# FOH mixer needing some technical insight



## Musickid (Feb 16, 2009)

Hi my name is Jeff I am a freelance FOH engineer for a lot of larger churches. One of the chuches I work with just purchased a small line array system and i am trying to do the math for what amp I need and this is not my cup of tea. 

The array we have is 8 dual driver elements that run 350 watts RMS at 16 ohms 
we run 4 per side 

So from my limited knowladge on this that puts me at 1400 watts RMS at a 2 Ohm load right? 

IF this is the case i need to get a new amp the one we have is a crown microtech 2400 and that only runs 1000 watts rms at 2 ohms

So I have been looking for some other options and i found the 

Yamaha P7000s that runs 1600 watts rms at 2 ohms
QSC PLX3102 that runs 1550 watts rms at 2 ohms 

any other suggestions under 1k for an amp and is my math right?


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## avkid (Feb 16, 2009)

What is the array?


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## museav (Feb 16, 2009)

Agreed, what are the speakers? How are they wired? What output are you trying to get? How much headroom do you want in the system? Your math is a little off, four 16 Ohms speakers in parallel, if that is how they are wired, would be 4 Ohms rather than 2 Ohms. Also, there is no such things as RMS Watts, you are probably referencing the continuous or long term ratings.

Be careful of some of the amplifier specs, not all the amplifier power ratings use the same basis for the numbers and the frequency range and distortion numbers used to define the power rating may differ; an EIA 1kHz, 0.1%THD rating is not the same as a FTC 20-20kHz, 0.05%THD rating. For example, the PLX3102 is rated 1,000W/channel into 4 Ohms as an EIA rating and 900W/Channel into 4 Ohms as an FTC rating, since you are reproducing more than just a 1kHz signal I would tend to use the lower FTC rating.

Without knowing anything else and assuming the information is correct and I am interpreting it properly, I might look at an amp rated 2,000-3,000W into 4 Ohms. The amps you mentioned are rated closer to 720-950W/channel. You could certainly get by with that as long as that power supports the desired levels and headroom for the system, which it may. However, if you try to get everything you can out of the system the amps would clip well before reaching the maximum power the speakers can handle, so if you wanted to closer to getting all you can from the speakers then the amps discussed are too small unless you use them in bridge mono mode with one amp per array.

The concept of a church apparently just purchasing a line array system on their own is sort of scary, a concern heightened since the amp issues make it appear that no one really looked at the whole system. I just hope that someone made sure the arrays purchased are a good fit for the space and use and that someone is making sure that the arrays are installed properly, that the system is tuned correctly and so on.


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## Musickid (Feb 16, 2009)

I am a certified rigger so they are installed right. The amp we were givin to use was from a local production comapny. As for the system we are mobil and the system is actaully over kill for our current space. I have worked with a lot of arrays i just have never had to be the one to choose the amps becase we always relied on contracted compaines or the manufacturer. I am primarily a sound engineer, but my credits are all in studio sound. So like I said this is a little out of my relm of know how. I appreciate the help. 

okay so if I run two P3500S in bridge mono should that be good?


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## avkid (Feb 17, 2009)

You still haven't answered our question.

What is the array?


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## Musickid (Feb 17, 2009)

It is a TVi array composed of 8 W208 elements. 4 Per cluster.


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## avkid (Feb 17, 2009)

Musickid said:


> It is a TVi array composed of 8 W208 elements. 4 Per cluster.


 Are you going to be running bi-amp or passive?

BTW:
http://www.tviaudio.com/product-p/w-208.htm


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## museav (Feb 17, 2009)

Four boxes seems like a rather small array, the smallest TVi shows or presents on their web site seems to be six boxes. TVi also seems to recommend at least two amps per array to allow amplitude shading the bottom boxes and when run full range they recommend 2,100-2,800W @ 4 Ohms for four boxes and 1,050 to 1,400 @ 8 Ohms for two boxes.

You mentioned that array potentially being overkill for the application. That also relates to where some of the questions may be. It could be that you could get along just fine with less amplifier power,as long as it provides the desired result and no one tries to get more out of the system than it can readily handle. On the other side, having lots of power can be great as long as it is used for headroom and not necessarily just to get the system louder. It is finding that envelope of what the system needs to do that is a major factor in determining what power is appropriate for a specific application. If that is not known then you have to work from some assumptions and that is what the numbers presented above represent, recommended power based on an assumption of how the system would be operated.

Glad to hear they have a certified rigger involved, that does not happen nearly often enough.


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