ProSoundWeb Community
Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => LAB Lounge => Topic started by: Yosi Melamed on December 09, 2011, 08:03:55 AM
-
I'm trying to set the limiters on my amp (Crown XTI 4002).
I'm calculating the values for the setting on the amp and something doesn't add up...
My speakers are 225W continuous, 8 ohms,
My amps voltage gain is 34.2db.
Using this formula 20*log(sqrt(P*Z/0.6))-G, I get 0.57dBu... So far so good.
Now I want to enter the numbers into the amp, but the amp takes db units, which I guess is in relation to its maximum output voltage no matter the load, or it is in relation to power output and then the calculation takes the load into effect.
Generally speaking if my amp has 650W @8 ohms and my speakers only take 225W, I think it should be in the neighborhood of -4 to -5 db but I don't know how to calculate it exactly, help...
One more, I'd like to set a high pass filter to try and prevent physical damage, what would you suggest I put in the frequency, slope and crossover type to accomplish that?
The Frequency Response of my speakers is:
55 Hz - 20 kHz (± 3 dB)
This is the frequency response chart if it helps
(http://cosmossound.co.kr/product/speaker/images/Ps12_chart1.gif)
-
Are you using these speakers with a subwoofer?
-
Are you using these speakers with a subwoofer?
Fullrange.
I forgot :-)
-
I've got one XTI 1002, and I control it using System Architect, rather than twiddling with the front panel. I don't know if it's available from the front panel, but in SA, you can set the limit as an actual voltage, which should be much easier to figure out. (Though I don't know if they're doing the fiddly stuff Ivan Beaver was mentioning on the Itechs here as well)
-
I'm trying to set the limiters on my amp (Crown XTI 4002).
I'm calculating the values for the setting on the amp and something doesn't add up...
My speakers are 250W continuous, 8 ohms,
My amps voltage gain is 34.2db.
Using this formula 20*log(sqrt(P*Z/0.6))-G, I get 1.02dBu... So far so good.
Now I want to enter the numbers into the amp, but the amp takes db units, which I guess is in relation to its maximum output voltage no matter the load, or it is in relation to power output and then the calculation takes the load into effect.
Generally speaking if my amp has 650W @8 ohms and my speakers only take 250W, I think it should be in the neighborhood of -4 to -5 db but I don't know how to calculate it exactly, help...
One more, I'd like to set a high pass filter to try and prevent physical damage, what would you suggest I put in the frequency, slope and crossover type to accomplish that?
The Frequency Response of my speakers is:
55 Hz - 20 kHz (± 3 dB)
This is the frequency response chart if it helps
(http://cosmossound.co.kr/product/speaker/images/Ps12_chart1.gif)
Is that the ACTUAL response of the loudspeaker-or a response with procssing applied?
It makes a difference.
Maybe the make/model number of the loudspeaker would help
-
I've got one XTI 1002, and I control it using System Architect, rather than twiddling with the front panel. I don't know if it's available from the front panel, but in SA, you can set the limit as an actual voltage, which should be much easier to figure out. (Though I don't know if they're doing the fiddly stuff Ivan Beaver was mentioning on the Itechs here as well)
Just for the record-I have been playing around with the Itechs and the limiters some more. The usable numbers I enter to get the required output voltage, at the point of limiting that I want, are nowhere near the numbers that you would come up with by calculating using the wattage and the impedance of the loudspeaker.
In a couple of systems with the calculated voltages, the system would not get loud enough. But when the MEASURED voltages are used (actual input numbers are ignored-and adjusted to get the proper output voltages), the system get a good bit louder. One system gained 8dB. That is nothing to sneeze at.
This has happened on several large projects with the same result.
I don't know what it is, but I am sticking by my measured results and ignoring the calculated voltages.
To me the funny thing is when you pull up a JBL preset for a particular model (I only did this on a couple of models-so cannot speak for all of them), the amps limit the output well below (typically 6dBish) the continuous rating of the loudspeaker-this would be 12dB below the peak rating.
-
Is that the ACTUAL response of the loudspeaker-or a response with procssing applied?
It makes a difference.
Maybe the make/model number of the loudspeaker would help
this is a link to my speakers (http://cosmossound.co.kr/product/speaker/ps12.htm)
The frequency responce chart is there too and I have no idea if it's with processing or not.
-
this is a link to my speakers (http://cosmossound.co.kr/product/speaker/ps12.htm)
The frequency responce chart is there too and I have no idea if it's with processing or not.
Part of the reason I ask, is that the stated response (-3dB) is 55Hz, while on the response chart the -3dB point is below 40Hz.
Why would they cheat themselves on a spec like that? Maybe either the graph or the spec is wrong. It happens.
-
Part of the reason I ask, is that the stated response (-3dB) is 55Hz, while on the response chart the -3dB point is below 40Hz.
Why would they cheat themselves on a spec like that? Maybe either the graph or the spec is wrong. It happens.
Assuming the worst is the safest way to go so lets do that.
Is there a quick way to measure it directly from the speakers?
-
OK, I got it... I saw on another thread in another forum that the XTI takes dBfs units and not dB, this explains why I was confused about the units and what data to give the amp.
So the maximum power output 650W equals 0 dBfs, from here it's just a plain dB ratio formula from here (http://en.wikipedia.org/wiki/Decibel) and I get:
10 X Log(P1/P2)=10XLog(225/650)= -4.61 dBfs
Any suggestions on the HP filter for my speakers (http://web.archive.org/web/20020215124727/http://www.cerwinvega.com/products/professional/ps12.html)?
-
Part of the reason I ask, is that the stated response (-3dB) is 55Hz, while on the response chart the -3dB point is below 40Hz.
Why would they cheat themselves on a spec like that? Maybe either the graph or the spec is wrong. It happens.
Dear Ivan,
I sent an eMail to CV about that, maybe they can shed some light on the subject.
I also went through the charts of other ProStax speakers and found that the chart of the PS-12 (http://web.archive.org/web/20020215124727/http://www.cerwinvega.com/products/professional/ps12.html) to be identical to the chart on the PS-152 (http://web.archive.org/web/20020214114852/http://www.cerwinvega.com/products/professional/ps152.html) and it also fits that model's specs for the highs as well, so maybe it's just a simple mixup only with the charts.
In the mean time what do you say we consider the 55Hz -3dB as the correct data?
-
Connect a tone generator and run a moderately low signal thru. Sweep downwards from 100hz till you see the cone motion get to a minimum (it helps if you paint a small white dot on the cone) that frequency will be fB (frequency of the box) then set your high pass to that point. Woofers tend to "unload" below fB in ported boxes so excursion and distortion increase quickly. If you don't have a tone generator the you can download free versions off the net and run it thru your computer:)
I didn't hear from CV back yet but what do you think about the method above to measure the frequency for the HP filter?
-
Dear Ivan,
I sent an eMail to CV about that, maybe they can shed some light on the subject.
I also went through the charts of other ProStax speakers and found that the chart of the PS-12 (http://web.archive.org/web/20020215124727/http://www.cerwinvega.com/products/professional/ps12.html) to be identical to the chart on the PS-152 (http://web.archive.org/web/20020214114852/http://www.cerwinvega.com/products/professional/ps152.html) and it also fits that model's specs for the highs as well, so maybe it's just a simple mixup only with the charts.
In the mean time what do you say we consider the 55Hz -3dB as the correct data?
I would start with a Butterworth 24dB slope at 50Hz.
-
I would start with a Butterworth 24dB slope at 50Hz.
Thank you, will start from there.
When I use a limiter, how doe's it effect the need for amp headroom?
If I understand correctly, speakers have allot of headroom for transient peaks, if I'm using a limiter, wouldn't it essentially get rid of those peaks, ending up with a system that has less volume or less kick?
-
Thank you, will start from there.
When I use a limiter, how doe's it effect the need for amp headroom?
If I understand correctly, speakers have allot of headroom for transient peaks, if I'm using a limiter, wouldn't it essentially get rid of those peaks, ending up with a system that has less volume or less kick?
As with many things in the business-limiters are misunderstood and misapplied and expected to do things they shouldn't.
Basically there are 2 things that kill a loudspeaker-excessive temp (power over time-NOT just power alone) and overexcursion.
You can easily drive a loudspeaker really hard with even 1/2 cycle and destroy it-if it goes past its mechanical limits.
A "proper" limiter is fairly complicated-has many parameters and is not easily setup with a good bit of data. You really need a fast limiter (to help control the mechanical limits of the loudspeaker and to help keep the amps out of clipping)-a slow attack (to help control the thermal heating) and a level and freq dependant limiter (to help control the excursion).
And some people would argue you need more. But how many people have the skill/knowledge/data to properly set them up? Certainly not the average user. And with more control-comes more complexity and the possiblity of screwing it up.
A limiter is something you should never need-it is there "just in case". If you are RELYING on the limiter (always into limiting for example) then you need a bigger rig-and are just asking for trouble.
Think of a limiter like the air bags in your car. Do you rely on the bags every time you stop? Are you constantly running into things to stop? Safe driving (like safe operation of your PA) is what keeps them from going off.
And don't expect the air bags to totally protect you. They offer a "measure" of protection. If you drive your car off a cliff-don't complain when the air bags didn't work properly and you are dead.
There is no magic-limiter setting-kick drum mic-subwoofer- etc etc. It is a whole lot of "it depends" and variables.
OPERATING your system within normal limits is the key-NOT relying on some form of "protection". Just like sex-but we won't go there.
-
As with many things in the business-limiters are misunderstood and misapplied and expected to do things they shouldn't.
Basically there are 2 things that kill a loudspeaker-excessive temp (power over time-NOT just power alone) and overexcursion.
You can easily drive a loudspeaker really hard with even 1/2 cycle and destroy it-if it goes past its mechanical limits.
A "proper" limiter is fairly complicated-has many parameters and is not easily setup with a good bit of data. You really need a fast limiter (to help control the mechanical limits of the loudspeaker and to help keep the amps out of clipping)-a slow attack (to help control the thermal heating) and a level and freq dependant limiter (to help control the excursion).
And some people would argue you need more. But how many people have the skill/knowledge/data to properly set them up? Certainly not the average user. And with more control-comes more complexity and the possiblity of screwing it up.
A limiter is something you should never need-it is there "just in case". If you are RELYING on the limiter (always into limiting for example) then you need a bigger rig-and are just asking for trouble.
Think of a limiter like the air bags in your car. Do you rely on the bags every time you stop? Are you constantly running into things to stop? Safe driving (like safe operation of your PA) is what keeps them from going off.
And don't expect the air bags to totally protect you. They offer a "measure" of protection. If you drive your car off a cliff-don't complain when the air bags didn't work properly and you are dead.
There is no magic-limiter setting-kick drum mic-subwoofer- etc etc. It is a whole lot of "it depends" and variables.
OPERATING your system within normal limits is the key-NOT relying on some form of "protection". Just like sex-but we won't go there.
Brilliant. Thank you, Ivan.
-
As with many things in the business-limiters are misunderstood and misapplied and expected to do things they shouldn't.
Basically there are 2 things that kill a loudspeaker-excessive temp (power over time-NOT just power alone) and overexcursion.
You can easily drive a loudspeaker really hard with even 1/2 cycle and destroy it-if it goes past its mechanical limits.
A "proper" limiter is fairly complicated-has many parameters and is not easily setup with a good bit of data. You really need a fast limiter (to help control the mechanical limits of the loudspeaker and to help keep the amps out of clipping)-a slow attack (to help control the thermal heating) and a level and freq dependant limiter (to help control the excursion).
And some people would argue you need more. But how many people have the skill/knowledge/data to properly set them up? Certainly not the average user. And with more control-comes more complexity and the possiblity of screwing it up.
A limiter is something you should never need-it is there "just in case". If you are RELYING on the limiter (always into limiting for example) then you need a bigger rig-and are just asking for trouble.
Think of a limiter like the air bags in your car. Do you rely on the bags every time you stop? Are you constantly running into things to stop? Safe driving (like safe operation of your PA) is what keeps them from going off.
And don't expect the air bags to totally protect you. They offer a "measure" of protection. If you drive your car off a cliff-don't complain when the air bags didn't work properly and you are dead.
There is no magic-limiter setting-kick drum mic-subwoofer- etc etc. It is a whole lot of "it depends" and variables.
OPERATING your system within normal limits is the key-NOT relying on some form of "protection". Just like sex-but we won't go there.
Great writing :)
You made me laugh for a good few minutes, Thank you.