Low Input Sensitivity on Amps

[KBToys82]

So I've been reading up a lot more about proper gain structure and I see those that believe the amps must be full or almost fully open to be putting out sound, and those that say the amp sensitivity knob should be the last item you adjust.


2 years ago, we had a company come in and give our school a tiny makeover on amps, speakers, and a mixer. Well, we have 2 QSC GX5's driving what appears to be 4 Yamaha C112A's.

We also have a GX3 driving 2 BR15's that we use for monitor's when needed.

Anyway, if I set all my mic level's so that my Mackie ProFX16's meter is reading around 0db, then the amp has to be set to -16 to -14db (1/4 way of the knob) and even some of that is pushing the volume for our small 400 seat auditorium.

If I was to have the GX5's up at the halfway position or -10db, than my mixer levels have to be way down and I start hearing a lot of noise in the sound.

My issue is that I've read that running amps that low can cause over heating. It doesn't help that the company installed all 3 amps directly on top of each other. My first step is I'm going to separate them by 1U and install a vent panel in the rack. I'm also going to look into getting a rack cover for the amps so that no one can adjust the amps after they are set so they don't destroy the speakers.

I know proper gain structure can be a loaded question, but what are your thoughts.

Unfortunately anything that has to be purchased is probably going to by coming out of my pocket, so I really can't afford any new gear at the moment.

 

[MNicolai]

Audia has some excellent reading on their website about matching amplifier gains to console meters. They assume you have a DSP in the middle but their presentation of information is very direct, not-tweaky, and not based on voodoo hi-fi-like assumptions about gain structures.

If you want to space the amplifiers out, use 1U blanks instead of vents. Vents allow the air cycle to "short-circuit", causing the hot air expelled out of the back of the amplifier to circle around through the vent to the front of the amplifier and get sucked back in through the front. You want a distinct passage of air flow where cool air comes in the front, hot air goes out the back, and there's no easy path for hot air to circulate back to the front of the amplifier.

Tripp-Lite has some excellent diagrams in their literature about rack cooling efficiency.

 

[KBToys82]

Didn't even think about that.

I know in a perfect world, I would probably have something like the dvx DriveRack 260 for our front and rear speakers. Right now, the Front speakers are coming out of the board's XLR Main Out's and the rear speakers from the TRS Main Out.

I know that on those type of machine's, you can have the device attenuate the signal going to the amps so that they can work where they were designed, but my question is wouldn't you still be hearing noise from the machine with the amps at or near full? I would think that it would be the same amount of noise as with my mixer outputting a low signal as compared to the DriveRack outputting a low signal.


Either way, as of now, I'm just worried about over heating my amps. I was able to move them apart using blanks rather than with vents.

 

[Lextech]

Most of today's professional amplifiers are meant to tight stack,separating them might help but won't fix the problem. If heat is a problem you need to vent the rack. That is more important then space between them. The air cooling them moves through them, I think, I could be wrong, that these amps draw air in from the front and exhaust in the rear. If this is indeed the case getting that warm air out of the rack is what will reduce heat most efficiently. Middle Atlantic racks, a very common AV rack company, have tops that mount fans for a large segment on their product line, if you have MA racks the solution could be as simple as a fan top. If not, MA, Furman and others make fan panels in rack sizes. mounting one at the top of the rear of the rack will help.

 

[KBToys82]

The rack is wall mounted, so unfortunately I can't install a fan in the rear. It also has a door and when the system is in use, the door is usually slightly ajar. They do draw the air from the front, and I think with MNicolai's suggestion of not having a vented blank, it's would be really hard for the air to get from the back to the front. Sadly the installers didn't leave a lot of room at the top for air to escape and 1/3 of the rack is empty on the bottom. Next time I have rack open, I will see about getting an opening at the top to let the hot air out.


I'm still curious about does a DriveRack or comparable device have such a low noise floor that attenuating the signal out to the amps won't effect having the amps hearing the noise at higher db settings.

 

[FMEng]

What is the nature of the noise, hiss, hum, or buzz?

 

[KBToys82]

More hissing. Before I adjusted the amps to a lower db setting, I'm 98% it was a combination of ambient sounds and mechanical noise because it was set up to have not a lot of gain on mics and the amps at 50%. Now, the noise has dissipated because I sat down with a pink noise generator, got 0 dBu on the main out's and adjusted the amps accordingly.

 

[MNicolai]

In that case, you'll need to get an exhaust fan in there. Even if it's one of these with some standard computer fans loaded into it mounted at the top of the rack on the front. Cold air enters the rack through the amplifiers, rises up in the rack, and the fans on the front of the rack exhaust the hot air out of the front, where the heat rises again instead of getting sucked back in through the amps.

Any option you look at that relies on passive convection is going to cause the amplifiers to cook. You need to have a way to get that hot air sucked out of that rack.

 

[BobHealey]

Most of today's professional amplifiers are meant to tight stack,separating them might help but won't fix the problem. If heat is a problem you need to vent the rack. That is more important then space between them. The air cooling them moves through them, I think, I could be wrong, that these amps draw air in from the front and exhaust in the rear. If this is indeed the case getting that warm air out of the rack is what will reduce heat most efficiently. Middle Atlantic racks, a very common AV rack company, have tops that mount fans for a large segment on their product line, if you have MA racks the solution could be as simple as a fan top. If not, MA, Furman and others make fan panels in rack sizes. mounting one at the top of the rear of the rack will help.

Pretty sure 90% of the QSC I've run across take air in rear and exhaust out the front. Something to keep in mind when mixing gear in a rack or adding exhaust fans.

 

[Lextech]

Pretty sure 90% of the QSC I've run across take air in rear and exhaust out the front. Something to keep in mind when mixing gear in a rack or adding exhaust fans.

Okay so I went and dug out the GX-5 I have in storage and I was wrong. It draws air is one side of the front and exhausts out of the other side and the back. I figured this out by use of a kleenex. It was sucked in on one side of the front and blew away the other two ports. So the easiest thing for you to do is open the door all the way when the amps are in use. Next step, lower everything in the rack to the bottom. Last step add a fan. A roof fan would be best , but using a fan panel on the front top if you can't mount top is better then not moving the air. If nothing else open the door. If the air gets hot between the amps and the door and can't leave then you will draw it back into the amps.

 

[KBToys82]

Thanks everyone! In the future, if I can ever convince the admin to get a DSP in the rack, would something like the DriveRack 260 work for what I'm trying to do, which is boost the amps more and get more efficiency out of them? That as well as get the speakers tuned for the room.

 

[FMEng]

Adding DSP will not change the noise floor of the system or change the efficiency of the amps. The result would be exactly the same as properly adjusting the attenuators on the amp. The number one factor in the over all system's signal to noise ratio is the audio console and how its gain structure is set.

 

[avare]

So I've been reading up a lot more about proper gain structure and I see those that believe the amps must be full or almost fully open to be putting out sound, and those that say the amp sensitivity knob should be the last item you adjust.



My issue is that I've read that running amps that low can cause over heating. It doesn't help that the company installed all 3 amps directly on top of each other. My first step is I'm going to separate them by 1U and install a vent panel in the rack. I'm also going to look into getting a rack cover for the amps so that no one can adjust the amps after they are set so they don't destroy the speakers.

I know proper gain structure can be a loaded question, but what are your thoughts.

Where did you get the idea that the amplifier gain controls should be almost open? The input sensitivity has nothing to do with how low or high the amps are run for any given output level.

It seems that you are over thinking things with false information.

Do not take this negatively. Congratulations on asking here!

Andre

 

[KBToys82]

No, I know that you can still get full power out of the amps at even a very low setting on the amps, my question was asked because I don't even want to come close to over-heating the amps with such a low setting. The equipment we have is decent equipment, but it wasn't installed with a lot of thought from whatever company the school used. Like having the main speakers be mounted next to the prescienium even though we regularly have a podium on the stage directly underneath the speaker. The speakers are also way to powerful for our small theatre, no adequate venting of the wall-mounted rack and no EQ at all.

 

[KBToys82]

When I do sound for the musical, I run everything from the booth into a feedback destroyer (only for Crown PCC 160's) and everything else will be routed to a external reverb/effects processor and compressor before making its way down the snake to the on-stage mixer. I was just curious that since the amps/speakers are too big for the space, that if I got something like a DriveRack, then I could safely lower reduce the output coming out while running the amps at a setting that would be more efficient for them and also have no noise as was my original problem when the company had set the amps to high in the first place.

 

[FMEng]

There is no such thing as the amps being too big for the space. As I said before, you don't need a DriveRack to adjust the gain structure. The gain setting on the amp has NOTHING to do with how much heat the amp produces or what the peak power level of the amp is. The only reason to use a DriveRack would be if you need equalization and/or protection limiting.

I would not put the reverb and compressor between the console and the amps. Feed the reverb from an post-fader aux mix and return it into a channel. That will allow you to have some channels get reverb and others not, and easily mix how much reverb you get. The compressor belongs on an insert for the main buss, or on an insert on a sub-master, so that you have benefit of using the console master fader without affecting how much compression you have. These two changes will also reduce or eliminate any noise issues you have.

Spend your effort getting the heat out of the rack. That's your real problem.

 

[KBToys82]

I would much prefer to put the reverb as a post-fade mixer, my only concern is the students who will be running the board during the show forgetting to turn up/down the aux control for the individual actors since some times only specific people will need the effect. I'm already into researching the MIDI controls and changing them via Qlab.

 

[FMEng]

The reverb would work exactly the same as it does now. Instead of using the wet/dry control on the reverd unit, just use the fader on the channel used for reverb return. Set the aux sends once and leave them alone.