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Help! Am I powering my subwoofer properly?

Jbl Stx 828s Crown Xti 6002 Dual 18 Subwoofer Amp

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#1 Matt_Coax

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Posted 26 June 2014 - 01:05 AM

Hi All,

I recently purchased a JBL STX 828s dual 18" subwoofer. I'm currently powering it with a Crown XTi 6002 in bridged mono. The speaker power rating is 2000/4000/8000 (Continuous/Program/Peak) rated at 4 ohms. The XTi 6002 puts out 6000 watts at 4 ohms bridged.

I spoke to JBL tech support and they  told me that I'm overpowering the sub by 2000 watts at 6000 vs. The speaker program rating of 4000w. They recommended that I limit the power output.

When I calculate the the voltage to limit, I get 126.5 volts (sqroot of 4000w x 4 ohms) - the problem is that the limiter on the 6002 only goes up to 100 volts on the limiter.

Can someone please explain if the calculation jbl gave me is correct and also, if my amp configuration is correct. Thanks for the help!




#2 joust

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Posted 26 June 2014 - 10:09 AM

Just a comment on Bridge mode, with low impedance speakers and low frequency delivery (Subs) :

When amp specs say : Bridge 4 ohms, it also means that you have a Dual (Stereo) 2 ohms! 2 ohms per side (or 4 ohms Bridge!) is brutal (at any high power levels!) for any amp. When using an smps/bca amplifier (ie XTI6002) you need to remember that 'all' the power to the output section has to pass through the smps. At 6000W, that's a lot of power! If you permit, 6000W/120V(AC) = 50A. Does the AC distribution supply have the capacity of delivering this 50A? I don't think that the 6002 can deliver the Bridge 6000W or Dual 3000W without any problems!

Second of all, speaker impedance : Physics says that the power is dependant on frequency and impedance of the load : ZL = 2(pi)FL; This implies that the lower the frequency (F), the lower the impedance of the coil (ZL); So, at very low frequencies, ZL is not 4 ohms anymore but possibly, lower! At this point, the amp is in unknown territory : It may go thermal at any moment, if the output levels are strong. Probably, the smps (and not the output) will go thermal initially. The XTI6002 doesn't have lower than 4 ohms Bridge or Dual 2 ohms specifications. Be careful!

My rule of the thumb :
Amp wattage continuous = peak power speaker
Amp wattage program (or 1/2 continuous) = program power speaker or 1/2 peak power speaker (normal operation)

Probably, Avi will be able to propose the correct configuration for the limiter.

Alain

#3 dakos

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Posted 27 June 2014 - 06:51 AM

Hey there Matt and welcome,
The JBL reps were pretty conservative IMHO, I believe you should limit to a much lower number and I'll explain. No point in me just writing it all over again so first you need to read this:
http://www.crownaudi...490

OK, now you know if we only want to protect our speakers we need to set the limit point to the continuous speaker power rating. If we also want continuous stable SPL through our show (avoiding power compression), we need to further limit our continuous power to 1/2-1/4 the continuous speaker power rating.

Now there are many kinds of limiters but for our purposes there are two, RMS and peak. Peak limiters can be configured in many ways. On the XTi series the limiter is set as a Peak limiter by default at the clipping point of the amp so the amp never clips and you get the full rated power cleanly. To limit the amp the way you want we need to configure the amp's limiter as an RMS limiter, that could, if you're not careful, make the amp clip. Alternatively you could purchase an external limiter such as the Symetrix 501 ($150 used on eBay) that would do a great job.

Before we dig into the calculations which approach would you like to take? The XTi only limiter or the XTi and an additional limiter approach, the latter being the better in protecting your speakers?

View PostMatt_Coax, on 26 June 2014 - 01:05 AM, said:

When I calculate the the voltage to limit, I get 126.5 volts (sqroot of 4000w x 4 ohms) - the problem is that the limiter on the 6002 only goes up to 100 volts on the limiter.
Are you sure? To the best of my knowledge the XTi's take dBfs units for their limiter setting, not volts nor watts. Please recheck and let me know...

Do you already own the amp or are you just thinking of buying one?
Avi





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