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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 Topic: Calculating limiters
    Posted: 23 November 2014 at 12:24pm
I've just taken delivery of a 4-channel TSA 4-300 amp from Thomann, to power my first small system comprising 1x 12" tapped horn sub & 2x 8"+1" mid-tops. I have a DCX for processing duty but need some guidance on correctly setting the limiters so I don't fry any voice coils.

The amp is rated at 350W per channel into 8 ohms (for the mid-tops) & 1000W bridged into 8 ohms (for the sub). Input sensitivity is selectable between 0.775v & 1.4v, voltage amplification is given as 35.3dB.

The mid-tops are Brooke INH-8 rated at 200W. The sub is a THAM-12 loaded with a PD.12SB30 which is rated at 400W AES, 1.6kW peak (6dB crest), but I know I will also have to take excursion into account for the sub?

I've tried using the calculator here --> http://www.poulpetersen.dk/Appn/gblimthc.html <-- but the numbers don't add up to what the spec sheet for the amp says.

Any guidance?


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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: 23 November 2014 at 4:16pm
Okay, so I've discovered that the meters in the DCX are dBFS & that as the unit is rated for maximum input & output of +22dBU that means 0dB on the DCX is +22dBU. This explains quite a bit!

But it also means that the maximum -24dB setting on the limiters isn't very useful, as that means any limiter setting beneath -2dB can't be done? Because just to get to 0dB you need to dial in -22dB onto the limiters?

Luckily I can switch the amp from 0.775v input sensitivity to 1.4v input sensitivity though, which I think will help.

If I put the numbers using 1.4v sensitivity into --> http://www.poulpetersen.dk/Appn/gblimthc.html <-- they still don't add up, but I get the following;

Mid-tops

Impedance 8 Ohms
Max. power 200 Watts
Max. voltage 40 Volts

Input sensitivity 1.4 Volts
Output power 350 Watts
Nominal load 8 Ohms
Voltage gain 31.549019599857427 dB
Threshold 2.7106677228653835 dBU

Sub

Impedance 8 Ohms
Max. power 400 Watts
Max. voltage 56.568542494923804 Volts

Input sensitivity 1.4 Volts
Output power 1000 Watts
Nominal load 8 Ohms
Voltage gain 36.10833915635467 dB
Threshold 1.1616481230079592 dBU

The voltage gains don't match up with the amplifier spec sheet, which claims 35.3dB?

But would I then set the limiters at something around 22 - 2.7 (-19.3) for the mid-tops & 22 - 1.16 (-20.8) for the sub? Intuitively that seems correct, as it limits before any of the channels on the amp clip.
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mini-mad View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mini-mad Quote  Post ReplyReply Direct Link To This Post Posted: 23 November 2014 at 4:31pm
Im gonna watch this carefully as i still find it hard at times to get my head around this limiter thing...

If it sounds like a gorilla is trying to escape, turn it down.
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jammin75 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jammin75 Quote  Post ReplyReply Direct Link To This Post Posted: 23 November 2014 at 4:33pm
whats a limiter ???  LOLLOL
feel the vibes !!!   "Who Feels it Knows it"            Strong like Lion              
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mini-mad View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mini-mad Quote  Post ReplyReply Direct Link To This Post Posted: 23 November 2014 at 4:34pm
Originally posted by jammin75 jammin75 wrote:

whats a limiter ???  LOLLOL
If it sounds like a gorilla is trying to escape, turn it down.
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jammin75 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jammin75 Quote  Post ReplyReply Direct Link To This Post Posted: 23 November 2014 at 4:36pm
never used them yet Big smile
feel the vibes !!!   "Who Feels it Knows it"            Strong like Lion              
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mini-mad Quote  Post ReplyReply Direct Link To This Post Posted: 23 November 2014 at 4:47pm
I lent a small system out for a house party recently... asked them what they are hooking up to it "laptop and iphone" was the answer i got so gave them a cable to go straght into the amp. Knowing the output of either device would not be enuff to fully drive the system was a sort of limiting factor for the whole system. So, no matter how hard they drove the iphone or laptop they could not do any damage to the small rig.

And none of them were intelligent enuff to downlkad anything to artifically boost the laptops output the get more volume :-)
If it sounds like a gorilla is trying to escape, turn it down.
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jammin75 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jammin75 Quote  Post ReplyReply Direct Link To This Post Posted: 23 November 2014 at 4:51pm
im always out with my set(s) if i get it wrong then its my fault  LOL   over 50 gigs and everything still runnin sweet Thumbs Up
feel the vibes !!!   "Who Feels it Knows it"            Strong like Lion              
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lost eden View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote lost eden Quote  Post ReplyReply Direct Link To This Post Posted: 23 November 2014 at 5:03pm
Yeah, I have to contend with the possibility of DJM's that kick out +26dB!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GEB Quote  Post ReplyReply Direct Link To This Post Posted: 23 November 2014 at 10:44pm
I usually set my limiters to "smack round the back of head with a rolled up copy of soundonsound!"
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Grubbah Quote  Post ReplyReply Direct Link To This Post Posted: 24 November 2014 at 3:46am
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



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Post Options Post Options   Thanks (0) Thanks(0)   Quote bitSmasher Quote  Post ReplyReply Direct Link To This Post Posted: 24 November 2014 at 4:31am
Get a multimeter and make some sine waves
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