Help me understand system limiting- setting it

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BJ James

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Jan 11, 2011
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I've been spending the past week or so trying to get a grip on setting up limiters.
Thus far I get the concept that all amps in the system must be set up to have matched voltage gains (x40/32db or x20/26db). With this, I also get that the gain controls on the front of the amp do not lower the available output of the amp, they only lower(or raise) the input sensitivity of the amp to reach full power. Just about everything beyond that has gone over my head. I want to be able to figure this out.

I have a manufacturers DSP with a preset loaded for one of their systems. For an example I would like to take the mid driver, and the limiting scheme from the processor. Here are the numbers from the DSP output limiting:

******************************
"1x 12 Woofer: 57 Volts @ x40 gain = 1.425 Volts
@ x20 gain = 2.85 Volts
Threshold: +4 dB
Ratio: 20:1
Attack: 60 mS
Release: 600 mS"
*******************************
What I think I see here is the DSP is allowing only 1.425 volts max to the amp. This is where I get fuzzy. If all the amps in the system have the same gain (x40 or 32db) and the DSP is only allowing 1.425 volts to the amp...does it really matter if I use an amp with way more wattage than the driver can handle?
The driver in this case is rated at 1000 watts program @ 8ohms. I'm not sure if you can determine that by the DSP settings, but that's what the manufacturer shows. The amp I intend to use is a PL380, but I would like to know the math so I can figure it out for other amps too.
I think I have worn out my welcome with some of the manufacturer reps trying to get them to explain it to me.
Thanks in advance.
BJ
 
Re: Help me understand system limiting- setting it

You are correct- the amplifier's maximum output voltage (and associated wattage into a load) doesn't matter. Limiting the amplifier's input voltage limits its maximum output voltage (and again, the watts), which is your goal.

You do not need all amp gains to be the same, although it does help with calculations since one more variable is removed and confusion is reduced. When using carefully set limiters, it is best to calculate them with the amp gain turned all the way up, so no one turns the knob up later, effectively rendering your limiter useless.

There is only one problem: if you do wrap your head around the calculations involved with getting your DSP settings to equate to amplifier outputs, how do you know what actual limiter setting is best? You can't brickwall a speaker, it sounds really bad. So where do you engage a soft 'RMS' limiter, where do you engage a peak limiter, and what about a thermal limiter? Where does the speaker actually fail, such that you can prevent failure by preventing that condition? You don't want to limit too early, reducing maximum output, but limiting too late is essentially the same as no limiter.

I have a tip: think of amplifier output as voltage, because that's what it is. If you have 1 volt going into the amplifier and it has a 40x gain (32dB), then 40 volts are coming out, which is 200 watts at 8 ohms.

Formula for gain:
Gain (dB) = 20*log(voltage 2/voltage1)

Forumla for watts from voltage:
P (watts) = Voltage^2/R (nominal impedance usually)

I hope this helps.

Edit: and one more thing. You have to check what scale your DSP is using for output voltage. The common one is dBu, but some use dBv I think. One is referenced to 1 volt, the other to .775 volts, at some impedance, IIRC. You should research and see exactly what your DSP is using, because you have to turn your 'amp input voltage' which is your DSP output voltage into dBu again for entering into the DSP.
 
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Re: Help me understand system limiting- setting it

Silas gave you some excellent information.

The only thing that I would add is that you should be careful so that you aren't in limit the entire time. Although limit can protect your drivers from spikes a system that is in limit the entire time can possibly damaging your drivers. This happens when your complex sine waves of the "push-pull" in your divers become square waves causing the drivers to not travel their intended distances. I've only seen it happen once in a show to HF Drivers. The fault belonged to a guest engineer that couldn't keep his mix under control.
 
Re: Help me understand system limiting- setting it

BJ James said:
What I think I see here is the DSP is allowing only 1.425 volts max to the amp. This is where I get fuzzy. If all the amps in the system have the same gain (x40 or 32db) and the DSP is only allowing 1.425 volts to the amp...does it really matter if I use an amp with way more wattage than the driver can handle?
Click to expand...

To answer your specific question, no it doesn't matter the size of the amp as long as it's bigger and limited correctly. That said, you better limit it, and you better get the settings right.

This is purely an observation from my own experience, but if your amp is rated between the RMS rating of the speaker and two times that value, and you keep the amps out of clipping, setting limiters is not as critical. Again, just my personal experience. I'm only really anal about limiter settings with my stuff when the amp is 2-4x the rating of the speaker or more.
 
Re: Help me understand system limiting- setting it

When doing the calculations remember that the "amplifier" is a voltage amplifier-NOT a power amplifier.

This is important when you think of it this way. At any point below the max output capability the following is true. It starts to be a bit "untrue" win a dB or 2 of full output.

Let's say you put in 1 volt and the amplifier "puts out" 100 watts into 8 ohms.

Now apply the same 1 volt input-but hook up a 2 ohm loudspeaker. The "power" being applied across that load is 400 watts.

The voltage across the load is the same-irregardless of the impedance load (again below max outputs), so the voltage gain is the same, but the power output would be different.

So you need to be careful to take the impedance of the loudspeaker into account when figuring out the applied signal.

The more you think in terms of voltage-the better off you will be.
 
Re: Help me understand system limiting- setting it

Bennett Prescott said:
BJ,

I wrote this article to try and explain proper limiting. Hopefully it will make things clearer for you.

http://www.soundforums.net/content/149-Loudspeaker-Fundamentals
Or download the PDF from my website, http://www.bennettprescott.com
Click to expand...

Very informative replies everyone. I thank you very much for the explanations. Of course every reply brings up more questions.
Bennett, not to oversimplify, but in your article you inply that peak limiting is rather useless, and if I'm not mistaken, the numbers I posted would indicate my DSP uses peak limiting. Would you suggest I not use the peak limiting and just try and use proper size amps?
BJ
 
Re: Help me understand system limiting- setting it

BJ James said:
I've been spending the past week or so trying to get a grip on setting up limiters.
Thus far I get the concept that all amps in the system must be set up to have matched voltage gains (x40/32db or x20/26db). With this, I also get that the gain controls on the front of the amp do not lower the available output of the amp, they only lower(or raise) the input sensitivity of the amp to reach full power. Just about everything beyond that has gone over my head. I want to be able to figure this out.

BJ
Click to expand...

BJ,

I have attached an Excel spreadsheet I wrote that will help you calculate things like amp input level to reach limiting. You enter the things in gray for the amplifer, and the spreadsheet calculates the things in blue. As you play around with the spreadsheet, it should help solidify things for you.

View attachment Amplifier Gain Worksheet.xls

Finally, remember that your amplifier is a voltage source. If the amplifier puts out 1000W into 8 Ohms, but only 1900W into 4 Ohms, the amplifier's voltage gain has not changed from the 8 Ohm load to the 4 Ohm load, but the limiter has made the apparent voltage gain at 4 Ohm seem smaller on the spec sheet.
 
Re: Help me understand system limiting- setting it

BJ James said:
Bennett, not to oversimplify, but in your article you inply that peak limiting is rather useless, and if I'm not mistaken, the numbers I posted would indicate my DSP uses peak limiting. Would you suggest I not use the peak limiting and just try and use proper size amps?
Click to expand...
BJ, I never intended to reply that peak limiting is useless! However, the reason for peak limiting is to prevent over excursion. If you are not having an over excursion problem, or if you do not have enough amp power to exceed the excursion limits of your drivers, then you do not need to worry about it. Your DSP does not allow you to do thermal limiting, which I think is a greater benefit since one can use amp size as a "free" peak limiter, but you need thermal limiting to keep your drivers out of power compression and safe from real long term abuse. Almost everyone has enough amp to burn up their drivers, but I doubt most people have enough amp to destroy their drivers suspension.

Now if only I had limiters that allowed sidechain EQ to truly optimize to the thermal and excursion characteristics of the driver in the enclosure, as well as motor thermal modeling to allow me to adjust the peak limiter based on the likely impedance of the voice coil.

P.S. The limiters in the Crown iTech and iTech HD series amps are fucking great. I wish I had them damn everywhere. Saves me a lot of math, solves the problem.
 
Re: Help me understand system limiting- setting it

Bennett Prescott said:
Now if only I had limiters that allowed sidechain EQ to truly optimize to the thermal and excursion characteristics of the driver in the enclosure, as well as motor thermal modeling to allow me to adjust the peak limiter based on the likely impedance of the voice coil.
Click to expand...

I'd like this too but I'm not sure we'll ever see it "in the wild".... ie outside of the current proprietary/blackbox implementations.
 
Re: Help me understand system limiting- setting it

OK, I had to take a little break to absorb some of this. I'm still not there.
If I take 3 of these speakers- here's the DSP settings again:

******************************
"1x 12 Woofer: 57 Volts @ x40 gain = 1.425 Volts
@ x20 gain = 2.85 Volts
Threshold: +4 dB
Ratio: 20:1
Attack: 60 mS
Release: 600 mS"
*******************************
and I want to power the 3 off one side of a PL380, I take the Forumla for watts from voltage:
P (watts) = Voltage^2/R

Watts= 1.425x1.425 (2.03) / 8= 0.25

OK, that's wrong. I don't take the DSP settings to do this. I must take the amps setting to calculate that...no. Fuck

Edit:
ok it's the 57v number I need to multiply then divide by the speakers ohm rating (8), gives me 406. Really? the speaker has a rating of 1000 watts program but is limited at 400 watts? God I need help lol.

FYI the DSP unit in question is the Peavey VSX26. In the manual it says attack settings over 50ms are performing RMS limiting, settings below 50ms are considered peak limiting. How is one different from the other by a simple difference in attack?
 
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Re: Help me understand system limiting- setting it

Yes, 57 Vrms @ 8 ohm nominal is ~400W.

You have to realize what "1000 Watts Program" means is: "It's permissible to power this speaker with a 1000 Watt amp if you are playing typical program music with a crest factor of 10-15dB.

"500 Watts RMS" means: "This is the long term thermal rating (without likely failure) using a signal with 6dB crest factor (typical pink noise)."
 
Re: Help me understand system limiting- setting it

That's right. So I can use an amp "up to" 1000 watts per speaker which I guess is my headroom to allow peaks. And limit with a slow attack of 60ms to allow peaks through, but clamp down on extended abuse. OK, Let me absorb some more.
Thanks,
BJ
 
Re: Help me understand system limiting- setting it

BJ,

60 milliseconds is not a slow attack. Attack time for thermal limiting should be seconds. The DRPA does not have attack times of this magnitude available. The DRPA cannot do thermal limiting without destroying the output capability of the speaker.
 
Re: Help me understand system limiting- setting it

Bennett Prescott said:
BJ,

60 milliseconds is not a slow attack.
Click to expand...

I never understood attack and release values with out some reference to dB, i.e. dB/s or s/dB. With just time, no matter how hot the signal is, it's squashed in the same time. Seems this varies across the industry.


I think limiters (even peak ones, and maybe only peak ones since RMS seems to hard to roll on your own) are good to have simply because it's a stop light. This is especially true if you use much larger amps than the driver is rated for. Setting a peak limiter for the Program rating of a speaker with a slow attack shouldn't affect the sound to horribly, and it lets you know.. "hey you are pushing it."
 
Re: Help me understand system limiting- setting it

OK, It's starting to sink in. There are a lot of variables.
Let's see if I can figure out what I'm doing for the ribbons(tweeters).
Here are the DSP settings for them:

"Ribbon: 36 Volts @ x40 gain = 0.9 Volts
@ x20 gain = 1.8 Volts
Threshold: -2.0 dB
Ratio: 20:1
Attack: 10 mS
Release: 100 mS"

The ribbon is rated for 160W program (16 ohms). According to the the DSP specs, it wants to see wattage of around 80watts before limiting occurs.
I plan on running 4 off a channel of a PLX3002 which will then see a 4 ohm load. The PLX3002 is rated at 900watts/channel at 4 ohms. That's a little more than I should have. And worse still if I only run 2 or 3 ribbons per channel.
I guess the real answer is to have a smaller amp on the ribbons. I'm not sure if I should change anything in the limiting.... it's Peavey's DSP, with their settings, for their speakers.
What would you do?
BJ
 
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Re: Help me understand system limiting- setting it

BJ James said:
OK, It's starting to sink in. There are a lot of variables.
Let's see if I can figure out what I'm doing for the ribbons(tweeters).
Here are the DSP settings for them:

"Ribbon: 36 Volts @ x40 gain = 0.9 Volts
@ x20 gain = 1.8 Volts
Threshold: -2.0 dB
Ratio: 20:1
Attack: 10 mS
Release: 100 mS"

The ribbon is rated for 160W program (16 ohms). According to the the DSP specs, it wants to see wattage of around 80watts before limiting occurs.
I plan on running 4 off a channel of a PLX3002 which will then see a 4 ohm load. The PLX3002 is rated at 900watts/channel at 4 ohms. That's a little more than I should have. And worse still if I only run 2 or 3 ribbons per channel.
I guess the real answer is to have a smaller amp on the ribbons. I'm not sure if I should change anything in the limiting.... it's Peavey's DSP, with their settings, for their speakers.
What would you do?
BJ
Click to expand...

I'd get a different speaker system, but that's not what you want to hear.

Another serving of worms in a can for you:

The ribbons handle far less power down where the transition to the 12” sounds decent.
Given too much power they will die from over excursion crossed low, and heat death when crossed higher.

Peavey has different presets for various types of abuse levels, raising the crossover point for “rock and roll (or whatever they call it now)”, allowing the 12” to beam like hell compared to the wide dispersion ribbon .

The lower crossover frequency requires a more aggressive peak limiter to keep the ribbons safe.
Peavey took care of this problem by adding in hi mid cones between the 12" and ribbons on the step up model.

Now, back to the regularly scheduled discussion.
 
Re: Help me understand system limiting- setting it

Ya, let's not get this too sidetracked if you don't mind. This system fills a hole that my others can't. Just need help with my limiting (or limited :) knowledge.
Anyone?
Cheers,
BJ
 
Re: Help me understand system limiting- setting it

BJ,

All the deaths of ribbons in those cabinets that I have seen have been excursion, so I would use Peavey's settings. I have no idea how to check for excursion issues in a ribbon. In a cone I either measure or do it by ear...
 
Re: Help me understand system limiting- setting it

Thanks Bennett. I'm satisfied using PV's settings, just wasn't sure if I should do something different when using too large an amp. Back to original post, I guess the DSP is limiting what the amp see coming in, therefore limiting what goes out.
BJ
 
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