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View Full Version : Compression Drivers - Why so heavy?



BigT
11-01-2007, 12:43 PM
I am rather new at this horn thing. Today the compression driver I bought - a JBL 2445J - arrived. I have never seen one of these before. It's supposed to weigh 32lbs. but, my God, if feels as if it weighs 100! Why does it need to be heavier than a typical 18" woofer? There are lots of tweeters that can do the job that weigh so much less; why does a compresssion driver need to be so heavy? In other words, why does it need such a massive magnet? And how should this be supported? I certainly am not going to let it just hang there supported only by the bolts holding the horn.

Finally, how do you test a compression driver? Just attach an amplifier and let 'er rip? I asked in another post how hard was it to recone a woofer; how hard is it to replace a diaphram? It looks ok but there seems to be a few things around it that probably ought not to be there.

Maron Horonzakz
11-01-2007, 02:01 PM
Why did you buy the driver??? if you didnt know what it was?;)

SMKSoundPro
11-01-2007, 02:05 PM
Dear Big T,

I have many 2445 drivers and the question of support is right on the mark! It is heavy, no doubt about it, and there are newer lighter weight alternatives using n-dyn magnets. The 2445 is very old-school and I think a very worthy driver from 800hz - 16k. Then go into a couple of 2404 baby butt tweeters for the sizzle.

"And of course, never mount a large diaphramed compression driver without protection!"

I have a couple of pictures of the cabinets I am working on that show the plywood support of the driver.

Others will chime in and help to discuss your drivers.
Good Luck!
Scott.

John
11-01-2007, 02:15 PM
Nice!!!:applaud:

BigT
11-01-2007, 02:22 PM
Why did you buy the driver??? if you didnt know what it was?;)

I didn't say I didn't know what they were - I only said that this was the first time I had seen one.

I love your solution, Scott! I hadn't thought of that. JBL has a mounting brackett but I didn't want to spend as much for the brackett as I did for the 2445J.

I wished I could afford one of those lightweight 2450H/J drivers but they always go for more (on Ebay) than I'm willing to spend, whenever they come up which is not often enough.

How good are those "baby butts" (I like the nickname)? Everyone seems to want the 2405 (077).

SMKSoundPro
11-01-2007, 02:37 PM
Dear Big T,

In my 4733/4870 style of cabinet, Jbl used either a single 2404 baby butt cheek tweeter or two of the in the concert 4800 series.

I have a quite a few 2402 bullets and will cross them over at 7k using a 3105 crossover.

I just don't have enough 2404's and everybody seems to outbid me on them.

I built four of these cabinets out of baltic birch plywood, paying special attention to the joinery details. They are to go to work downstairs i my larger ballroom here at the nightclub for disc and live shows.

I am using these components because it what I have, and have found that JBL used them too at one point in time. Powered with Crown Macro-tech amps.

Andyoz
11-01-2007, 02:46 PM
Why does it need to be heavier than a typical 18" woofer?

I think the question is why does the magnet driving a very light tweeter diaphragm have to be nearly as heavy as one used to drive a larger (and heavier) woofer cone.

Good question and one I've been afraid to ask myself. :applaud:

Mr. Widget
11-02-2007, 12:44 AM
I think the question is why does the magnet driving a very light tweeter diaphragm have to be nearly as heavy as one used to drive a larger (and heavier) woofer cone.

Good question and one I've been afraid to ask myself. :applaud:The short answer is that it takes gobs of magnetic energy for these things to perform as designed... read over this tech note, it explains it far better than I could.

http://www.audioheritage.org/vbulletin/showthread.php?t=4410


Widget

Thom
11-03-2007, 06:56 AM
Not being a carpenter or cabinet maker I'm fond of unistrut and/or plumbers tape.

1audiohack
11-09-2007, 07:47 PM
:blink: So, after reading the "vbullet" tech note shown by Widget, and doing the math on something changing direction close to 20,000 times a second, the acceleration forces are approaching 100 G ?!?

Scratch head,,, if that is even close to right,,,

No wonder they need so large a motor to play so loudly at very low distortion levels, and small wonder they fill a small room at less than one watt!

Even with way heavier moving mass, the acceleration forces for even subs could be less,,, scratch head,,,:blink:

Hoerninger
11-10-2007, 02:11 AM
... and doing the math on something changing direction close to 20,000 times a second, the acceleration forces are approaching 100 G ?!?

Scratch head,,, if that is even close to right,,,
Ya, a car which reaches 80 mph in 6 seconds instead of 12 seconds needs a much stronger engine. Time has been halfed in this example, but when you consider 20 kHz against 200 Hz the time for one period is only one-hundredth.
May be this comparison gives a picture.
___________
Peter

1audiohack
11-12-2007, 09:14 AM
I'm pretty sure I get it. Its just amazing when you actually do the math.

clubman
11-13-2007, 02:37 PM
Finally, how do you test a compression driver? Just attach an amplifier and let 'er rip?

Well I wouldn't recomend that! First I would do a test with a OHM meter to see if you get a open, short, or value of the DC resistance (ohms)

If you get a OHM reading then there is a chance everything is fine. Allthough just cause it ohms out doesn't mean that it won't act up at a given freq. A speaker repair shop could do a freq. sweep to test it more indepth than just a ohm meter.

You can just hook it up to a amplifier as long as the program material you are runing through the amp is within the range of the driver, 800Hz and higher. Youl would not want to run full range into that driver unless you want to get new diaphrams anytime soon.

These drivers sound amazing, and will last for years. I am runing 8 of them matched up with jbl 2226G's in SR cabs in a night club powered with macrotechs.....never had to replace a diaphram in 3 years

Best luck to you with your drivers!

Steve Schell
11-14-2007, 12:35 AM
This is a great thread, BigT. First of all, I'm not surprised that that a 2445J prompted your initial question, as they seem almost absurdly huge and heavy. Fortunately they have been superceded by the modern neodymium magnet drivers which have only a fraction of the size and bulk of the ceramic magnet monsters.

In general, a compression driver needs a really big motor to cope with its task. The diaphragm is feather light, but the air load on it is great as the exit path on the surface of the phasing plug is only about 10% of the size of the diaphragm. Imagine pushing and pulling the diaphragm at super high speed against such a constriction!

Even the strongest compression driver motors cry uncle by 3 or 3.5 kHz., and the power response falls by 6dB per octave above that point. Up to that point they have an efficiency (ratio of electrical input to acoustical output) of 30% or so, really terrific compared to the dismal 1 or 2% efficiency of most direct radiator speakers. This is the reason for their existence, as they produce as much sound as a number of direct radiators, and at less distortion, cost and bulk.

To test a compression driver you need to use a crossover network to keep lower-than-designed frequencies out of the driver. You also need to install the driver on an appropriate horn, as the loading provided prevents overexcursion of the diaphragm.

kziss
11-14-2007, 11:52 AM
Hi Audition.
I am a Hammond / Leslie enthusiast from Melbourne , Australia.
I have learned how to work on and modify Hammonds through the work that I have done on my 1962 C3 organ such as replacing the aged wax capacitors of the Tone Wheel Generator ( TG ) with new correct micro farad spec capacitors , recalibrating the whole TG to produce a louder and warmer sounding TG output curve and repairs and modifications to the AO28 preamp. I play my Hammond organ through a Leslie 122 that I have done some modifications to , including adding a JBL 2482 treble horn driver and I may put in a JBL-E140 8 ohms bass speaker.

I am currently repairing the gooey black foam related damage done to the manual resistance wires of my newly purchased 1965 C3.

I am quite interested in learning about the modifications that Bill Beer did to Hammonds and Leslies in order to get his customized fat sound.
Information about the Bill Beer organs and Leslies seems very hard to find and some stock Hammond/ aLeslie purists dont like the Bill Beer sound , but from examples that I have heard , I quite like the fat sound of the Bill Beer organs and the Bill Beer Leslies.

I have read that Bill Beer used to recalibrate the TG to produce a fatter sound than the Hammond factory stock TG calibration .

Do you know anything about the output levels specs of the Bill Beer TG recalibration ?

Where there particular areas of the TG frequency ranges that Bill Beer boosted or attenuated in order to get his particular sound ?

Bill Beer was quoted as having measured the TG output levels of six ''hot bitchin'' B3 organs and that he then used these as the basis of his TG recalibration specs and he said that he could recalibrate the TG to make it sound ''fat and ballsy''.

I know that the Hammond factory used to calibrate the TG's to have stronger outputs in the bass and the lower midrange region until 1956 when the factory TG output curve was changed to have slightly lessened bass and lower midrange levels and that this new TG output curve remained until the end of Hammond TG organ production in 1975.

Do you know anything about thes six ''hot bitchin'' B3's that Bill Beer used as the basis of his TG output curves , and what production years these B3's were from ?

As well as recalibrating the TG and replacing the aged wax capacitors of the TG notes 49 to 91 of the pre 1964 organs , do you know if Bill Beer also modified the TG note filters to produce a more pure sine wave quality and less background rumble or hum ?

Do you know anything about how the frequency response of the solid state Bill Beer organ preamps differed from that of a stock AO28 preamp ?

I have been told that the Bill Beer biamped solid state high power Leslie amplifiers had a three position tonality switch , and that this switch selected:
1) the straight flat response
2 ) boosted bass ( for organ)
3 ) boosted bass and treble ( for guitar ).

Do you know what the frequency range and the decibel level of the bass boost for organ was and what the frequency range for the boosted bass and treble for the guitar was ?

Although Bill Beer used the JBL E-140 or the K-140 bass speaker and the JBL 2482 treble driver , do you know if there are better JBL or other brand bass speakers that are more efficient and that produce a deeper bass response than the JBL E-140 or the JBL K-140 bass speakers ?

I have searched the JBL website for a frequency response curve chart but all that I could find was a brochure that described the E-140 series and the K-140 series speakers together with some other speakers , but there were no frequency response curve charts on that brochure.

Do you have a frequency response curve chart for the JBL E-10 or the K-140 speakers ?

I have been told that Gregg Rollie from the early Santana years used a Bill Beer modified organ or a Bill Beer Leslie on the early Santana albums but no one has yet verified this .
I know that Tom Coster and other subsequent Santana organists used a Bill Beer organ.

Apparently Dick Sims used a Bill Beer organ and Leslie on Eric Clapton's early 70' albums.

I have been told that Steve Walsh used a Bill Beer organ and Bill Beer Leslie on the 1976 album "Leftoverture". Do you know if this is correct ?

Are you able to say who the other big name organists / bands that used Bill Beer organs and Bill Beer Leslies were ?

Dop you know what year the modified Bill Beer organs and Leslies first started appearing ?

I am a big fan of the mid - late 60's / 70's era Rock music.

I am quite interested in finding out what I can about these things and Bill Beer's legacy in improving the tonality of the Hammonds and Leslies.

Thank you very much in advance for a reply and for your patience with my many questions.

All the best.
Kon