JBL 4408 Crossover Cap Question

D

dolphy

New Member
Hey everybody,

I have a pair of JBL 4408 Studio Monitors. I am replacing the caps on the cross over but am having trouble finding replacements for the bigger ones. They are ceramic:

4.0 J 250V CMC
&
5.0 J 250V CMC

Since these are ceramic multilayer, do I have to use ceramic as replacements or am I fine with using electrolytic instead?

Any help is appreciated 1000 fold.

Thanks.
 
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Ceramic multi layer?

They look like cheap polypropylene film caps to me. 4 uf and 5 uf. Nothing special about them.

You would not want to replace them with electrolytic. That would actually be a step backards. Best to leave them alone.
 
dolphy said:
Alrighty. Thanks for the info.

I still plan on replacing the resistors on the crossover just for POM.
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Well if you are going to the trouble you might as well replace the existing films with new films. If you go with some mid grades like the Dayton or Solens they will likely sound a small amount better due to higher quality and much tighter tollerance. The cheap films usually do vary by 20% but the decent brands usually run like 1%. No need to spend the big bucks.

BTW, changing the resistors will not likely change the sound unless a) the originals are off in value or b) you replace with MOX resistors. Those do sound different, not necessarily better.
 
What Dave said. I'm doing a pair of L 46's (I think those are the studio/professional version that you have) and went with Mundorfs C cap for the LF, are inexpensive and had some Jantzens crosscaps, also not expensive. Although I went with the 12 watt Mills resistors. Not yet have them up as the woofer surrounds need replacement....sorta the "while you're at it" kinda thing. Heck, had to open them up anyway so for the minor bucks it costs do the crossovers.
 
Thik I got that wrong. The consumer version of yours are the L40 t3's, My l46's dont have the titanium tweets but are 2-way, 8" woofer.
 
Forgive me for my ignorance as this is my first time dealing with a crossover without electrolytics.

Since these are CMCs (Ceramic Multilayer Capacitor) I can replace them with film capacitors?
 
dolphy said:
Forgive me for my ignorance as this is my first time dealing with a crossover without electrolytics.

Since these are CMCs (Ceramic Multilayer Capacitor) I can replace them with film capacitors?
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The thing is, ceramic caps are those dinky little pico farad types. I don't think that's what you have. I could be wrong but I think the caps in question are films. Could it be that CMC does not stand for Ceramic Multilayer Capacitor but instead indicates the manufacturer?
 
Attached are the photos of the caps. I think you may be right on them being film.
 

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One thing I also neglected to mention is that I am replacing the sub-surrounds as well.

Is one of the reasons I should just leave them be is because they do not loose quality as much over time as electrolytics?
 
They should be OK, although some people believe you can get improved sound by upgrading to fancier caps of the same value (Jantzen, Sonicap, Clarity Cap, Solen, etc.)
 
dolphy said:
One thing I also neglected to mention is that I am replacing the sub-surrounds as well.

Is one of the reasons I should just leave them be is because they do not loose quality as much over time as electrolytics?
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dolphy, I just noticed your thread.

Don't be too hasty. This is not about 'losing quality'. You need to take some measurements and not just throw parts in there.

First of all, have you actually checked these caps to be sure they're still in spec? And checked the bypass caps for that too??? A Radio Shack Digital Multimeter Model 22-811 has this capability and is under $45USD. JBL was a real stickler for the precise point where the crossover started in their studio monitor networks, so it's kind of important; as well as using that .01µF polystyrene bypass cap to get the JBL 'studio monitor' sound.

If these caps have drifted, or you've removed the bypass cap, or the ohms have changed on the drivers, your crossover point of 2.5K is going to be off and the sound will be different. Are you actually using them as studios monitors, or just for listening to music?

How did the two 20 ohm and one 3.9 ohm resistors measure up?

I thought you could use this:

http://www.falconeddystudios.com/images/JBL-4408-TechServSheet.pdf

. . Falcon
 
Sure, worth checking the tolerances on your present caps. Appropriate. Since I have the crossovers exposed my point was that for little money it might be practical to replace the caps more as a future-looking feature. Cheap insurance. The technology has advanced in the decades following production. FWIW.
 
day67 said:
Sure, worth checking the tolerances on your present caps. Appropriate. Since I have the crossovers exposed my point was that for little money it might be practical to replace the caps more as a future-looking feature. Cheap insurance. The technology has advanced in the decades following production. FWIW.
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Absolutely.

Generally, film caps for drivers love to increase in value over time, which drops that x-over point.

I just finished recapping a set of Jensen TF-3's, and the film caps for the midrange drivers had nearly doubled in value since the day they were installed. The tweeter also increased by 30%. That doesn't sound like a lot, but it's a HUGE drop in a crossover when you're up at 13.8K, and the original cap value was .25µF. It pulled the crossover down to 9885Hz from 13825Hz when the cap increased to .35µF!

On the mids, one had gone from 4.0µF to 7.73µF, and the other went to 7.02µF. Either of those create a hell of a drop in the x-over. Thankfully, the TF-3's only have a First Order filter which means a gradual slope of only 6dB. But the starting point was now well below the published low frequency range of the mid drivers. Too much power being fed to them could have made them go P_O_O_F !!!!. . .and that's not a good thing with a vintage Jensen Alnico driver. Especially if all four of them 'poofed' at the same time. :tears:

So if you want to be sure your entire system is tuned, do yourself a HUGE favor and don't stop at your electronics hardware. Vintage speakers, their drivers, and crossover/network components need periodic servicing, too.

. . Falcon
 
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