An amplifier getting hot due to spacing or clipping?

Shane Presley

Shane Presley

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Jan 12, 2011
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Ottawa, Ontario, Canada
www.bssproductions.com
So to start off....forgive me if this topic was ever covered on the old forum. I was supplying my rig to a local fair who had a popular 80's band, hitting the kit pretty hard. Anyway, the top amp of my 4 x PLX3402 combination was hotter than the rest of the amps, and was giving a slight clip indication, without hitting the yellow inidcation led lamp. it was hitting green led / green led/ red light clip "flickering".. It was powering the mid/high section of 3 EAW KF730 cabs. So two questions - what would cause the amp to behave this way, and secondly, was the amp extra hot due to induction from the others, or was it over-heating from the light clipping. I checked the 3 other amps running the other mid/high signal, and they were all clean. I'm running them bridged mono. Should I space these amps out with a rack space in between each for better cooling?

thanks
SP
 
Re: An amplifier getting hot due to spacing or clipping?

Shane Presley said:
So to start off....forgive me if this topic was ever covered on the old forum. I was supplying my rig to a local fair who had a popular 80's band, hitting the kit pretty hard. Anyway, the top amp of my 4 x PLX3402 combination was hotter than the rest of the amps, and was giving a slight clip indication, without hitting the yellow inidcation led lamp. it was hitting green led / green led/ red light clip "flickering".. It was powering the mid/high section of 3 EAW KF730 cabs. So two questions - what would cause the amp to behave this way, and secondly, was the amp extra hot due to induction from the others, or was it over-heating from the light clipping. I checked the 3 other amps running the other mid/high signal, and they were all clean. I'm running them bridged mono. Should I space these amps out with a rack space in between each for better cooling?

thanks
SP
Click to expand...


Check with the amps owners manual to see if they recommend spacing the amps or not.

Sometimes on windy days...of which we have a lot of down here....you may need to orient your amps so they are "downwind". In other words, with the wind blowing the same direction as the fan direction.
 
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Re: An amplifier getting hot due to spacing or clipping?

If I'm reading this right, the rack contains just four PLX3402s bridged mono, and they're all tight packed. This is how they should be. Putting a space between the amps, if it is not blocked from airflow, could actually hurt the amp cooling by allowing freshly exhausted hot air to circulate back around to the intake of the amp.

Make sure all the amps are clean inside and out, that all fans are working properly, and that there is no short in any cable presenting an unusually large load to the amp. If all gets a clean bill of health, there is probably something else wrong with the amp.

Not to rant or anything, but I'm sure you are aware that running PLX amps bridged mono (into less than 8 ohms) is a bad idea. It would be wise to sell all the PLX and get larger amps; the profit from selling the PLX would pretty much cover the cost of a couple big amps anyways, and you'd lose a lot weight and save rack space.
 
Re: An amplifier getting hot due to spacing or clipping?

where is the air intake? QSC airflow is back to front, so do you have a nice big vent in the back of the rack? or are the amps relying on leaks in the case etc for new air?
are you sure the fan in that unit is working? if one stops (or becomes weaker) the others will draw air through the malfunctioning unit and cause it to seem ok for a bit but die prematurely..

Jason
 
Re: An amplifier getting hot due to spacing or clipping?

No panels on the back. The rack is open. Fans were working check, and they all seemed to be working at the same rpm. I guess il have to wait for the next BE to give them a run. I'm going to check cabling as well. I also was using a big fan at the front of the rack to blow the hot air out. Amp fronts were facing the rear of the truck about 1/3 from the front of the truck. (maybe 3 ft from the larger fan). The fan was pointed at about 60 degrees up towards the roof.
 
Re: An amplifier getting hot due to spacing or clipping?

Shane, You do know that hot exhaust comes out the front of QSC amps, right? You'd benefit more by placing the amp at the rear of the rack to help blow air through the amps.

But, this does not sound like an overheating problem. I've experienced this a few times with QSC PLX amps. IIRC, it's either low voltage causing them to clip soon, VHF causing them to clip, or a short somewhere. I've also had them do that with a 2ohm load.


Evan
 
Re: An amplifier getting hot due to spacing or clipping?

Contrary to what you may think, an amp may not produce maximum heat output at clip. In fact, more heat can be produced somewhere at mid level, since the transistors inside have to dissipate the heat instead of directing it to the speakers as it would at full power. Top amps tend to operate at a midrange level for longer periods than sub amps which vary from peak to idle.

The PLX amps do tend to run fairly warm, so unless it's extremely hot, it may not be of any concern or not.
 
Re: An amplifier getting hot due to spacing or clipping?

Evan Kirkendall said:
Shane, You do know that hot exhaust comes out the front of QSC amps, right? You'd benefit more by placing the amp at the rear of the rack to help blow air through the amps.

But, this does not sound like an overheating problem. I've experienced this a few times with QSC PLX amps. IIRC, it's either low voltage causing them to clip soon, VHF causing them to clip, or a short somewhere. I've also had them do that with a 2ohm load.


Evan
Click to expand...

Thanks Evan, yeah I was trying to pull the hot exhaust out of the truck, as opposed to forcing cooler air in ( I thought this would work against the fans built in to do this job)... I'll try the other way - forcing cooler air in. I am running the mid/high section bridged mono on the amp, so it should only be seeing about 6 ohms so I don't think it's getting near 2ohms. As for power, I had a 2 year old, 200amp single phase service to new 100 amp breaker (audio only) about 50 feet from my distro, which was in located in the truck, so about a 15 ft L14-30 away from this rack... no power loss
(BTW the amp in question was top amp, in the center, bottom rack)
 

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Re: An amplifier getting hot due to spacing or clipping?

Shane Presley said:
...I am running the mid/high section bridged mono on the amp, so it should only be seeing about 6 ohms so I don't think it's getting near 2ohms....
Click to expand...

Running an amp 6 ohms bridged is the same as running 3 ohms stereo, which is very close to 2 ohms. Especially considering that there may be impedance dips lower than nominal somewhere in the speakers' response, you are teetering on the brink of certain amp destruction. Maybe.

I'd switch that amp with another amp doing the same thing and see if the problem follows the amp or the load.
 
Re: An amplifier getting hot due to spacing or clipping?

That's a nice setup, Shane! I agree with Evan - I think your problem has nothing to do with over-heating and you should have the amp checked out once you have confirmed all your connections are proper.
 
Re: An amplifier getting hot due to spacing or clipping?

Thanks all, I appreciate the direction... Silas, any idea how I could accurately measure (or calculate) the load of the mid-high section of 3 KF730's ? I've always wondered if I'm heading into that "below 4ohm" territory... What would anyone's recommenfation be to amplify my current rig... can't you run 4 KF730's on two amps ( at a 4ohm load?) so wouldn't running 3 be safe, power wise... ? I have 6 per side, so running 3 and 3 I thought would be fine...
@ Chuck thanks, it's my G.A.S kicking up...lol I'd love to move those PL6.0II's for PL380's...
SP
 
Re: An amplifier getting hot due to spacing or clipping?

Silas Pradetto said:
Running an amp 6 ohms bridged is the same as running 3 ohms stereo, which is very close to 2 ohms. Especially considering that there may be impedance dips lower than nominal somewhere in the speakers' response, you are teetering on the brink of certain amp destruction. Maybe.

I'd switch that amp with another amp doing the same thing and see if the problem follows the amp or the load.
Click to expand...

Yes, but as you said there may also not be. 15.9 minimum versus 16 nominal. Not going to be an issue... All Shane's amps have been run like this for some time, so they'd have been teetering along for a long time now (and in significant numbers to the point that I would suggest that normal HF impedance is not the issue). There could be some driver/crossover/cabling/etc causing MF/HF impedance issues, but only swapping the loads and amps could really diagnose that.

From the KF730 spec sheet.

Screen shot 2011-08-10 at 11.51.05 PM.png

PS - I hope it's not the 3402 I sold you a while ago :lol:
 
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