Power Amp + Cabs Question

[christliche]

Was doing a little research for a potential new rig I'd like to throw together. Came across a review on the Orange OBC15 1x15 cab, since I already have one and want to get a second one, so this review was helpful. The cabs are 400 watt 8 ohm. The reviewer seems to be saying he runs the full wattage out of the power amp @ 4 ohms, I am just wondering if there is potential to damage the speakers by doing this. Looked up the specs for the QSC GX7, and it is 1200w @ 4 ohm with a single channel driven, and 1000w @ 4 ohms with both channels driven. Not sure if the reviewer was just mentioning the amp can do 1200w @ 4 ohms, or if he ran out from one channel with the cabs in series, or if he is utilizing both channels at the 1000w each @ 4 ohms to each 1x15.

"I have a SansAmp RPM into a QSC GX7 (1200w @ 4ohms) to power 2 OBC115 cabs. I'm loud enough for 2 half stacks and a heavy drummer. These cabs are rated at 400w each, but they are efficient enough that my power amp doesn't have to push them very hard to produce high volume."

This is pretty much the exact set up I was imaging, just want to avoid blowing anything up.

 

[BadExample]

You are wise to ask first.

The first thing to do is boil the "watts" down to the same type. Are the 15's rated at 400 Watts RMS?

 

[mikezimmerman]

The QSC is a two-channel amp that doesn't appear to support bridging the two channels together, so either he's

1) running each of the 8-ohm cabinets on its own channel, with the amp supplying a maximum of 725 watts to each cab (the "8 ohms both channels driven" spec), or

2) powering both cabs from one channel, which results in the channel seeing 4 ohms from the combined cams and giving a maximum of 1200 watts, which is 600 watts per cab (the "4 ohms single channel driven" spec).

Was doing a little research for a potential new rig I'd like to throw together. Came across a review on the Orange OBC15 1x15 cab, since I already have one and want to get a second one, so this review was helpful. The cabs are 400 watt 8 ohm. The reviewer seems to be saying he runs the full wattage out of the power amp @ 4 ohms, I am just wondering if there is potential to damage the speakers by doing this. Looked up the specs for the QSC GX7, and it is 1200w @ 4 ohm with a single channel driven, and 1000w @ 4 ohms with both channels driven. Not sure if the reviewer was just mentioning the amp can do 1200w @ 4 ohms, or if he ran out from one channel with the cabs in parallel, or if he is utilizing both channels at the 1000w each @ 4 ohms to each 1x15.

"I have a SansAmp RPM into a QSC GX7 (1200w @ 4ohms) to power 2 OBC115 cabs. I'm loud enough for 2 half stacks and a heavy drummer. These cabs are rated at 400w each, but they are efficient enough that my power amp doesn't have to push them very hard to produce high volume."

This is pretty much the exact set up I was imaging, just want to avoid blowing anything up.

 

[christliche]

The QSC is a two-channel amp that doesn't appear to support bridging the two channels together, so either he's

1) running each of the 8-ohm cabinets on its own channel, with the amp supplying a maximum of 725 watts to each cab (the "8 ohms both channels driven" spec), or

2) powering both cabs from one channel, which results in the channel seeing 4 ohms from the combined cams and giving a maximum of 1200 watts, which is 600 watts per cab (the "4 ohms single channel driven" spec).

My thoughts exactly. Would going either of these routes cause damage? Or would it be safer/wiser to find a different power amp that will deliver less wattage per channel/per cab?

 

[christliche]

You are wise to ask first.

The first thing to do is boil the "watts" down to the same type. Are the 15's rated at 400 Watts RMS?

I believe it is RMS, from Musician's Friend and Amazon tell me.

 

[BadExample]

You first need to get the watts we're discussing boiled down to the same unit of measure. Are the cabs 400 Watts RMS, Program or Peak? Without knowing that, we are guessing.

 

[BadExample]

Haha, we replied at the same time.

 

[BadExample]

Divide the above figures by 2 and you should be very close to the PA's output in Watts RMS.

If the cabs are indeed 400 Watts RMS, IMO one per channel would be your best bet. I used to be a firm believer in having an amp that can put out a little more power than the cabs can handle. I'm a bit slow, so after just a few years of reading @agedhorse 's comments on these types of threads, I've changed camps and now want to see the amp have a bit less power than the cab can "take."

WARNING: The cab's wattage rating is the the voice coil (usually) and the amount of power it can take before the copper melts. It does not (usually) consider the mechanical limitations of the driver itself. I believe our @agedhorse has some familiarity with QSC amps, and if he chimes in, you would be well served to take his advice.

 

[Deleted member 165050]

The "no frills" answer:

Dime that amp and you'll fry those cabs. Not enough power handling, and there is no way you can be sure you've pushed the cabs too far until they start making dalek noises, possibly accompanied by magic smoke.

 

[BadExample]

Good point

 

[Marko 1]

I'd put both cabs on one channel, 600watts per cab which imo is scary. One cab per channel = 725 per, which is scarier yet. My GX5 was great for PA or driving two 500watt 410s.

 

[Bass_Pounder]

Nowhere in that other guys post does he say that he is running the power amp wide open, or implying that the cabs can handle more then their wattage ratings.

There is no reason to believe that he is running that equipment at full tilt.

 

[BadExample]

I'd put both cabs on one channel, 600watts per cab which imo is scary. One cab per channel = 725 per, which is scarier yet. My GX5 was great for PA or driving two 500watt 410s (which is still more power than I'd use now).

I didn't take the time to find it on their website, but I believe the power ratings of that amp will be in "Peak" Watts. So you can divide by two to get that to Watts RMS. If the cab is rated at 400 Watts RMS, I'd feel better with an amp rated somewhat below the 400 WRMS. Who knows what the 400 WRMS cab can mechanically handle? The only way to find out costs one (presumably expensive) 1 x 15

 

[BadExample]

Nowhere in that other guys post does he say that he is running the power amp wide open, or implying that the cabs can handle more then their wattage ratings.

There is no reason to believe that he is running that equipment at full tilt.

Good points, friend.

 

[Marko 1]

I didn't take the time to find it on their website, but I believe the power ratings of that amp will be in "Peak" Watts. So you can divide by two to get that to Watts RMS. If the cab is rated at 400 Watts RMS, I'd feel better with an amp rated somewhat below the 400 WRMS. Who knows what the 400 WRMS cab can mechanically handle 400 WRMS? The only way to find out costs one (presumably expensive) 1 x 15

I have one (GX5), pretty sure it's rated RMS/Continuous, not Peak. He'll cook or destroy those speakers imo.

 

[BadExample]

I have one (GX5), pretty sure it's rated RMS/Continuous, not Peak. He'll cook or destroy those speakers imo.

I think walking 12 ft away from the preamp while using a 10 ft. instrument cable will likely provide the pop of death to the cab

You would certainly know better than I on how the amp is rated. I'll take your word for it

 

[ctmullins]

Remember that amplifier power ratings are like automobile horsepower ratings. Just because you have a Hemi with a 450hp engine doesn’t mean you can’t mosey on down to the grocery store. It only puts out 450hp when it’s being run flat out. And no sane person would run it flat out while doddling around town.

Likewise, the amp, your preamp, your bass, and your fingers can and should control what portion of the 600 watts is actually going to the cabinet(s).

 

[agedhorse]

Nowhere in that other guys post does he say that he is running the power amp wide open, or implying that the cabs can handle more then their wattage ratings.

There is no reason to believe that he is running that equipment at full tilt.

Not running the power amp's controls wide open doesn't reduce its ability to deliver rated power.

I have one (GX5), pretty sure it's rated RMS/Continuous, not Peak. He'll cook or destroy those speakers imo.

It is rated are continuous average power (same as "RMS")

Remember that amplifier power ratings are like automobile horsepower ratings. Just because you have a Hemi with a 450hp engine doesn’t mean you can’t mosey on down to the grocery store. It only puts out 450hp when it’s being run flat out. And no sane person would run it flat out while doddling around town.

Likewise, the amp, your preamp, your bass, and your fingers can and should control what portion of the 600 watts is actually going to the cabinet(s).

If this were always the case, I wouldn't see so many speakers damaged from too much power, right?

 

[fourstringburn]

It wouldn't be a bad idea to use a small format mixer or compressor between the power amp and preamp. I use the Behringer RX1602 for visual level control between 2 of my preamps/compressor signals. This also has an effects send with level control which could be used to a FOH direct out.

A rackmount compressor/limiter would be good because you have meters to see how much signal is going to the power amp plus the limiting feature can control signal overloads. A mixer or compressor has input and output level adjusting capabilities to set levels to the power amp without over loads which you don't want when running high power.

What Aged Horse said about " Not running the power amp's controls wide open doesn't reduce its ability to deliver rated power." is correct. The level controls on power amps are attenuation controls, not power reduction.

Depending on the amplifiers input sensitivity voltage level which typically is selectable from .775 volts (0dB) to 1.23 volts ( +4dBu), full power is achieved when the input drive level voltage from a preamp/mixer/or compressor output is reached at the power amp's input sensitivity voltage level.

You can set the mixer/compressor final output level below 0 dB output on the meters and that will reduce the drive level to the amp thus reducing the amps power output.

 

[Downunderwonder]

Nowhere in that other guys post does he say that he is running the power amp wide open, or implying that the cabs can handle more then their wattage ratings.

There is no reason to believe that he is running that equipment at full tilt.

True but why ask if you aren't looking for maximum MOAR.