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School Gym Sound Challenge
School Gym Sound Challenge
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Old 25th March 2017, 05:29 AM   #1
Teachnew is offline Teachnew
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Default School Gym Sound Challenge

I just bought some equipment for my school gym sound system and would appreciate any advice you could offer to help me set it up properly. My school needed a PA system that would be used by many staff members/students in their gym classes, assemblies etc. and was VERY simple to use (idiot-proof). The school's budget for this was VERY limited. Here's a list of components I have to work with:

•2 Speakers-EVI-15 250/1000 8 Ohms
•Behringer Amp- KM1700 2 x 500 Watts into 8 Ohms
•DBX GoRack-Connected between the mixer and amp for EQ, Feedback Supression, and volume limiting
•Mackie PRODX8 Mixer with Bluetooth (staff and students connect their phones to play music via Bluetooth)
•Shure BLX288/PG58 Dual Wireless Handheld Mic System
•A timer that turns the system on every morning and off every evening.

Pressing Questions:
-The system makes a loud popping sound when it powers off. Is there any way to prevent this? Perhaps something that delays the amp power off?
-I'm using the GoRack as a limiter to prevent the speakers being damaged. Should I leave it as is, or crank it to full volume and use the amp gain dials to limit maximum volume?

Thank you in advance for any input or tips to make our school PA system sound it's best.
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Old 25th March 2017, 08:08 AM   #2
steveu is offline steveu  United States
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I'd connect one bit at a time to find out what is popping, and replace it or add a muting circuit. I could probably modify the culprit to prevent the noise but you do whatever works for yourself.
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Old 25th March 2017, 08:09 AM   #3
silverprout is offline silverprout  France
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Quote:
Originally Posted by Teachnew View Post
•A timer that turns the system on every morning and off every evening.

Pressing Questions:
-The system makes a loud popping sound when it powers off. Is there any way to prevent this? Perhaps something that delays the amp power off?
You should put a another timer on the Behringer Amp... delayed to switch on after your main timer and off before your main timer.

PS : the amp timer should trigger a switcher box placed on the speaker wires.
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Last edited by silverprout; 25th March 2017 at 08:26 AM. Reason: PS : the amp timer should trigger a switcher box placed on the speaker wires.
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Old 25th March 2017, 09:43 AM   #4
JMFahey is offline JMFahey  Argentina
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What silverprout said, (to avoid modding them as steveu suggested which is probably beyond what you can comfortably do) .

Never trust a volume control dial "numbers" as a safe Power control, doubly so in inexperienced hands and even worse if Teens/Students who are always "enthusiastic" and selectively deaf: they do NOT hear distortion, amp grossly clipping, etc. so USE any Limiters you have available

If no sophisticated equipment available, set it "by ear" : rise volume until it barely starts turning ugly and then set limiter threshold until you clearly hear it "going down one notch" .
ON/OFF it to confirm when ON it IS audibly lowering overall volume to clean undistorted one; if in doubt make it lower volume even more.

Try to set the system "as a Teen would", you should not be able to make it clip.

Only safe way to leave a system unattended
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Old 25th March 2017, 02:15 PM   #5
conanski is offline conanski  Canada
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Quote:
Originally Posted by Teachnew View Post
My school needed a PA system that would be used by many staff members/students in their gym classes, assemblies etc. and was VERY simple to use (idiot-proof).
•2 Speakers-
•Behringer Amp-
•DBX GoRack-
You should have bought powered speakers.. that would have combined all 4 of these devices and features into 2 boxes and you wouldn't have to figure out what settings are needed to protect the equipment.



Quote:
Originally Posted by Teachnew View Post
•A timer that turns the system on every morning and off every evening.
You need a power sequencer so that the amplifier can be turned off first and turned on last every time, that will eliminate the pop.
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Old 25th March 2017, 04:35 PM   #6
Teachnew is offline Teachnew
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Thanks for the advice everyone.
The EV passive speakers were already installed in the gym, but if we ever upgrade, I'll be sure to install powered speakers. I'll look into getting a cheap power sequencer.
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Old 25th March 2017, 04:56 PM   #7
simon7000 is offline simon7000  United States
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Powered loudspeakers require conduit for the power line. Often very costly.

In a school installation you often require UL or equivalent approved gear. You might be surprised at how much gear does not meet the safety standards.
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Old 26th March 2017, 04:48 AM   #8
calneva is offline calneva
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It was implied earlier, but you need to power on the mixer first, then the amp. Power off the amp first, then the mixer. The avoid the pop, the amp has to be last on and first off.

Some people use the mnemonic MA-AM or "ma'am" to remember. But it's not just the mixer, it would mean the Go Rack and wireless receivers too. Again, the amp has to be last on and first off.

A power sequencer does take care of this if it has to be automated. Just put the amp in the last delay group so it's last on and first off.

The limiter does not protect speakers, much. Clipping signal is what damages speakers, not so much driving them too loud. So if your mixer is clipping, it can damage the speakers even if the amp is turned down. The EVI-15's have a "PRO Driver Protection Circuit" which I believe only protects the horns. There are more sophisticated driver protection systems such as the Eminence D-fend SA300.

That isn't to say that a limiter isn't useful, but I would set the gain structure to avoid clipping rather than to limit sound pressure level (SPL or "volume"). Set the gain on the channels so the highest signal levels don't clip. Then, because the Go Rack itself has variable gain, make sure the input isn't driven to the point where it clips there. Last, set the input gain levels on the amp so it doesn't clip.

The reason you want to avoid turning the input gain on the amp way down is that will make it more likely the mixer will be turned up to the point where it will be clipping.

But you also don't want to make the mistake of a person who is trying to get maximum SPL and they ensure the amp input gain is cranked all the way up. In that condition, the amp can be clipping when the master faders are only half-way up. Trimming the input on the amp doesn't make it any less powerful. It just sets the level of input so the amp can make the most of it.

The limiter helps in a similar way, but if you're driving it near the limiter, you're just compressing the signal, which doesn't lower the signal level, it just makes the dynamic range, the range between the quietest signal, and the loudest, less.
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Old 30th March 2017, 06:24 AM   #9
Teachnew is offline Teachnew
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Thank you very much for your thoughtful suggestions. I'll do as you suggested tomorrow!
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Old 30th March 2017, 02:18 PM   #10
conanski is offline conanski  Canada
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Quote:
Originally Posted by calneva View Post
Clipping signal is what damages speakers, not so much driving them too loud. So if your mixer is clipping, it can damage the speakers even if the amp is turned down.
Good advice Cal except for this bit which is simply not true. The only way to blow speakers is by overpowering them. A clipped signal will not blow a speaker if the overall signal power level is within the handling capacity of the speaker. Guitar players routinely do this and they don't have any problem with blown speakers.

However your post does expose how difficult it is to correctly configure a system like this to be safe from abuse and ignorance. Even if it is initially setup correctly it only takes 1 button twiddler to come along and make a total mess of it. This is why I suggested that self powered speakers would be a much better solution because even if a user completely messes up the mixer that shouldn't blow up the speakers which have built in protection that cannot be defeated or bypassed . When inexperienced users are at the controls the less control they have the longer gear lasts, so in this case everything that should be set and not changed should get locked away in a closet, if I were setting up this gear the only thing users would have access to are the handheld microphones.
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