Amp Clipping Issue

[panteragstk]

Hey guys. I have a Crown XLS1500 powering my dual infinity subs running at 2 ohms stereo. I have piratically zero issues other than in the most demanding of movies the amp will clip. I'm trying to figure out what is causing this.

At fist I thought I had the gain too high. After lowering it all the way to half the issue still happens when torture testing (war of the worlds). I haven't been able to get any other movie to recreate the issue, but I still want to make sure there isn't an issue.

My knowledge (what little there is) tells me that when lowering the gain and raising the level in the AVR to compensate I'm not really changing anything all that much.

So, the question is how do I stop this from happening? It doesn't occur all that often, but I'd like to prevent it all together.

Where should I start assuming there is even anything I can do about it?

Thanks

 

[markw]

Simple answer: You can't. Everything has it's limits and you've found your systems limit.

 

[panteragstk]

Simple answer: You can't. Everything has it's limits and you've found your systems limit.

Crap. Why did you have to tell me what I already knew. I was really hoping I was wrong.

Now, what to do to fix it? New amp? Different drivers? Stop trying to murder them with WOTW?

 

[avengineer]

One thing you can do, and I've done exactly this before, is install a compressor/limiter just ahead of the sub power amp, and calibrate it to limit the maximum input voltage to the power amp at a point just below clipping. I've used an old dbx 160 (search eBay), set to a high ratio. You have to play with the settings a bit, because ideally it should only act as a clipping protector, not a compressor. The function is very similar to a THX concept that has largely gone away, they had a sub over-excursion limiter for a while, haven't seen it in anything in years though.

This is a band-aid, a cheap fix that gets you out of clipping on extreme soundtracks without doing anything on less taxing ones, but really, you want more power, or subs that present less load, or are more efficient. Make sure you calibrate the system too, getting sub levels right. If you add the limiter, PM me for specific setup help. The system I used it on had an issue with custom designed subs that had limited maximum SPL at extreme LF. The user couldn't change the subs (they were built-in), and this saved the day.

 

[avengineer]

BTW, my sub stress test is the tower collapse, Return of the King. That track is off the seismic charts.

 

[Irvrobinson]

I hate to bring this up with avengineer around , but you might want to make sure it is the amp that's clipping. Most of these clipping detection circuits can't tell the difference between a clipped input from the pre-amp and the output stage of the amp actually clipping. Given that we're talking subs it could very well be that the amp is actually clipping, but your AVR could be the problem too. One challenge - it's not easy to check for pre-amp clipping.

 

[panteragstk]

I hate to bring this up with avengineer around , but you might want to make sure it is the amp that's clipping. Most of these clipping detection circuits can't tell the difference between a clipped input from the pre-amp and the output stage of the amp actually clipping. Given that we're talking subs it could very well be that the amp is actually clipping, but your AVR could be the problem too. One challenge - it's not easy to check for pre-amp clipping.

That's really what I was wondering, if any other component could be the issue. The drivers could very well be pulling all 1550w from the amp if the track is demanding enough, but I've used this amp in pro settings with WAY more power hungry (maybe) subs and couldn't get it to clip. I really think the output level to the amp is the cause. I wonder if it would help to go from dual unbalanced input to a single unbalanced input. Since the receiver basically has an internal y adapter that wouldn't change anything other than only one input being used on the amp. What I'm wondering is if the AVR is somehow over-driving (right word?) the output of both unbalanced connectors instead of just splitting the single signal to both outputs.

I'll try a single output/input and put the amp in bridged mode and see what happens.

 

[slipperybidness]

I've used this amp in pro settings with WAY more power hungry (maybe) subs and couldn't get it to clip.

But, did you play the same tracks on those more power hungry subs? Those LFEs often hit much lower notes than any musical passages.

 

[panteragstk]

But, did you play the same tracks on those more power hungry subs? Those LFEs often hit much lower notes than any musical passages.

No, it was when we were using the amps on an in-store demo for our live sound room. We did use some track with some very low frequencies (sub 20hz) that made other amps clip so it was a good test. It was more a frame of reference than anything, basically saying that these amps are very good. Since this series of amps is very high quality, I was just hoping the amp isn't to blame, but the signal into the amp.

Could balanced vs unbalanced make a difference? I don't think it would, but it'd be worth a try if I can't fix it another way.

 

[M Code]

I hate to bring this up with avengineer around , but you might want to make sure it is the amp that's clipping. Most of these clipping detection circuits can't tell the difference between a clipped input from the pre-amp and the output stage of the amp actually clipping. Given that we're talking subs it could very well be that the amp is actually clipping, but your AVR could be the problem too. One challenge - it's not easy to check for pre-amp clipping.

Absolutly correct..
The AVR preamp could be clipping..
Or the subwoofer drivers bottoming out..

Each component should be checked for its optimum operating range.

Just my $0.02...

 

[PENG]

How do you know it clips, was it just by ear or you have clip warning lights on the power amp? If you know it clips for sure, to eliminate the pre-out side of things, can't you just lower the output level and increase the Crown's gain and see what happens?

 

[Irvrobinson]

How do you know it clips, was it just by ear or you have clip warning lights on the power amp? If you know it clips for sure, to eliminate the pre-out side of things, can't you just lower the output level and increase the Crown's gain and see what happens?

I was assuming he was seeing the clipping LED flash on the front panel of the amp.

 

[avengineer]

I hate to bring this up with avengineer around ,

Ok, i deserved that.

but you might want to make sure it is the amp that's clipping. Most of these clipping detection circuits can't tell the difference between a clipped input from the pre-amp and the output stage of the amp actually clipping. Given that we're talking subs it could very well be that the amp is actually clipping, but your AVR could be the problem too. One challenge - it's not easy to check for pre-amp clipping.

Couple of things about the XLS1500. The internal clip detector is tuned to the amp and should completely ignore preamp clipping. Second, there's an internal clip protection peak limiter available, you just need to turn it on. Page 8, manual.

The stated input sensitivity is 1.4v for full power, assuming that's with the front panel two controls all the way up, so preamp clipping should not be the issue. Ok, theres a chance. You could tell by finding material that causes the problem, the turning down the amps input controls while leaving the preamp/AVR volume control alone. If you still hear the problem, but now its a lower volume, you have preamp output clipping. If not, the problem is down stream from the preamp.

However, at 775wpc into 2 ohms, are you sure you aren't bottoming out the subs?

 

[Irvrobinson]

Couple of things about the XLS1500. The internal clip detector is tuned to the amp and should completely ignore preamp clipping.

I think you'll find that all of these circuits will trip if there are clipped waveforms anywhere in the signal path.

, there's an internal clip protection peak limiter available, you just need to turn it on. Page 8, manual.

Good point. The OP should engage this capability and see what happens.

 

[slipperybidness]

To the OP:

When you say "2ohm stereo", you do mean that the amp "sees" a 2ohm load? Not 2ohm speakers wired in parallel (resulting in a 1ohm load)?

Just want to be sure we are all on the same page here.

 

[avengineer]

I think you'll find that all of these circuits will trip if there are clipped waveforms anywhere in the signal path.

How do you know this? I'm not saying you're wrong, but it's counter-intuitive based on how clipping indicators usually work. I'm not even sure how you could differentiate a clipped waveform from an arbitrary one. Most clipping indicators are based on either a fixed voltage threshold or a difference between an input and output waveform.

Please clue me in how this could happen.

 

[Gordonj]

...Or the subwoofer drivers bottoming out..

Uhmm, there is a good point to this. Maybe it is just the sub over extending due to the volume levels at the extreme low LF....

 

[markw]

At fist I thought I had the gain too high. After lowering it all the way to half the issue still happens when torture testing (war of the worlds). I haven't been able to get any other movie to recreate the issue, but I still want to make sure there isn't an issue.

After reading these two sentences, I'd be thinking that it might be the source software itself.

 

[avengineer]

After reading these two sentences, I'd be thinking that it might be the source software itself.

...feeling very stupid now. Yup, missed that.

 

[panteragstk]

It is indeed the clip/peak limiter engaging that made me aware. That and the fact that the LFE signal was gone from what I was hearing. I've tried lowering the gain and turning up the level in the AVR, but it still happens. I'll keep going to see if I can get it to stop.

I too think it is just the software as mark stated, just trying to avoid the issue completely. Whether or not this is possible is another question. I'll update my progress.

Yes, 2 ohm stereo means two channels at 2 ohms each, not 1 ohm. I think my amp would cry if I did that to it. Thanks for saying that though. You never know how simple things can slip through the cracks when troubleshooting.