Pushing power amp too hard?

jamisonsalamand · #1 12-24-2011, 03:41 PM

I've been using a Crown XLS 402 to power my cab, and at practice the other day it seemed quieter than normal. During some songs it would also just stop putting out anything to the cab even though the input lights kept flashing, and I'd have to restart it once or twice to get it working again. The sudden shutoff seemed to happen during notes lower than the E string. Perhaps the overall volume level seemed lower because the drummer was playing harder during that practice, but maybe it's related.

Was/am I pushing it too hard? The level on that channel was all the way up, though I can run it in bridge mono mode to get 900 watts at 8 ohms instead of 300. I don't want to put the 200 rms cab at risk with 900 watts, however(getting one with more power handling soon).

**B-string** · #2 12-24-2011, 03:51 PM

Maybe you already damaged your cab and the amp went into protect. 200 rms cab can easily be blown with 300 watts of bass!

RickenBoogie · #3 12-24-2011, 04:18 PM

200 watt cab can be blown with 100 watts. Are you bumping the lows on your pre? That costs alot of power too.

jamisonsalamand · #4 12-24-2011, 04:19 PM

I've heard a lot of talk about this from people with years of experience that make me pretty confident 300 clean watts won't hurt something with 200 rms.

jamisonsalamand · #5 12-24-2011, 04:20 PM

Everything is flat except some boosted mids on my pickups' preamp

Phalex · #6 12-24-2011, 04:38 PM

I think the question is what kind of protect mode did the amp go into? It could be as simple as a bad cable.

Try different input and speaker cables. If it still does it, try it on a different speaker cab.

I have thought more than one amp has been on the fritz, and it usually comes down to something relatively cheap, and or stupid.

**B-string** · #7 12-24-2011, 04:39 PM

Quote:

Originally Posted by jamisonsalamand

I've heard a lot of talk about this from people with years of experience that make me pretty confident 300 clean watts won't hurt something with 200 rms.

I have over 40 years experience with not only live sound on huge venues but as a bassist, they are dead WRONG!

Live bass guitar without lots of compression and limiting will blow them very easy. This gets covered A LOT in the stickies (FAQs) If you would like a little light reading.

jamisonsalamand · #8 12-24-2011, 05:17 PM

Well since it got brought up, how would I tell if I've damaged the cab? It sounded like s*** in the first place.

BassmanPaul · #9 12-24-2011, 05:30 PM

Quote:

Originally Posted by jamisonsalamand

Well since it got brought up, how would I tell if I've damaged the cab? It sounded like s*** in the first place.

Quite possibly because you were feeding more power than the cabinet can handle. The power ratings on cabinets are usually the THERMAL ratings of the combined drivers. That's the point where they begin to melt. Drivers tend to run out of excursion, their mechanical limits, at about half of their thermal power rating. A 200W driver can typically use about 100W.

jamisonsalamand · #10 12-24-2011, 05:50 PM

Are you sure? It's rated at a continuous rate of 200 and general information tells me that gives it a peak of 800. Stuff can't seriously be recommended to run at only half the rms, or I'm extremely surprised

Zooberwerx · #11 12-24-2011, 05:56 PM

Quote:

Originally Posted by jamisonsalamand

Was/am I pushing it too hard? The level on that channel was all the way up, though I can run it in bridge mono mode to get 900 watts at 8 ohms instead of 300. I don't want to put the 200 rms cab at risk with 900 watts, however(getting one with more power handling soon).

Probably not a good idea as it presents a less-than-ideal gain structure and is more likely to clip with comparable preamp settings. Irrespective of attenuator settings, a power amp is capable of providing full output if fed a hot enough input signal. Sounds as if your amp has gone into some sort of protective mode as designed. The fact it fires back up and runs normally (?) is a good sign.

Riis

Downunderwonder · #12 12-24-2011, 05:59 PM

Sure he's sure. You have a lot to learn about power and excursion and rms power. Read the stickies and come back in a week.

Dave M · #13 12-24-2011, 06:34 PM

Let's re-start this at ground zero.
All this forum (besides the OP) knows is of one specified component in the signal chain. That is the XLS 402.

OP: my suggestion is to post the entire signal chain, and its settings, i.i., bass and its electronics, outboard preamp, power amp (we already know), speaker cab(s). Maybe even the speaker cable(s) used.
From this description it's more possible to work out some reasonable deductions on what is actually happening, and cause(s).

Reading the stickies can offer some usable info, but don't take it all as fact. There are unabashed opinion, conjecture, and misleading info in areas there, too.

jamisonsalamand · #14 12-24-2011, 06:53 PM

Quote:

Originally Posted by Downunderwonder

Sure he's sure. You have a lot to learn about power and excursion and rms power. Read the stickies and come back in a week.

Just read some of them, and it only confirmed my thoughts on power ratings. For example, it's safer to run a 300 watt cab with a 1000 watt amp.

My signal chain is a Schecter Studio 6 with Bartolini split coils and Bartolini 5.4 preamp. I'm using an Ampeg Micro SV and putting the line out to the Crown amp. Then it's driving an Ampeg SVT210AV. The ampeg preamp is flat with the gain around 11 o clock. The Bart preamp is almost flat, with the mid switch at 500hz and the mid boost around 2 o clock. The power cable is a speakon to 1/4 adapter using the cable that came with the ampeg head

Downunderwonder · #15 12-24-2011, 06:58 PM

You could easily be overdriving the Micro SV input with the preamp.

Lets not do the "is it safer to run a 300W cab with a 1000W amp" thing here. It isn't safer. Read more. Eventually you'll sift the good stuff from the b/s.

jamisonsalamand · #16 12-24-2011, 07:10 PM

I just went through probably five pages on the subject and no one voiced the same opinion as you are here, so I don't know how I'm supposed to find more elaboration on your opinion. If you could explain, it would be great.

BurningSkies · #17 12-24-2011, 07:29 PM

Your cab probably sounds like crap because you're exceeding the cab's excursion limits. That the step right before or during the time you ruin your speakers. The excursion limit tends to be well below the rated thermal limits of a cab. Also, after a point, your cab only gets more distorted and no louder. Get more speakers and don't ask two small 10 inch speakers to do more than they can, and quit pushing your power amp as hard as you are, which is causing it to shut down.

**B-string** · #18 12-24-2011, 08:11 PM

Alright so you didn't get to the crux. The "Run more power amp than speaker input rating.", was misused from a white paper published by JBL. It was published to reduce HORN FAILURES in PA speakers. Did NOT pertain to low frequency speakers. I assume you are playing bass guitar through this rig not full range FOH signals. The only way to use over-powered amp to speaker ratios with low frequency drivers with PA systems is to compress and apply limiters to the signal. Most PA must also use high pass filters to filter out things like the very low frequency of string to pickup movement before the note is actually struck. Bass guitar is not the same as the processed signal PA's produce. We as bass guitar players must recognize the mechanical limits of our cabs, which can have very little to do with the "RMS" rating of a speaker cab usually referenced between 500 and 1000Hz.
That help any?

Downunderwonder · #19 12-24-2011, 08:17 PM

Quote:

Originally Posted by BurningSkies

Your cab probably sounds like crap because you're exceeding the cab's excursion limits. That the step right before or during the time you ruin your speakers. The excursion limit tends to be well below the rated thermal limits of a cab. Also, after a point, your cab only gets more distorted and no louder. Get more speakers and don't ask two small 10 inch speakers to do more than they can, and quit pushing your power amp as hard as you are, which is causing it to shut down.

That's a really great summing up.

Read more and more and more until you understand everything backward to frontwards, and agree with it. Then you will be zen with your rig.

billfitzmaurice · #20 12-24-2011, 08:35 PM

Quote:

Originally Posted by jamisonsalamand

Are you sure? It's rated at a continuous rate of 200 and general information tells me that gives it a peak of 800. Stuff can't seriously be recommended to run at only half the rms, or I'm extremely surprised

Very few drivers can make use of more than half their RMS rating. More than that ends up as heat in the voice coil, which can lead to long term damage even at inputs less than the thermal rating.

Quote:

I've heard a lot of talk about this from people with years of experience that make me pretty confident 300 clean watts won't hurt something with 200 rms.

True, if you don't actually use more than 100w. The problem is: how do you know?