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Author Topic: Calculating Speaker Limiter Settings  (Read 35568 times)

Chris Delcambre

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Calculating Speaker Limiter Settings
« on: March 23, 2011, 03:43:10 PM »

Hi Everyone,

I have calcuated the speaker limiter settings for my Peavey QW (4 QW4s over 4 QW218s)system driven with 8-XTI4000(Subs and Lows) and 2-XTI1000(Highs 1 per channel).

Here are the specifications I used to calculate the settings. Please let me know if I have done something wrong. I am new at doing things like this. I have been running PA for 10+ years but never gotten into actual configuration settings.

XTI1000

30.5dBu Voltage Gain
275W @ 8ohms
1.4 Volts

XTI4000

34.2dBu Voltage Gain
3200@4ohms
1.4 Volts

QW4

Hi's- 200W@8ohms
Lows- 2400W@4ohms

QW218
Subs-3200W@4ohms

The limiter settings I calculated are:
Hi's 0dBu
Lows 3.84dBu
Subs 5.09dBu

My processor is setup for +4dBu input.

Thanks for you help.
Chris
« Last Edit: March 24, 2011, 10:25:54 AM by Chris Delcambre »
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Mac Kerr

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Re: Calculating Speaker Limiter Settings
« Reply #1 on: March 23, 2011, 05:41:05 PM »

Here are the specifications I used to calculate the settings. Please let me know if I have done something wrong. I am new at doing things like this. I have been running PA for 10+ years but never gotten into actual configuration settings.

XTI1000
30.5dBu Voltage Gain
275W @ 8ohms
1.4 Volts

XTI4000
34.2dBu Voltage Gain
3200@4ohms
1.4 Volts
SNIP...
The limiter settings I calculated are:
Hi's-0dBu
Lows-3.84dBu
Subs-5.09dBu

My processor is setup for +4dBu input.

Thanks for you help.
Chris

The first thing to keep in mind is that when you use mono bridge mode the amp gain goes up by 6dB, so anywhere you are using a bridged mono amp you want to start by lowering the gain by 6dB.

With a sensitivity of 1.4V each amp will reach its maximum output with about a +5dBu input level. (after the bridged amps have been turned down 6dB)

With that in mind, it seems like you want to limit your high frequency amp to about 1.4dB below its maximum output (200W), your low frequency amp to about 1.25dB below its maximum output (2400W), and your sub amp to its maximum output (3200W).

How did you calculate the values you listed?

Mac

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Chris Delcambre

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Re: Calculating Speaker Limiter Settings
« Reply #2 on: March 23, 2011, 06:53:52 PM »

The first thing to keep in mind is that when you use mono bridge mode the amp gain goes up by 6dB, so anywhere you are using a bridged mono amp you want to start by lowering the gain by 6dB.

With a sensitivity of 1.4V each amp will reach its maximum output with about a +5dBu input level. (after the bridged amps have been turned down 6dB)

With that in mind, it seems like you want to limit your high frequency amp to about 1.4dB below its maximum output (200W), your low frequency amp to about 1.25dB below its maximum output (2400W), and your sub amp to its maximum output (3200W).

How did you calculate the values you listed?

Mac

Thanks mac. I used the formulas on this website.

http://www.poulpetersen.dk/Appn/gblimthr.html

Also you mentioned that when bridging you must reduce gain by 6dB. Do I do that on the input gain of the amp, processor, front of amp channel gain??? Also why is this???

If I am understanding you right, then if the output of my processor is set to +4dBu then I would set the limiter to -2.6 dBu on the highs, lows to -2.75dBu, and the sub amp output no limit???

Thanks for the help Mac. I am new at getting this in depth with the actual electrical workings of the gear and am trying to get an understanding it.
« Last Edit: March 23, 2011, 07:00:50 PM by Chris Delcambre »
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Ivan Beaver

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Re: Calculating Speaker Limiter Settings
« Reply #3 on: March 24, 2011, 07:28:36 AM »

Thanks mac. I used the formulas on this website.

http://www.poulpetersen.dk/Appn/gblimthr.html

Also you mentioned that when bridging you must reduce gain by 6dB. Do I do that on the input gain of the amp, processor, front of amp channel gain??? Also why is this???


The reason the amp picks up 6dB of gain is that when you bridge an amp-the output voltage is doubled.  Doubled voltage = 6dB.  So your limiters need to be set 6dB lower in front of the amp.

Now if you turn the amp gain back 6dB or the output of the processor, then you can leave them the same.
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Nick Hickman

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Re: Calculating Speaker Limiter Settings
« Reply #4 on: March 24, 2011, 09:14:10 AM »

Hi Chris,

If you know the amplifier's gain, you can forget about sensitivity and maximum output power for the purposes of calculating limiter thresholds.  (The gain figure tells the whole story; the sensitivity and max power don't, though you can use them to calculate an estimate for gain.)

The limit thresholds you show (assuming no bridging) are all about 4dB below the driver power ratings you show but, looking at the box specs, you seem to have chosen the "program" ratings, i.e. 3dB above continuous, so the thresholds are only 1dB below the drivers' continuous ratings.  If it were me, I'd want to be a couple of dB lower.

If P is the power limit in watts, Z is the driver's nominal impedance in ohms, and G is the amplifier gain in dB (not dBu!), the limiter threshold in dBu is just 10*log(P*Z/0.6)-G.

As Mac mentioned, amp gain goes up 6dB when bridged.

BTW, don't use a hyphen in "Subs-5.09dBu": it looks like a minus sign!

Nick
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Chris Delcambre

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Re: Calculating Speaker Limiter Settings
« Reply #5 on: March 24, 2011, 10:25:14 AM »

Hi Chris,

If you know the amplifier's gain, you can forget about sensitivity and maximum output power for the purposes of calculating limiter thresholds.  (The gain figure tells the whole story; the sensitivity and max power don't, though you can use them to calculate an estimate for gain.)

The limit thresholds you show (assuming no bridging) are all about 4dB below the driver power ratings you show but, looking at the box specs, you seem to have chosen the "program" ratings, i.e. 3dB above continuous, so the thresholds are only 1dB below the drivers' continuous ratings.  If it were me, I'd want to be a couple of dB lower.

If P is the power limit in watts, Z is the driver's nominal impedance in ohms, and G is the amplifier gain in dB (not dBu!), the limiter threshold in dBu is just 10*log(P*Z/0.6)-G.

As Mac mentioned, amp gain goes up 6dB when bridged.

BTW, don't use a hyphen in "Subs-5.09dBu": it looks like a minus sign!

Nick

Is it 10*log(P*Z/0.6)-G or 20*log(P*Z/0.6)-G??? The website I used to calculate those settings said 20... That may be part of the problem.
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Chris Delcambre

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Re: Calculating Speaker Limiter Settings
« Reply #6 on: March 24, 2011, 10:26:53 AM »

The reason the amp picks up 6dB of gain is that when you bridge an amp-the output voltage is doubled.  Doubled voltage = 6dB.  So your limiters need to be set 6dB lower in front of the amp.

Now if you turn the amp gain back 6dB or the output of the processor, then you can leave them the same.

Which way is proper or does it matter???
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Nick Hickman

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Re: Calculating Speaker Limiter Settings
« Reply #7 on: March 24, 2011, 11:20:54 AM »

Hi Chris,

Is it 10*log(P*Z/0.6)-G or 20*log(P*Z/0.6)-G??? The website I used to calculate those settings said 20... That may be part of the problem.

In that form, it's 10 times.  Where were you looking?  It may also be presented as 20*log(sqrt(P*Z)/0.775)-G.  The two are equivalent.

Nick
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Chris Delcambre

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Re: Calculating Speaker Limiter Settings
« Reply #8 on: March 24, 2011, 11:30:00 AM »

Hi Chris,

In that form, it's 10 times.  Where were you looking?  It may also be presented as 20*log(sqrt(P*Z)/0.775)-G.  The two are equivalent.

Nick

http://www.poulpetersen.dk/Appn/gblimthr.html
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Jon Barnhardt

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Re: Calculating Speaker Limiter Settings
« Reply #9 on: March 24, 2011, 12:09:15 PM »

I am also trying to grasp this and was hoping to supply my numbers/equipment to try to make it make sense to me...

I have two 8 ohm subs that I want to limit to 1200 watts each running off of a PLX 3602 bridged at 4 ohms. (2 of the subs on the amp bridged mono)

Using the above equation I get the following:

P = 1200
Z = 8 ohms
G = 36 (according to the specs I can find)

10*log(1200*8/0.6)-36

I add 6 to the gain of the amp since it is bridged and it gives me:

10*log(1200*8/0.6)-42

which equals 0.041

OK, now what do I do with that number to make it a worthwhile setting in my BBE DS26? Setting the limiter at 0.04 would make it turn on almost instantly... By ear I would assume the limiter on the DS26 would need to be between 4 and 6 to be effective, but I may be all wet here...

Thanks for any help.
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Re: Calculating Speaker Limiter Settings
« Reply #9 on: March 24, 2011, 12:09:15 PM »


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