problem. I'd rather have the limters begin to work and have more clean gain left in the system than have the system go into ragged clipping all at once.

 

Regardless of which way you lean, this is an excellent example of manufacturers publishing a recommendation that's based on a subjective topic that's open to debate and interpretation, and should not be taken as 'written in stone'.

[dboomer]

 

The dbx Driverack PA has a neat little back page discussion about setting up a system. It advocates setting up the entire system so everything clips at the same time. Andy and I don't agree with this, Don does. Andy and my argument is that if everything clips simultaneously, you have no gain left for the limiters to work in.

 

 

In your example you have to be totally mismanaging the mixer to consider this a factor. I say ... don't be an idiot, learn how to run the mixer!

 

You guys always miss the point that clip limiters in amps are NOT tied to an input threshold. The limiters always work the same way ... that is they trigger when the rails are maxed.

 

You also seem to forget that you should be using the limiters in the DSP and not the ones in the amps (because they'll be taking care of themselves and prevent clipping of the amp (regardless of where that coincides to your speakers)

 

The only real world issue about turning down amps is ... if you turned one down so far as to clip the input electronics stage that precedes the attenuator.

 

While I said earlier that background noise is the issue ... in this case it is complicated by the DSP. Digital gear has a different kind of noise that to my ears is a much bigger problem than hiss AND if you don't run digital gear up hot it flat doesn't work as well and sound as good as it could.

[Not Serial]

these discussions remind me of the 74 page threads on how to set your distributor advance curve in a muscle car v8, except less heated. ive been there and probably will again - i LOVE old cars/trucks.

 

sometimes people just like to talk about these subjects without any real negative agenda. it is a fine detail in a complex system much like the advance curve in a muscle car that gets picked up a lot.

 

many ideas together help the understanding of things. thats my 2 cents.

[Big Al]

This is getting a bit above my head, but I'm learning a lot.

[twostone]

 

This is getting a bit above my head, but I'm learning a lot.

 

 

Good news you did come to right place if you think these guys talk to technical try ProSound they really go over your head.

[congaron]

So does your brain clip at the same time your facial expression does?