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Proper PA power?
- Thread starter girhen
- Start date
Bingo! Expansion, re-assignment, equipment upgrades, etc....you have to know what you're doing. It's not for the faint of heart or those who refuse to study the available material.
Regarding amp attenuators: they really do serve a pupose and not as volume knobs in PA / FOH applications. I've seen a couple of references where, irrespective of amp brand / model, attenuators are set wide open (no attenuation) and volume is controlled solely at the board. Fair enough...it works but plagued with problems. First off, the practice usually results in decreased signal / noise ratio and loss of linearity of volume response between a properly structured mixer and amps. On the bright side, there is a number of tutorials which guide you step-by-step through the gain structure process including mixers, processors, limiters, etc. Takes some time and patience but, upon completion, you end up with a "set it & forget it" rig...plug it in, cable it up, power up, and hoist the main sliders. I like the dbx tutorial, myself. BTW, did you know the recommended attenuator setting (starting point) for the QSC RMX series is 37% ?!? I didn't believe it so I ran it by hand with a pink noise CD. Turns out they were spot-on.
Riis
Yeah, perhaps it was overkill using the Carvin DCML3800 for the lead vocalist's wedge at the corner coffeehouse. But the headroom was unbelievable! The baristas were swooning at the sound of all that power in reserve.
I run pink noise or music into my mixer.
Trim the channel to 0. Set ch fader at 0, EQ flat. Unplug speakers from amps.
Set master fader at 0. Bypass any limiters etc. ..... Then turn master fader up to just below clipping in the mixer master meter. Then turn up amp attenuators until clipping light comes on, then back off a notch. Then un bypass limiters and set limiting to kick in at that point. make note of attenuator settings and turn them off. Plug speakers back in and turn amps back up and master fader down and listen.
Then adjust FOH EQ to taste while playing some music and recheck again.
My goal is to have the same amount of headroom in the mixer and amps.
Now I know I can push the master up to +15 and no clipping. I also always have amps on stage where I can see the clip lights from where the mixer is located.
Most of the time I'm running 0 average with peaks of +6 to +8 at the most on the mixer master. Usually below that in smaller venues.
DBX and Rane both have guides on their web sites explaining this procedure. My QSC PLX amps usually end up being about 2/3 to 3/4 on the attenuators.
Trim the channel to 0. Set ch fader at 0, EQ flat. Unplug speakers from amps.
Set master fader at 0. Bypass any limiters etc. ..... Then turn master fader up to just below clipping in the mixer master meter. Then turn up amp attenuators until clipping light comes on, then back off a notch. Then un bypass limiters and set limiting to kick in at that point. make note of attenuator settings and turn them off. Plug speakers back in and turn amps back up and master fader down and listen.
Then adjust FOH EQ to taste while playing some music and recheck again.
My goal is to have the same amount of headroom in the mixer and amps.
Now I know I can push the master up to +15 and no clipping. I also always have amps on stage where I can see the clip lights from where the mixer is located.
Most of the time I'm running 0 average with peaks of +6 to +8 at the most on the mixer master. Usually below that in smaller venues.
DBX and Rane both have guides on their web sites explaining this procedure. My QSC PLX amps usually end up being about 2/3 to 3/4 on the attenuators.
Someone who knows how to operate a sound system will want and be able to make use of a good 3dB to 6dB of amplifier power above the continuous power handling rating of a typical, commercially available, speaker system rated in the manner in which typical, commercially available, speaker systems tend to be rated.
I'm talking about really using that much amp, right up to the clip limiters on the loudest transients.
This will not substantially increase the risk of damage to the equipment, and will in fact make the most common mode of failure less likely. It will also provide wider dynamic range, lower distortion, and higher overall SPL than a system run with less than optimal power.
In addition it will allow a FOH engineer to figure out what's going on with the PA without having to listen through clipping and/or a sputtering power supply.
Repeat after me: You never want the amp to be the limiting factor.
If you are blowing stuff up it is because you do not know how to configure or operate your PA, not because of 3dB in the amp rack one way or the other.
I'm talking about really using that much amp, right up to the clip limiters on the loudest transients.
This will not substantially increase the risk of damage to the equipment, and will in fact make the most common mode of failure less likely. It will also provide wider dynamic range, lower distortion, and higher overall SPL than a system run with less than optimal power.
In addition it will allow a FOH engineer to figure out what's going on with the PA without having to listen through clipping and/or a sputtering power supply.
Repeat after me: You never want the amp to be the limiting factor.
If you are blowing stuff up it is because you do not know how to configure or operate your PA, not because of 3dB in the amp rack one way or the other.
Sticky worthy! Note that system volume is adjusted via channel sliders for balance and main sliders for FOH...that's it.
Riis
Use the channel pad in conjunction with the trim.
PS: what are you using that makes your bass signal that overly hot?
Hmm, somethings not right. If your miced up bass cab signal is to hot for the channel trim/pad to handle, you may want to have a tech check it out, because something must be wrong with your MixWiz16:2.
So the strength of your bass signal to the board defines the gain setting on your FOH amps? That's a little strange; it seems to me that your FOH mixer would be clipping like mad if that were the case.
If you run your own system, so you paid for the kit, and you operate it, then you can have over or underpowered amps because you have ears that will protect your investment. My systems have more amp power available than I need, and I like it this way because they just seem to go on and on. However - when somebody else runs my system, they could if they wish - severely overdrive my speakers, so I alter the gain structure by turning down the amp controls so that even with the desk showing many red leds, the amps are not going to be able to damage my speakers. when I use it, I can push if I want to, and I know my ears will shout warnings before damage is caused. Loudspeakers rarely die without protesting and pleading that can be heard if you know what it sounds like.
"Power amplifiers, however, have an input sensitivity rating that is much lower than that. The input sensitivity rating on power amps is the minimum voltage required on the input to cause the amp's output to put out full clean power. Most power amps have an input sensitivity rating of about +3dBu-+8dBu. This is why power amplifiers typically have input attenuator controls on them, which are the volume knobs on the front panel. A VERY common misconception is that if you don't set the input attenuators all the way up that you won't get full output from your amps. However, power amps are fixed gain devices...they will put out full power no matter where the input attenuators are set, however, depending on where they're set, it will just take a much larger signal to push the amp to full power. Judging by this statement, we now know that all the input attenuators do is raise and lower the amp's input sensitivity. The input sens rating that is advertised in the power amp's user manual is the sensitivity of the amp's input with the attenuators fully clockwise(all the way up). This is why they're there. I mean, for example, let's say that your mixer is capable of putting out +22dBu of signal, yet your most sensitive amp clips at +4dBu at it's input with the attenuators fully up. Well, now you're sacrificing +18dBu of gain/headroom you could be getting out of your mixer because once your amp is at it's clip point...that's all you get. This is why the input attenuators are there...so that you can match your amp's input sensitivity to your mixer/crossover.
The reason proper gain structure is important is so that we get max gain from the mixer, giving us more dynamic range(headroom) and a lower noise floor. Max gain from the mixer will increase your signal to noise ratio, giving you better seperation between instruments. It also makes your channel EQs MUCH more effective. I see a LOT of people use WAY too much boost from the EQ itself than from the gain control. If you flatten the EQ, then maximize the gain from the trim control, your EQ will be MUCH more effective to where on some instruments you may have to use cut more than boost, which enables you to get even more gain from the channel's trim control. Too much boost from the EQ rather than from the trim will cause phase distortion, which will cloud up, or muddy up your signal to where it almost sounds buried in the mix. "
The reason proper gain structure is important is so that we get max gain from the mixer, giving us more dynamic range(headroom) and a lower noise floor. Max gain from the mixer will increase your signal to noise ratio, giving you better seperation between instruments. It also makes your channel EQs MUCH more effective. I see a LOT of people use WAY too much boost from the EQ itself than from the gain control. If you flatten the EQ, then maximize the gain from the trim control, your EQ will be MUCH more effective to where on some instruments you may have to use cut more than boost, which enables you to get even more gain from the channel's trim control. Too much boost from the EQ rather than from the trim will cause phase distortion, which will cloud up, or muddy up your signal to where it almost sounds buried in the mix. "
Add a limiter to protect your speakers. FWIW - some clubs hide their limiters in locked racks, so no one can mess with them.
All I was trying to say ( and you will forgive my lack of terminology and actual knowledge, after all I am a bass player lol) is that it is not always proper gain staging to run all your power amps wide open. My board works perfectly, trim pots and all. I also have a permanent SE to run it for me on gigs and mostly I just mess with it in rehearsals, when we are usually just mic'ing vocals.
