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Calculating limiters

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lost eden View Drop Down
Young Croc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote lost eden Quote  Post ReplyReply Direct Link To This Post Posted: 24 November 2014 at 7:02pm
Thanks for the clear explanation & examples Grubbah! I had to read through it several times but I think it's finally making sense to me. Knowing that voltage gain is different depending upon load for this amp clears up a lot, but I guess if I'd thought about it I should've realised this myself by the simple fact that the rated power into different loads/bridge configurations aren't clean multiples!

I ran through the calculations again, assuming each Brooke cab on a 350W/8Ω channel & the sub on 2x channels bridged to 1000W/8Ω, I come up with this;

Brooke INH-8

0.775 V input sensitivity
36.68554626329598 dB voltage gain
-2.4258589405731716 dBu limiter

-24.4258589405731716 dB limiter on DCX

=======

PD.12SB30

0.775 V input sensitivity
41.244865819793226 dB voltage gain
-3.974878540430603 dBu limiter

-25.974878540430603 dB limiter on DCX

==============================================

Brooke INH-8

1.4 V input sensitivity
31.549019599857427 dB voltage gain
2.7106677228653835 dBu limiter

-19.2893322771346165 dB limiter on DCX (same as your example, so I think I'm doing this right!)

=======

PD.12SB30

1.4 V input sensitivity
36.108339156354674 dB voltage gain
1.1616481230079492 dBu limiter

-20.8383518769920508 dB limiter on DCX

==============================================

So if I set the amp to 0.775V input sensitivity, I can't dial in the limiters in the DCX low enough (their range is -24dB to 0dB) & would have to use its output gains to 'make up' the discrepancy.

But if I set the amp to 1.4V input sensitivity. I can dial in a -19.3dB threshold for the limiters for the Brooke's & a -20.9dB threshold for the limiter for the sub, leaving the output gains at 0dB.

Correct so far?

And am I right in thinking that as these limiters in the DCX are 'zero' attack, these settings will allow me to play band-limited pink noise right up to the continuous power handling of the speakers, but won't allow crests above the continuous power handling (eg the sub is rated for 1.6kW 6dB crests, but with the -20.9dB limiter any signal that would result in the amp's output to the sub going above 400W (even for a moment) will be clobbered)?


Edited by lost eden - 24 November 2014 at 7:10pm
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junoprobelaunch View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote junoprobelaunch Quote  Post ReplyReply Direct Link To This Post Posted: 05 July 2018 at 9:30am
Originally posted by Grubbah Grubbah wrote:

So first off you need to learn the relationship between input sensitivity and voltage gain.

Input sensitivity is the amount of signal required to put the amplifier at full power. Voltage gain is the steps of gain it takes to reach full power.

They are conversely interlinked.. i.e. the higher the input sensitivity, the lower the voltage gain. The higher voltage gain, the lower the input sensitivity.

Now the Thoman manual is actually a little miss-leading in that the voltage gain displayed is when the amplifier is on a 4 ohm load with an input sensitivity of 0.775V.

0.775V = to 0dBU (signal at which amplifier will be at full and producing 550W

Change the input sensitivity to 1.4V and the voltage gain drops to around 30.5dB 

1.4V = +5dBU (ish).. So your amplifier requires an extra 5dB worth of signal to reach its full power and its dropped by 5dB on the voltage gain.. therefore this effects your limiter point, by 5dB

In this particular case, the amp doesn't produce double its 8 ohm power on a 4 ohm load, otherwise you would expect the voltage gains to be relatively the same on either load... in this case on a 8 ohm load with an input sensitivity selected at 0.775V the voltage gain is actually around 36.6dB

Because logarithmically input sensitivities and voltage gains are interlinked, we use the voltage gain vs the RMS power of our drivers to determine a limiter point. The limiter calculator you use is fine, though if you certain of the voltage gain;

http://www.funktion-one.com/settings/ - this is a little quicker. 

A lot of amplifiers have either selectable voltage gains or input sensitivities. The difference is as follow; 

Say you had a 350 - 400W RMS speaker, with an amplifier that had a selected gain of 32dB, it would be roughly 1.4V input signal for an amplifier to achieve 350 - 400W... which is roughly 5dBU, so your limiter point would be  roughly 5dBU or -17dBFS on the ultra drive. ** Rounding up decimal places here **. So actually it doesn't matter what size amplifier you put on this 350W/400W speaker (providing it matches or exceeds 350/400W) your limiter point will not change as long as the voltage gain is the 32dB

For example, say my amplifier was 2000W @ 8 ohms, with a selected gain of 32dB, it requires around 3.1V (+12dBU) to reach its full 2000W... however to reach 350 / 400W, it still requires 1.4V (+5dBU)... double that, stick a 4000W @ 8 ohm amplifier on your tiny driver... at a voltage gain of 32dB, it needs around 15.5dBU to reach the full 4000W... it still only needs 1.4V (+5dBU) to hit 350 / 400W.

This is not true with selectable input sensitivities;

Take our 2000W @ 8ohm amplifier... Lets pretend you have selected 0.775V as an input sensitivity. It requires 0dBU to reach its full 2000W, so our limiter point is around -7dBU, which will limit the amplifier around 350/400W... Now lets take out 4000W @ 8ohm amplifier. You select 0.775V as an input sensitivity and your limiter point becomes around -10dBU! This is because both amplifiers require the same amount of signal to full output, however achieve different outputs with alternating voltage gains! Where our 2000W amplifier would have a voltage gain of around 44dB, the 4000W amplifier would have a voltage gain of 47dB. 

If your amplifier doesn't have selectable voltage gains or input sensitivities, you can alter this by using your the output gains on your LMS. Consqeuntly, every time you change your output gains on your LMS, you need to alter your limiter point!

SO lets say are running your TA amp on a 8 ohm load on 1.4V (31.5dB voltage gain), the limiter point for your Brooke mid top is +2.7dBU or -19.3 dBFS. This is at 0dB on your LMS output gain. If you move this to -1dB, this correction technically means you are running at 30.5dB voltage gain, therefore your new limiter point will be +3.7dBU or -18.3 dBFS. Its the same in reverse order... notch your LMS output gain to +1, this correction means you are running at 32.5dB voltage gain, therefore your new limiter point would be +1.7dBU or -20.3dBFS

Can you see how this starts to relate to calculating limiter points? 

My advice would be;

. Choose at what input signal you like to run your system
. Work out the limiter points
. Balance your system how you see fit and remember all alterations with gain need to be accounted for




That was literally so helpful, even four years later 
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lost eden View Drop Down
Young Croc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote lost eden Quote  Post ReplyReply Direct Link To This Post Posted: 09 July 2018 at 11:39am
And my drivers are all still alive four years later Smile
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bass*en*mass View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bass*en*mass Quote  Post ReplyReply Direct Link To This Post Posted: 11 July 2018 at 8:02pm
theres a simple calculator available: 
works offline too.. ;) 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote toastyghost Quote  Post ReplyReply Direct Link To This Post Posted: 11 July 2018 at 8:53pm
Originally posted by bass*en*mass bass*en*mass wrote:

theres a simple calculator available: 
works offline too.. ;) 


But most boxes (or drivers) cannot handle the AES rating for any length of time so a simple calculator can lead to dead kit
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bass*en*mass Quote  Post ReplyReply Direct Link To This Post Posted: 11 July 2018 at 9:26pm
can, not must, as a starter it helps imo..
real world impedance in most cases is higher than nominal afaik anyways hence nominal limiting with a simple calculator seems better than no starting point :) 


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