Are These 4312a Control Monitor Crossovers?

[brettMc]

Hello,

Bought a second set of what I think are JBL 4312a Control Monitor Crossovers but not sure the item was properly marked. I figured if I was going to attempt my first ever hands-on upgrade of a crossover network that I should do them on a second set...in case I totally mess it up.

Please take a look at the images and let me know if these are the proper crossovers for these speakers. Your expertise is much appreciated!

Speakers:
77579

Second-hand Crossover:
77580

[grumpy]

yes.

Network 66324 is 3112D and called out on the 4312A Technical Manual (as is network 3112C)

[brettMc]

yes.

Network 66324 is 3112D and called out on the 4312A Technical Manual (as is network 3112C)

Could your recommend where I can find all the replacement parts I might need and a good soldering iron? Also what gear do I need to test the speakers? Tone genrator? Some sort of impedance tester? And maybe a hot air gun to unglue some of the parts from the board?

I could also use some tips on 'do's and fonts' before I do anything....

Thanks so much!

[brettMc]

...also....

Does anyone know if the tweeter and midrange are wired out of phase with the woofer or is it just the midrange that's out of phase?

[Odd]

JBL 4312A Technical Manual

[Doctor_Electron]

Looking at the technical manual the LF and MF are not wired at 180 degrees because the LF to MF section is first order. And also it appears that the HF is also not wired at 180 degrees even though that section is second order. As I think I understand it the MF to HF phase relationship is not as audibly critical as that of the LF-HF transition.

If you've ever had a mid driver such as a 104H connected wrong or happen to do it in the future, you will immediately appreciate the need to wire the mid out of phase with the LF when using a second order network.. It sounds absolutely hideous.

[brettMc]

Looking at the technical manual the LF and MF are not wired at 180 degrees because the LF to MF section is first order. And also it appears that the HF is also not wired at 180 degrees even though that section is second order. As I think I understand it the MF to HF phase relationship is not as audibly critical as that of the LF-HF transition.

If you've ever had a mid driver such as a 104H connected wrong or happen to do it in the future, you will immediately appreciate the need to wire the mid out of phase with the LF when using a second order network.. It sounds absolutely hideous.

Thanks for the reply! I unfortunately do not understand, does this mean the tweeter is out of phase? Or none of the drivers are out of phase?

sorry. Thanks!

[grumpy]

If you have the wiring, the wires with the black stripes go to the "-" terminal.
If you don't, you'll need to follow the schematic.

Parts-express.com (among others) has all that stuff. Not sure I see the point
in "re-capping" a fairly new network unless something is missing/damaged.

If you've not soldered before, I'd practice on something else first...
it would be easy to ruin a circuit board.

Testing? to what end? pink noise (FM static) and a working set of ears is a good start :)

[brettMc]

If you have the wiring, the wires with the black stripes go to the "-" terminal.
If you don't, you'll need to follow the schematic.

Parts-express.com (among others) has all that stuff. Not sure I see the point
in "re-capping" a fairly new network unless something is missing/damaged.

If you've not soldered before, I'd practice on something else first...
it would be easy to ruin a circuit board.

Testing? to what end? pink noise (FM static) and a working set of ears is a good start :)

Thanks for the reply!

Love these speakers but they are definitely midrange forward and tend to be harsh in the upper mids for sure. So I am recapping them with more laid back caps, with tighter percent tolerances and ditching the ceramic caps for film to help possibly tame some of the upper-mid distortion. Plus the L-pads are scratchy and causing some dropouts.

I don't know if I have ever soldered anything in my life, but I will proceeded with care and clip nothing so original parts remain reusable. This also why I purchased this second set of crossovers in case I am way out of my depth.

All of the support and guidance from you out here in JBL fan land will be in much need and greatly appreciated.

while the JBLs are down I am purchasing a pair of Infinity 7 Kappas just to see what the whole Emit ribbon tweeter is all about...

Thanks,
Brett

[grumpy]

Might try contact cleaner on the L-pads, but can't hurt to replace them if you can find similarly spec'd
items that will fit. A worthwhile effort, as setting levels with dirty or damaged L-pads is
a waste of time (in my mind). I would definitely do this first and then adjust/listen.

I'm a bit more skeptical regarding the caps, as they already appear to be mylar film
(yellow), and probably polyester film (blue, .01 bypass). If you are actually
hearing distortion (as opposed to a midrange level prominence), it is more likely
the L-pad or driver... but have fun experimenting :)


Thanks for the reply!

Love these speakers but they are definitely midrange forward and tend to be harsh in the upper mids for sure. So I am recapping them with more laid back caps, with tighter percent tolerances and ditching the ceramic caps for film to help possibly tame some of the upper-mid distortion. Plus the L-pads are scratchy and causing some dropouts.

I don't know if I have ever soldered anything in my life, but I will proceeded with care and clip nothing so original parts remain reusable. This also why I purchased this second set of crossovers in case I am way out of my depth.

All of the support and guidance from you out here in JBL fan land will be in much need and greatly appreciated.

while the JBLs are down I am purchasing a pair of Infinity 7 Kappas just to see what the whole Emit ribbon tweeter is all about...

Thanks,
Brett

[brettMc]

Might try contact cleaner on the L-pads, but can't hurt to replace them if you can find similarly spec'd
items that will fit. A worthwhile effort, as setting levels with dirty or damaged L-pads is
a waste of time (in my mind). I would definitely do this first and then adjust/listen.

I'm a bit more skeptical regarding the caps, as they already appear to be mylar film
(yellow), and probably polyester film (blue, .01 bypass). If you are actually
hearing distortion (as opposed to a midrange level prominence), it is more likely
the L-pad or driver... but have fun experimenting :)

The little blue caps appear to be ceramic. They say .01k 250v CVC (or CMC)...hard to make out. From my research that denotes ceramic, and from further research ceramic is more likely to be microphonic and more likely to create some distortions.

You are the second person who has stated that the larger yellow caps are good quality. But they are, at their oldest, around 30 years old. Would very much like to hear what you think.

And yes I have already found the appropriate L-pad replacements. So why not new instead of clean the old ones up and hope for the best.

Thanks,
Brett

[Mr. Widget]

You are the second person who has stated that the larger yellow caps are good quality. But they are, at their oldest, around 30 years old. Would very much like to hear what you think.
I'll be the third. :D

Anything is possible, but I think issues with the drivers are far more likely than the caps.


Widget

[brettMc]

I'll be the third. :D

Anything is possible, but I think issues with the drivers are far more likely than the caps.


Widget

Thanks for bringing that up. How can I objectively test the drivers, with what tools and what should they measure?

One woofer was professionally reconned using stock JBL parts. I have several sets of the ti tweeters... How can I tell if all the drivers are acting properly?

I also hear that sometimes spiders need re-stiffening on older drivers, would this be something that I need to look into?

Thanks!

[Mr. Widget]

Thanks for bringing that up. How can I objectively test the drivers, with what tools and what should they measure?
There are numerous measurement platforms at all price ranges. Typically you get what you pay for.

I would run frequency measurements, impedance curves, and if JBL published distortion plots are available, I'd measure the second and third harmonic distortion curves. Of course your measurements will only be as good as you are, so you may want to find someone with experience.

I use CLIO on a laptop and Studio Six on iOS. I have calibrated mics for each.


Widget

[grumpy]

The JBL p/n (61098) for those little blue caps are polypropylene.
No one would use ceramic caps for bypassing larger values in a crossover.

Definitely replace the L-pads if you have them in-hand.

If you're hell bent on replacing the caps, you could both get higher quality
capacitors (obviating the need for the bypass caps at all), and double up
on the values of the yellow ones and get two of each: to replace each single
cap with a pair in series. Then introduce a dc bias voltage (9v battery works)
through a several-Mohm resistor to the point between the caps. Google
"charge coupled crossover" or "dc-biased crossover".

[brettMc]

The JBL p/n (61098) for those little blue caps are polypropylene.
No one would use ceramic caps for bypassing larger values in a crossover.

Definitely replace the L-pads if you have them in-hand.

If you're hell bent on replacing the caps, you could both get higher quality
capacitors (obviating the need for the bypass caps at all), and double up
on the values of the yellow ones and get two of each: to replace each single
cap with a pair in series. Then introduce a dc bias voltage (9v battery works)
through a several-Mohm resistor to the point between the caps. Google
"charge coupled crossover" or "dc-biased crossover".

I have often wondered about DC biasing. But since I have never done any of this sort of work before I would need some serious input from you all. Does the DC biasing just 'precharge' the caps? How could I do this and still be able to change the battery easily? Where does the battery need to go in the circuit?

As for doubling up do you mean instead of one 10uf 250v get two 10uf 250v and wire those in series to replace the original single 10uf? Then I would assume pull the bypass cap and simply bridging that gap with wire? I am totally willing to spend $30-$40 a cap. Please share your approach.

Thanks for your interest and time!

[grumpy]

I have often wondered about DC biasing. But since I have never done any of this sort of work before I would need some serious input from you all. Does the DC biasing just 'precharge' the caps? How could I do this and still be able to change the battery easily? Where does the battery need to go in the circuit?

As for doubling up do you mean instead of one 10uf 250v get two 10uf 250v and wire those in series to replace the original single 10uf? Then I would assume pull the bypass cap and simply bridging that gap with wire? I am totally willing to spend $30-$40 a cap. Please share your approach.

Thanks for your interest and time!

Did you check google, like I suggested? Double up means 2x, or using two 20uf caps in series (in your example).

Do not put a short where the bypass caps are/were. The battery is always in-circuit... use a connector for it of your choosing. The - battery terminal goes to ground (negative speaker terminal). The + battery terminal is connected (using large value resistors such as 3 Mohms) to each junction between the double valued caps... one such resistor per junction.