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Setting Crossover Gains by the numbers
Rich G.
Posts: 7
I'm trying to figure out how to set the crossover gains by the spec sheet numbers for various amps in a Tri-Amp'd system.
I was thinking that all I would need to do is look at the amplifier gains and then center them about the mid frequency gain for the simple case of a complete stereo system... but when I try to use the DriveWare wizard I see this is not the case.
For example, let's say we have amps with 32, 34, and 36 db gain on the Hi, Mid, and Low when turned full up. I would think the proper crossover gain settings would be +2, 0, and -2 db... but when I run this example using QSC PLX1802, RMX2450, and RMX5050* which have these gain specs the values from DriveWare Wizard are +2.9, 0, and -1.5. Am I missing something here? Do I need to factor in the input sensitivity (which are 1.28, 1.23, and 1.42 V for these amps)? If so, how?
I also see that if bridge mono sub is chosen for any given stereo setup then the low frequency crossover band subtracts 6db. I can understand this as the channels are summed in mono (adding 3db) and the power delivered to a given speaker will be doubled when bridged (adding another 3db).
*These are just for example
I was thinking that all I would need to do is look at the amplifier gains and then center them about the mid frequency gain for the simple case of a complete stereo system... but when I try to use the DriveWare wizard I see this is not the case.
For example, let's say we have amps with 32, 34, and 36 db gain on the Hi, Mid, and Low when turned full up. I would think the proper crossover gain settings would be +2, 0, and -2 db... but when I run this example using QSC PLX1802, RMX2450, and RMX5050* which have these gain specs the values from DriveWare Wizard are +2.9, 0, and -1.5. Am I missing something here? Do I need to factor in the input sensitivity (which are 1.28, 1.23, and 1.42 V for these amps)? If so, how?
I also see that if bridge mono sub is chosen for any given stereo setup then the low frequency crossover band subtracts 6db. I can understand this as the channels are summed in mono (adding 3db) and the power delivered to a given speaker will be doubled when bridged (adding another 3db).
*These are just for example
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Comments
When looking at numbers it is NEVER as simple as it seems. For instance...if you think turning the knob all the way up will be the only way to get full output you would be mistaken... if you look at the amp input sensitivity then you will see the amount of voltage necessary to drive the amp top full output potential... so if your mixer were to put out .7746 volts @ 0dBu (typical) and your amp was a Crown MA3600vz... with .775 volt input sensitivity, you could drive the amp to maximum output potential with just over unity gain (the -0- on the master fader...) and if you look at the specs That .775V would provide 26dB of gain.
There is still the sensitivity of the given drivers... typical would:
horns :
105-107dB 1 metrer 1 watt
Mids
98-100dB 1 meter 1 watt
subs
94-98dB 1 meter 1 watt
The Driverack, when all the numbers are provided, (all amps and speakers are listed in the menu) calculates the variables for you.
Gadget
In my case I'm using amps & speakers that are not in the Driverack's database. This leaves me to calculate the values on my own... but again, dbx has not provided an equation to describe how the gain values should be set.
Other than using the gain structure method for setting up the system (very easy and sensible) you can find an amp with the same sensitivity and use that as for the speakers... use the 3dB down points (usually published) and any provided xover recommendations...
Short of that, we offer setup suggestions all the time, but as I said...the gain structure method still remains the single best setup method unless you are using, for instance, JBL speaker, and the JBL amps they spec for their setup....
Oh and dbx DOES provide the gain structure.. we have simply taken that a few steps farther here:
viewtopic.php?f=61&t=959
Gadget