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Author Topic: Help with DSP tunings for custom 218 (JBL srx 728, martin audio blackline 218)  (Read 12177 times)

Conrad Muzoora

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Here is my conundrum. I bought these replicas of Martin audio Black line series 218 subs (1pair). Found they had very inferior low wattage drivers inside them. I replaced the drivers with good RCF LF18g-401 (http://www.rcf.it/en_US/products/precision-transducers/low-frequency-transducers/lf18g401) 900wrms each driver.
My target was to make something that sounds like the famous jbl srx728s subs. I run these subs on a crown ma9000i (4ohm per channel) and i use a Dbx driverack 260 low pass 90hz high pass 40hz. Problem is they start making funy cracling noises even when driven at moderate levels and i have so far had to replace a spider on one of the drivers due to over excussion damage. They sound nice but am unable to utilise all their power potential. I have played with changing the HPF and LPF xover freq in vain.
Photos attached, the boxes measure 41 inches high 20 inches wide and 28inches deep. There are 4 ports on each compartment which are triangular 6x6x7.5 inches and the ports are all 17 inches deep.
Any suggestions on the kind of dsp settings i can employ is very welcome.

Conrad
www.kooleventug.com
« Last Edit: June 16, 2011, 04:59:03 am by Conrad Muzoora »
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Art Welter

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Here is my conundrum. I bought these replicas of Martin audio Black line series 218 subs (1pair). Found they had very inferior low wattage drivers inside them. I replaced the drivers with good RCF LF18g-401 (http://www.rcf.it/en_US/products/precision-transducers/low-frequency-transducers/lf18g401) 900wrms each driver.
My target was to make something that sounds like the famous jbl srx728s subs. I run these subs on a crown ma9000i (4ohm per channel) and i use a Dbx driverack 260 low pass 90hz high pass 40hz. Problem is they start making funy cracling noises even when driven at moderate levels and i have so far had to replace a spider on one of the drivers due to over excussion damage. They sound nice but am unable to utilise all their power potential. I have played with changing the HPF and LPF xover freq in vain.
Photos attached, the boxes measure 41 inches high 20 inches wide and 28inches deep. There are 4 ports on each compartment which are triangular 6x6x7.5 inches and the ports are all 17 inches deep.
Any suggestions on the kind of dsp settings i can employ is very welcome.

Conrad
www.kooleventug.com
Conrad,

The speakers have an Xmax of 8.5mm, so if you see movement over 17mm peak to peak (just over 5/8th inch) they are into non linear operation.

Xlim is supposed to be 50 mm peak to peak, that is where some part of the speaker breaks.
If they are making "funy cracling noises " below about 20mm peak to peak the speakers may be defective.

The 18 used in the JBL SRX 728 has a lot more Xmax, and a larger box, you won't get as much output from your speakers.

Depending on the particular box/speaker alignment, the speaker may run out of Xmax way before it reaches full power, many 18" speakers can handle 1000 watts, but exceed Xmax at only 150 watts or so.

Check the box for good air seals, air leaks can cause the tuning to be higher. Air leaks are easy to detect using sine wave tones. While checking for air leaks, check out the cabinet tuning, the cone will move the least at Fb (box tuning frequency) which should be around 40 Hz given the cabinet and port dimensions.

A 24 dB BW filter about 5 Hz below Fb should protect the LF end of the excursion, but a ported box also has an area of high excursion above Fb. If the Fb is 40 Hz, around 50-60 Hz will have the most excursion, right where lots of kick drum energy is centered.

Assuming you don't find gross air leaks, since the problem is excursion related, you will have to limit peak power to keep the speaker from going much over Xmax in the problem frequency areas.

Art Welter

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Conrad Muzoora

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Conrad,

The speakers have an Xmax of 8.5mm, so if you see movement over 17mm peak to peak (just over 5/8th inch) they are into non linear operation.

Xlim is supposed to be 50 mm peak to peak, that is where some part of the speaker breaks.
If they are making "funy cracling noises " below about 20mm peak to peak the speakers may be defective.

The 18 used in the JBL SRX 728 has a lot more Xmax, and a larger box, you won't get as much output from your speakers.

Depending on the particular box/speaker alignment, the speaker may run out of Xmax way before it reaches full power, many 18" speakers can handle 1000 watts, but exceed Xmax at only 150 watts or so.

Check the box for good air seals, air leaks can cause the tuning to be higher. Air leaks are easy to detect using sine wave tones. While checking for air leaks, check out the cabinet tuning, the cone will move the least at Fb (box tuning frequency) which should be around 40 Hz given the cabinet and port dimensions.

A 24 dB BW filter about 5 Hz below Fb should protect the LF end of the excursion, but a ported box also has an area of high excursion above Fb. If the Fb is 40 Hz, around 50-60 Hz will have the most excursion, right where lots of kick drum energy is centered.

Assuming you don't find gross air leaks, since the problem is excursion related, you will have to limit peak power to keep the speaker from going much over Xmax in the problem frequency areas.

Art Welter

Dear Art,

Thank you so much for this very educative reply to my post. After a check of the box (Dont have any sine wave generator) i did not identify any major air leaks, apart from a little at the speakon connectors. Do you think i should seal the speakons? Also do i employ some eq at around 50-60 hz to reduce the gain in that area in order to protect the speakers? Or do i set the voltage limiter in the MA9000i at some value to limit the peak excussion? The problem usually arises when i power them with the big amp, there is no issue when i power them stereo on my plx 3602 - other than the reduced output ofcourse!

Conrad
www.kooleventug.com
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Paul G. OBrien

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Here is my conundrum. I bought these replicas of Martin audio Black line series 218 subs (1pair). Found they had very inferior low wattage drivers inside them. I replaced the drivers with good RCF LF18g-401 (http://www.rcf.it/en_US/products/precision-transducers/low-frequency-transducers/lf18g401) 900wrms each driver.
My target was to make something that sounds like the famous jbl srx728s subs. I run these subs on a crown ma9000i (4ohm per channel) and i use a Dbx driverack 260 low pass 90hz high pass 40hz. Problem is they start making funy cracling noises even when driven at moderate levels and i have so far had to replace a spider on one of the drivers due to over excussion damage.

What you have discovered is that simply installing "good" drivers in a box does not make a "better" speaker. Upon modeling your combination in WinISD Pro the problem is obvious.. the box is too big for these drivers. They first reach Xmax at 52hz with as little as 700w and with the full potential of the MA9000(1500w/driver) behind them they are well over xmax everywhere except at box tuning which is 37hz. The solution(if you want increased power handling) is to build smaller boxes with higher tuning.. something on the order of 4.5cu ft per driver with 11" long ports, that produces a system that will only be just a hair over xmax(9.3mm) at 72hz with 1500w input, but of course box tuning is quite a bit higher at 47hz so it's not exactly an SRX killer in terms of low end extension. You could also modify the boxes you have by adding a dense styrofoam block to displace some of the volume but you still have to shorten the ports which will likely be harder unless they happen to come out relatively easy. All this just shows what an accomplishment the JBL differential drivers are.. controlling driver excursion at low frequencies and high power levels is not trivial.
« Last Edit: June 17, 2011, 12:08:03 pm by Paul G. OBrien »
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Paul G. OBrien

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Another compromize config would be do drop box volume to about 5.5 cu/ft per driver and shorten the ports to about 13", with this the drivers will take 1000w before exceeding excursion and the system -3db point is 42hz. If I guesstimated the internal box dimensions correctly a styrofoam block 4.5" thick and the full X-Y dimensions of the chambers will displace 1 cu ft, this you could glue to the back of the chamber. After shortening the ports with whatever combination of cutting tools it may take.. maybe one of those new disk grinders type wood saws for example, if you then add a 4th order BW highpass at 35hz and limit the amps 1.5 -2db below max the drivers will be pretty well protected I would think and you'll have about as good as it gets in terms of power handling and extension.
« Last Edit: June 17, 2011, 12:11:17 pm by Paul G. OBrien »
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Conrad Muzoora

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Wow, Quite very useful info Paul,
I greatly appreciate. After taking a good look at the ports and the tools i have, i dont think this is going to be an easy task shortening them. These are the hard options left.

1. Leave the speakers as they are and drive them with a less powerful amp ( I currently have a plx 3602 and am looking at acquiring a crest 8002 one speaker per channel) and hopefully this will be enough power to get them to their "best". ( They really sound nice)

2. Sell off the drivers and attempt to get a better driver for this box. Question is what drivers in a reasonable budget can be able to perform to the full power of my MA9000i with reasonable low extension (Say 38hz -3db)? Am very open to suggestions.

NB: Am willing to spend in the range of 1000$ for all 4 drivers (Here in Uganda i can't afford to purchase the JBL SRX728 because its prohibitively expensive due to the shipping and taxes but i can ship drivers from the US relatively cheap)

Conrad
www.kooleventug.com

 
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