recommendation for a sub amp for JBL6118

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#22
BigE said:
According to what I have been reading, the 337 is the maximum frequency that the sub can be crossed over, given the spacing between the sub and the low mid.
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That is irrelevant however because you never want the sub operating at frequencies where it begins to become directional, you want it to stay well down in the omnidirectional realm where it can't be located, and with a very capable 15" driver above it there is absolutely no reason to have a sub/low crossover higher than 80hz.

BigE said:
I've also read that for any given frequency, it is best if the sub/low mid are at a distance of 1/2 - 1/4 wavelength.
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No you don't want them at a 1/4 to 1/2 wavelength apart at crossover freq you want them within 1/4-1/2 wavelength... and less(closer) is better.

BigE said:
Applying similar logic to the low-mid and horn, the max frequency will be around 680 Hz,
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680hz may be the minimum low-mid crossover you can use since this also depends upon the capabiities of the mid driver... which is what?
 
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#24
Thanks for pointing that out. you are right. I inverted the formula in the spreadsheet.

The mid unit is the JBL AL6115, from their application engineered series. It can go to 2.5K, 3 dB down. The driver is a 15" JBL 2246H ( 8 ohms).
 
#28
Ok.. so that Altec horn will only play down to 1200hz or so right? If so use those numbers as a starting point for a basic setup for the system. Configure 24db Linkwitz-Riley crossovers for 80hz and 1.2khz, set all outputs to the same phase and remove any delays that were applied. Fire it up and set the balance between the 3 bands to whatever sounds natural, and then you really need to setup a test mic and measure 1 channel of your system at a time. Are you familiar with REW(Room EQ Wizard)?
 
#29
Altec says that the 511B will play down to 500 Hz. They say that the 811B will play down to 800 Hz. Since is it for playing in my basement, I don't doubt that. On a stage, with much higher power levels, that may be quite different.

I am not familiar with REW yet, but it is installed on my laptop, and ECM8000 measurement mic, A/D and SPL meters are ready. The sub amp just lost a channel, so I am not quite ready to roll. Perhaps late tomorrow, it should be repaired.
 
#30
800hz would be great, I know for a fact the JBL2226 drivers work very well up to that range so you should be able to get excellent results from this system once you can see where it needs some EQ. You will also be able to see how correctly setting the time delay between the horn and the other reflex drivers flattens response at the crossover. Good luck and be sure to let us know how it goes.
 
#32
I just checked the time alignment settings:

Sub: 2.46 msec
LMid: 7.53 msec
Horn: 8.96 msec

But this was before the change in cross over frequencies. They are set now, to 80 and 750.

Do these numbers make sense? or should the auto align be redone?
 
#33
You should redo the alignment. Assuming the front of the Altec horn is roughly aligned with the front baffle of the 15" cab, that would put the voice coil of the compression driver further back than that of the 15 or 18 so only the 15 and 18 should get any delay applied to bring them into time alignment, but then I'm assuming you have everything stacked vertically on top of one another. A physical measurement of the offset between the voice coils should give you a ballpark idea of how much delay will be needed, but the crossovers used also affects phase shift so the actual delay required is often a little more than you calculate by hand. Put the measurement mic at ear level at the listening position and do only 1 stack at a time.
 
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