atcomsystems.ca/forum Forums Wiring - Telecom Carriers - Trunks & Services Security, Alarms and Paging sound system problems

We are having a weird problem at work...our auditorium system consists of a Mackie DX8 and 3 Crown Com-Tech 1600 amplifiers, all 70 Volt.
If someone bumps the podium mic, or goes to plug a 1/8 inch cable into their laptop through a direct box, it will often cause a loud pop, then the system surges and shuts off. I can't figure out why or how to fix this, besides turning the channels all the way down when someone is plugging in. The devices in the rack are connected to power strips on 2 different circuits. Sometimes the breaker on the power strip will trip, but usually the breaker in the panel trips instead.

Any ideas?

First off you should always have the volume off on empty inputs. What is shutting off? Is you direct box a passive device or does it have isolation? Have you checked the current load to make sure you are not overloading your circuits? Put and amp meter on the feed and monitor the current and see if it spikes when this happens.

Exactly!

Sam

I just took a peek at the manual for the Crown Com-Tech amps. Check out the current draw for a 1600 at 50% duty cycle [on page 43 on manual K-80636]. That "pop" when somebody plugs into a live input is easily equal to 50% duty... probably more!

If these are the 120V units, it is close to 20 amps. If they run on 240V, it is only about 9 amps.

But blowing the breaker is only a symptom. As others have said, you need to figure out how to stop people from plugging into inputs that are live. They have actually been lucky so far...

Jim
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Speaking from a secure undisclosed location.

Merritt,
The breaker will trip thus shutting the mixer and amplifiers off. The direct boxes are passive, these are the ones we use: https://www.artproaudio.com/products.asp?type=90&cat=13&id=104

Those channels should be near off or completely off when plugging in or unplugging anything. Also check the level controls on those amps, you could try turning them down so the power to the system is not so high on those loud sounds.

Sounds like you need a limiter to me...

-Hal

Very good advice Hal that may even save the speakers.

The mixer does have all kinds of built in stuff through the software so I can try a limiter.
I talked to Crown and they said at peak those amps can draw up to 13 amps. The way we have spaced out the power is with the wireless mics, one mixer, and 2 amps on one circuit, and another mixer, cd player and 2 more amps on the second circuit (the rack has equipment for 2 rooms in it)
Unfortunately we only have 2 circuits for the rack, without putting in anything new. I should have the electricians come check with a clamp on meter when we are running it...

If each amp can draw 13 amps at peak I think they should each be on a dedicated circuit. Have 2 new circuits run in for the mixers and other stuff.

The manual says it is closer to 20 amps, but if they told you 13, so be it. It wouldn't be the first time a manual was wrong...

Jim
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Speaking from a secure undisclosed location.

You are chasing the symptom rather than the cause Jeff. With a 70 volt system when something like low frequency high level pop comes along the transformers on the speakers will saturate and pretty much present a dead short on the amp output that is probably being driven to more than it's rated 70.7v output also. One would expect the amp to go into protection but it might not be as fast as the breaker in the panel that is supplying it. I would not be surprised if the instantaneous mains current draw during an episode like that is several times the rated peak spec. Hence the breaker trips.

If you have DSP, absolutely insert a hard limiter that clamps just about where the clip indicator on the Crowns illuminate as a start,(if you have no other means to determine levels within the system) then back it down from there until you have eliminated your problem. The limiter(s) should be last in line and right before the inputs to the Crowns.

-Hal

If you haven't already, make sure the High Pass filter is dialed in on those channels. It's probably good to have it as a default, unless you specifically need to go lower then that.. 80hz would be a good place to start, and move up from there..

There might also be a HP filter on the amps.. If you're using ceiling speakers, they probably don't go down that low anyways, and you should set your HP filters appropriately to protect them.

That is a good point. Because of the transformers on the speakers saturating at low frequencies as I said above you want to high pass and 80Hz is a good number. The better and heavier the transformers are the better the low frequency response so if they used really good transformers on the speakers and an amp that is direct drive with no output transformer you could get decent low end and not have to (or want to) use any high pass. But usually you find that most ceiling speakers are el cheapo and only go down to around 100Hz.

Those Crown Com-Tech amps are top of the line and direct drive 70.7V so you are in good shape there.

-Hal

I don't have the specs for the speakers off hand but I know they are JBL.
I will mess around with the DSP that is built into the DX8 mixer.

If they are JBL Control series they are very good with excellent transformers built in. I have installed exactly what you have, Crown Com-Tech 800 with JBL Control 24CTs in the ceiling.

Nothing to do with your problem but did you know that those Com-Techs require Class 1 wiring to the speakers?

-Hal

Hal, would that be due 70.7 AND the probable 15 amp capability on the amp output? Close enough to 110VAC and 15 amp branch to scare me!

These are not 24CT, they are square...

Hal, would that be due 70.7 AND the probable 15 amp capability on the amp output?

No, nothing to do with what you are saying at all, in fact most amps today that are even larger only require CL2 wiring. Those Com-Tech amps are a few years old (8-10) and back then installers either didn't know or didn't care about speaker wiring unless you had an inspector who was sharp enough to know LV wiring and knew to look at the back panel of those amps where it says "Class 1 wiring required".

The class of wiring required is determined by UL or other testing lab specs which use a complicated formula of output current, current limiting, voltage and voltage limiting. The engineers that design commercial amps today are smart enough to design something that can deliver full power yet (and I'm guessing here) react quickly enough to a fault so that only Class 2 wiring needs to be used.

When I used those amps the wiring was in conduit and Greenfield whips to each speaker. I had a couple of thousand feet of 18ga TFFN in red and black twisted up which I pulled for the wiring.

If this ever should become an issue Jeff, it would, without a doubt be less expensive to just replace those amps though I would be crying because they are bullet proof.

-Hal

They used West Penn C206 unjacketed speaker wire, says CL2 on it...
According to the manual, class 1 wiring must be used if the amp is configured for 100 volt output...

Huh, now you made me look at the manual and you are absolutely right, that is what it says but it's in the text of the instructions, not in the specs. Thing is though they have "Class 1 wiring must be used" printed next to the outputs and that's what everybody sees. Nothing about only with bridged mono.

I suppose if it became an issue you could drag out the manual but according to the NEC it has to explicitly say that you can use CL2 (or CL3) wiring and I am not seeing that. The rule according to the NEC is that the wiring requirement must be printed on the unit and if it says nothing you have to assume Class 1. In this case Crown only says in the manual that you have to use Class 1 if you configure the amp in bridged mono where the outputs will be more than 100v. They don't say what the wiring requirement is for 70.7v. So because they say nothing we are back to Class 1 and that's what it says on the back panel so what's an inspector going to think even if you show him the manual?

-Hal

Like Hal said: "Article 640.9 (C)Output Wiring and Listing of Amplifiers. Amplifiers with output circuits carrying audio program signals shall be permitted to employ Class 1, Class 2, or Class 3 wiring where the amplifier is listed and marked for use with the specific class of wiring method."

So Jeff, time to buy a sheet of rub-on alphabet!