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Is the Limiter in an XLS Series amp the same thing as a Compressor?

limiter compressor sound level peak control

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#1 HammerSlammer

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Posted 13 June 2014 - 02:18 PM

Hi. I've been reading a lot of posts and responses on this site. I must say that I am thoroughly  impressed with the care, politeness, accuracy and simplicity that goes into answering of the user's questions. There are a couple explanations that I have walked away from, having no more understanding than I did when I started reading. One in particular is the explanation of Crest Factor. Some additional reading elsewhere may clear things up for me.

My question is about the built-in limiter in my XLS 2500 and XLS 1000. I just want to get a clear understanding of how it works and how to apply it correctly. I know that a compressor can amplify the quiet parts and surpress the loud parts of pre-recorded music (if I understand it correctly).

Reaching my hands on occasion, has been a bunch of songs  burned on a CD to play at some events. Without a compressor, one song would come through so low that I would have to increase the levels to maintain the right acoustic dB level for the gig. On the same CD, the next song would begin to play and it would be so loud that I'd have to quickly move the levels down. So I either have to have a real quick hand, or use a compressor so I can be on auto-pilot as far as that's concerned.

Does the limiter on the XLS work like this? I'm thinking it does in a way more electrically to protect the speakers, rather than to maintain and even-out the acoustic dBs to be at a certain level. If I do use the limiter in the XLS, then my overall acoustic dB level would be lowered?

Can you explain how this works in a simple way, and also explain how to set the limiter on the XLS?
Will the integrated limiter benefit me in either of the following situations; please explain:
(1) 500W RMS speaker 1000W Program 1500W Peak- with an XLS 1000 bridged (under-powered)
   "        "   "   - with an XLS 2500 bridged (over-powered)

#2 dakos

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Posted 13 June 2014 - 02:57 PM

Hello,
Thank you for your compliments :)

The limiter in the XLS is a Peak limiter. The setting is very simple, either ON or OFF. It's basically used to make the amp not go into clip so you get the rated output, in a safe manner. If you match the amp to speakers correctly, it's going to be a bit more difficult to damage your speakers.
For more info on the limiters of the XLS:
http://www.crownaudi...&showtopic=4482

Crest factor: is the ratio between the RMS or average power in the signal to the Peak power in the signal.
In DC there are no peaks so peak and RMS are the same therefor the crest factor is 0dB.
In a sine wave there is a 3dB difference between RMS and Peak. That's a crest factor of 3dB.
In music the difference between RMS and Peak is between 6dB to 15dB and even more. So that's a crest factor of 6-15dB.

The solution to your variable volume problem is not using the built in limiter, nor using a compressor but rather to backup and rip all your CD's into your backup hard drive, it takes only a few minutes per CD, once you have them all in your HD, you need to run a small program called ReplayGain on it. It averages all the tracks added to it and the result is the same level on all your tracks. Just keep your backup software on the laptop you take with you to your gigs and you're set.
Avi

#3 HammerSlammer

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Posted 13 June 2014 - 04:48 PM

Hey thanks for the quick response. I was wondering how many days would pass before I heard from anyone. Yipee!
To make sure I understand you (and referencing the scenario in my post):
1.) if I have the limiter on in the case of the XLS 1000 and 500W RMS speaker, there will be no clipping with the XLS, having the channel 1 level all the way up?
2.) never occurred to me to re-do a CD that somebody hands me on the job. I knew I could re-master it, but never thought doing it on the spot. Thx; I will make that a standard practice. I always assumed, when they were handing it to me that they knew what they were doing when they made the CD. Always find out when it's too late.
3.) have read what you wrote about Crest Factor almost word for word more than a dozen times, still don't get it. I suppose there's too much terminology and signal/electrical concepts that I haven't grasped yet.

After I posted this, I started reading more of the endless amount of posts and answers.
I read in one of the posts that the sound is degraded in 4 ohm bridged mode of the XLS 1000 and XLS 1500 for subs. Is that really true?

#4 dakos

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Posted 14 June 2014 - 05:07 AM

I believe there is some kind of error with the specs you quoted so please tell me exactly which speakers you have and what you use to drive them (mixer, processors, source) so I'll be able to verify your amp to speaker matching.

Yes, it's true that the SQ degrades when you lower the ohm connected. That's true for every amp on this planet, not only class D :) I would be very surprised if anyone can hear the difference in a true A/B test. "Real" audiophiles like to say that they hear a difference you can only see in the lab but in our every day life in pro audio, those little things make no difference.

I'll try to explain the crest factor again in as simple words as I can, it's still in words so could still be a bit complex. To get it in the most simple way you need a frontal lecture or a video which I didn't try to look for until now. I'll start with the basics:

this is a picture with a DC voltage and AC voltage in the same magnitude, let say just for ease of explanation that they are both 10V, BUT the fundamental difference between AC and DC is while the DC is +10V in relation to 0V, the AC voltage is +10V in relation to 0V in its positive side of the amplitude and -10V on it's negative side of the amplitude. It's also sometimes referenced as 20V peak to peak.

Posted Image

Are we good thus far?

Now if we take the AC and simply run it through a diode rectifier circuit you get rectified voltage or rippled DC:

Posted Image
Another image that shows Pure AC, Rectified or rippled DC and finally DC. all in the same magnitude...


Posted Image


Now when we want to calculate how much POWER a curtain signal carries, in DC it's pretty easy, you probably know the formula, P=VI. (P=power in watts, V=Volts, I=Current in amps). In AC and in rippled DC it's a bit more difficult to calculate but the illustrative way is much simpler. The power a curtain signal carries is equal to the "area captured under the graph" for that signal. Now if we try to draw a line of DC voltage that would capture the same amount of "area under the graph" as an AC voltage (or rippled DC) we will get:

Posted Image

In this picture we have an AC signal with a magnitude of 170V or 340V PTP (peak to peak). The "area captured under the graph" (The signal's power) for that AC signal is colored in purple. In order for the DC signal to capture the same "area under the graph" as the AC signal, it needs to be of lower magnitude, that's the area colored in green and why it's only 115V (If we were to actually calculate the RMS voltage of that 170V AC sine wave signal we would find it's actually 120.2V and not 115V).

The relation between the 170V peak of the AC signal to the 115V (or 120.2V to be exact) of the average DC signal (also called RMS), that capture the same "area under the graph" or in other words, create the same power, is called Crest Factor and it is 3dB for a sine wave signal).

Did we hit the nail on its head?

If anyone has some corrections about anything I wrote, I will be more than happy to amend my post :)

Avi

#5 HammerSlammer

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Posted 17 June 2014 - 02:19 AM

Your efforts involved with that explanation were not misguided, and they were step-by-step right on. Thank you for that.
As far as the amp-matching, I'm going to let that question rest for now and read some other posts on the subject. If I have something specific I need clarity on, I'll post a nice, juicey question. :unsure:


My last questions (for a while)
1.) As far as the output degradation at 4 ohms: does that mean it would be even worse at 2 ohms?
2.) Why am I seeing ads for a Crown NXLS and also for an XLS?



Hey thanks for taking the time to answer my questions. Not just mine, but the hundreds of others you have taken the time to answer too. You seem to really take an interest in helping people out.

#6 joust

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Posted 17 June 2014 - 09:18 AM

A little history :

Early 2003/4 : The XLS 3U rack amp is shipped; Using conventional analog psu and output circuitry; (XLS402, 602, 802)
Around 2008 : The XLS 2U rack amp is shipped; Using conventional analog psu and output circuitry; (XLS202, 402, 602, 802, 5000)
Around 2012 : The XLS Class I is shipped; Lite DSP; Using smps and BCA type output circuitry; (XLS1000, 1500, 2500)

As far as the ads go, the latest XLS uses the NXLS suffix. Verify the model number.

Alain

#7 dakos

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Posted 17 June 2014 - 02:14 PM

You are most welcome, again :)

Theoretically speaking, the lower you go in the ohm rating, the lower the sound quality gets. Same as before, changes in the signal that can be measured only in the lab and don't effect our every day life don't account for much.

If you haven't found it already then look here for an explanation on amp to speaker matching:
http://www.crownaudi...490

Avi





Also tagged with one or more of these keywords: limiter, compressor, sound level, peak control