compression driver cut off points using active dsp avoiding damage

charlie2

Member
2012-10-27 8:45 am
Uk
When assesing manufacture data of drivers regards recomended crossover points, they usualy ascertain/recomend a certain point.

Now if those recomendations are using a specific octave re 6db 12db butterworth linkz reily, how does one translate this?

hope this makes sense i am still learning

the steeper the slope the lower you can go but to what point? and how does wattage figure into this.

Take the jbl 2426 compression driver as a ballpark

800hz or higher 12db/octave slope minimum

power capacity 70w continuos program above 800hz
....................100w ...................................1.2KHZ

Frequency range 500hz .....-20'000hz

JBL link

https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwjLmtObyPDKAhUBkg8KHW_MAosQFggdMAA&url=https%3A%2F%2Fwww.jblpro.com%2Fpages%2Fcomponents%2Fcmp_drvs.htm&usg=AFQjCNF1UWb-9yjGvaJFfr9W226Dh_tADg

so how low can i go at what wattage and db spl
 
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POOH

Member
2010-10-05 9:06 pm
Ohio
When assesing manufacture data of drivers regards recomended crossover points, they usualy ascertain/recomend a certain point.

Now if those recomendations are using a specific octave re 6db 12db butterworth linkz reily, how does one translate this?

hope this makes sense i am still learning

the steeper the slope the lower you can go but to what point? and how does wattage figure into this.

Take the jbl 2426 compression driver as a ballpark

800hz or higher 12db/octave slope minimum

power capacity 70w continuos program above 800hz
....................100w ...................................1.2KHZ

Frequency range 500hz .....-20'000hz

JBL link

https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwjLmtObyPDKAhUBkg8KHW_MAosQFggdMAA&url=https%3A%2F%2Fwww.jblpro.com%2Fpages%2Fcomponents%2Fcmp_drvs.htm&usg=AFQjCNF1UWb-9yjGvaJFfr9W226Dh_tADg

so how low can i go at what wattage and db spl

Depends on the horn too. Double the flare rate keeps you out of trouble provided the compression driver is up to snuff. The little JBL probably sounds best above 1200 cycles in a suitable horn. 100 watts at 800 cycles in a living room would be torture.
 
Depends on the horn too. Double the flare rate keeps you out of trouble provided the compression driver is up to snuff. The little JBL probably sounds best above 1200 cycles in a suitable horn. 100 watts at 800 cycles in a living room would be torture.

100 at 800 would be awful but for home use, it can be pushed that low (if you're running a CD at 100W in home.... you're obviously not interested in fidelity). The 2426 can go quite a bit lower than some smaller CDs.
 
IIRC there is a power spec for even lower usage in the data sheet of this driver. Something like 10 Watt at 500 Hz which would still be *%/&" loud for home use IMO.

Regards

Charles

Edit:
Had to search intensively but now I found it: "4) A 2426 can be used to 500 Hz; however, power capacity will be reduced to 20 W continuous program in the region between
500 Hz and 800 Hz."

IMO THD will probably increase as well down below.
 
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the steeper the slope the lower you can go

This is actually not true in general. Sure, if you use a steeper slope you get more attenuation at a given frequency that is far below the crossover point. But AROUND the crossover point the attenuation is typically LESS, given the same class of crossover function, e.g. comparing a Butterworth 3nd order to a Butterworth 5th order for example. The reason for this is that the HIGHER order crossover maintains its flat passband response LONGER (closer to the crossover point) and only then turns down more steeply. So the driver would actually be getting MORE power in a small band of frequencies near the crossover point (just above it) when the order of the crossover is INCREASED! This is very counter intuitive, but it's nonetheless true.

Using the above reasoning I hope you can see that your statement that "the steeper the slope the lower you can go" is not accurate.
 

charlie2

Member
2012-10-27 8:45 am
Uk
thanks for replies.

yes it is for home use (but like to crank it up now and again) without fear of destroying drivers

I was and am intrested how low the compression driver can be used, not so sure how many watts I will be pumping into or db level required as more to avoiding damage to it with the lower frequencies.

thinking out aloud about crossing over lower to a dual 15" one upto 300hz the other upto 1200hz

or 15" dual upto 500hz(above) 800hz lobbing inter-fears

with a celestion cdx1745 for highs

thinking along the lines of

jbl 4435 but vertical
usher d2
Manfrotto FIG RIG 595B

cheers guys
 
thanks for replies.

yes it is for home use (but like to crank it up now and again) without fear of destroying drivers

I was and am intrested how low the compression driver can be used, not so sure how many watts I will be pumping into or db level required as more to avoiding damage to it with the lower frequencies.

thinking out aloud about crossing over lower to a dual 15" one upto 300hz the other upto 1200hz

or 15" dual upto 500hz(above) 800hz lobbing inter-fears
Charlie,

The 2426 titanium diaphragm can withstand a fair amount of hammering against the phase plug before destruction, though once that level of excursion has been hit, the distortion (clacking sound) is quite apparent if you have normal hearing. At lower frequencies, the driver will not be limited by power, but by excursion- it does not take much voltage (watts) to hit the excursion limit.

At 1200 Hz (pretty high for a 15") a 2426 will keep up to the 2.5 way 15" with a 12 dB or greater slope.

Art
 

ufo2

Banned
2016-01-31 9:57 pm
hm.. everytime a compression diapraghm have broken here i only measure a minute change in DC resistans. like from 6.7ohm to 6.5. no visible damage on the voice coil, and long before i reach volumes anything close to calculated. even with active xo and recommended slopes. always perplexes me.
 
What horn will you be using with the celestion cdx1745? The horn will influence the frequency response too... for example if you look at the driver datasheet:
http://www.parts-express.com/pedocs/specs/294-2124-celestion-cdx1-1745-specifications.pdf
the bottom most plot shows the response measured on a horn. Note that the response is almost 10dB down at 1kHz...

Keep in mind that the MFG recommended crossover point is 2200Hz! This is a cheap 1" diameter compression driver. Even on a horn with a much lower cutoff frequency I would not recommend using it below 1500Hz @ 12dB/oct no matter how little power input you will be giving it.

For comparison, the JBL system you wish to copy uses a high quality JBL compression driver on a large "butt-cheeks" style CD horn and they cross over at 1kHz @12dB/oct, likely with a shallow (low Q) knee to reduce excursion around the crossover point.

As another example, I am building a 2-way directivity-controlled monitor using a special 18" woofer and an 18" horn. Using a high-quality 1" compression driver, the lowest I am considering crossing over is 1kHz. This is not only because of the limitations of the 1" c.d. itself, but also because the pattern control of the horn starts to break down below 1kHz and this is a good compromise between how high I can go with the 18" driver and how low I can go with the horn.

Finally, are you aware of this Econowave project:
http://gainphile.blogspot.com/2010/11/s15-econowave-dsp.html
they mention your c.d. as a possible alternative and give some praise. Crossover used in that project is LR2 @ 1250Hz, which has a very shallow rolloff, meaning that the response of the c.d. is reduced around the crossover point like I mention above. But they use a DE250 c.d., not the Celestion...
 
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charlie2

Member
2012-10-27 8:45 am
Uk
Maybe there was not enough horn loading at the lowest frequency used, causing excessive excursion.

How would one calculate this given an unknown horn/cd waveguide with access to REW for example.

The reason I ask is I took a punt on some cheap wave guides from ebay £6.95 each including postage/shipping

but without frequency specification,

they are knockoffs but of what i am not sure of mnc waveguide.jpg as of yet

33305 Plastic Screw Horn Flare Speaker Speaker DJ Music 420x160x210 mm | eBay

maniac sound system
 

charlie2

Member
2012-10-27 8:45 am
Uk
What horn will you be using with the celestion cdx1745? The horn will influence the frequency response too... for example if you look at the driver datasheet:
http://www.parts-express.com/pedocs/specs/294-2124-celestion-cdx1-1745-specifications.pdf
the bottom most plot shows the response measured on a horn. Note that the response is almost 10dB down at 1kHz...

Keep in mind that the MFG recommended crossover point is 2200Hz! This is a cheap 1" diameter compression driver. Even on a horn with a much lower cutoff frequency I would not recommend using it below 1500Hz @ 12dB/oct no matter how little power input you will be giving it.

For comparison, the JBL system you wish to copy uses a high quality JBL compression driver on a large "butt-cheeks" style CD horn and they cross over at 1kHz @12dB/oct, likely with a shallow (low Q) knee to reduce excursion around the crossover point.

As another example, I am building a 2-way directivity-controlled monitor using a special 18" woofer and an 18" horn. Using a high-quality 1" compression driver, the lowest I am considering crossing over is 1kHz. This is not only because of the limitations of the 1" c.d. itself, but also because the pattern control of the horn starts to break down below 1kHz and this is a good compromise between how high I can go with the 18" driver and how low I can go with the horn.

Finally, are you aware of this Econowave project:
Gainphile: S15 - Econowave DSP
they mention your c.d. as a possible alternative and give some praise. Crossover used in that project is LR2 @ 1250Hz, which has a very shallow rolloff, meaning that the response of the c.d. is reduced around the crossover point like I mention above. But they use a DE250 c.d., not the Celestion...

cheers charlie for the input (the driver in question regards lowest cut-off is the JBL 2426)

The celestion cdx-1745 for highs crossed over maybe around 3000hz or even higher in a 4 way

JBL 2426 for mids

sorry for any confusion

The celestion cdx-1745 is the sams as a b+c de250 (Polyimide) to my internet research
 
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charlie2

Member
2012-10-27 8:45 am
Uk
Measure the horn/driver raw response, you will see a point where the frequency dips very rapidly, that is Fc, the horn cutoff frequency. Excursion will rise rapidly below Fc.

cheers Weltersys

here is the waveguide in the link earlier

mounted to JBL 2426 RAW NO EQ

[IMGDEAD]http://i68.tinypic.com/33usumr.jpg[/IMGDEAD]

Black 500hz LR 48dB

RED 400HZ LR 48DB
about 650hz cut off I think
 
cheers Weltersys

here is the waveguide in the link earlier

mounted to JBL 2426 RAW NO EQ
about 650hz cut off I think
Looks like 600 Hz Fc to me.
By "raw" I meant no crossover filter (perfectly safe at 2.83 volts for an 8 ohm, 4 volts for 16 ohm, though a fraction of a volt is plenty to test response), but the 400 Hz steep filter is low enough to see that the horn/driver response has gone south below 600 Hz.

The driver should be safe with a 1200 Hz crossover of 12 dB per octave or more, but the horn's vertical pattern that low will be a lot wider than the 15", but polar response is another issue entirely ;).

The multiple phase wraps in your last post are due to an incorrect delay time.

Cheers,
Art