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View Full Version : Short Intro and Quick Question on JBL 4343 Crossover Design



pen_man
03-10-2011, 11:47 AM
Before I get to my question, please allow me to provide a little bit of extraneous background information. I have owned a pair of JBL 4343 Studio Monitors for about 4-5 years now. I got them from the original owner and they even came with the original boxes. They are the grey utility cabinets and are all original, though I wish they were in wood veneer . Because they needed foam surrounds on the cone drivers, I never got around to hooking them up; instead I tweeked my Klipsch Khorns and continued to build around the Khorns. Out of the blue, I decided to refoam the drivers and give them a listen. I substituted the 4343's for the Khorns in my biamp setup where I use a Crown DC300 on the LF and a pair of Marantz 9's on the HF's along with a pair of Heathkit XO-1's for the crossovers and a pair of Dukane Ionovacs to supplement the HF's. Though I loved my Khorns, hearing the 4343's sealed the deal for me as to which pair of speakers I decided to go with in the future. I was amazed and could not believe the reveling nature and detail combined with the smoothness of the 4343's when combined with my other gear.

Now that that is out of the way, here is my quick question. Based on my reading of some threads in connection with the 3143 crossover network, the threads seem to say that in bi-amp mode the crossover caps are eliminated from the network. Is this a correct reading? Though this seems counter to my understanding of the schematic, I readily admit I am a novice at these sort of things. If this is indeed correct, I see no sonic benefit to replacing the caps if one intends to bi-amp. Is this correct logic or I am I missing something (as is most likely the case).

ratitifb
03-10-2011, 12:26 PM
Based on my reading of some threads in connection with the 3143 crossover network, the threads seem to say that in bi-amp mode the crossover caps are eliminated from the network. Is this a correct reading? take a look on 3143 X-over schematic http://www.jblproservice.com/pdf/Network%20Schematics/3143%20Network.pdf and understand that in bi-amp mode, all the "LF" section is bypassed
like the 52F capacitor in the "MF" section (actually the low cut off frequency of the high pass section in the midbass frequency range) ;)

pen_man
03-10-2011, 01:02 PM
take a look on 3143 X-over schematic http://www.jblproservice.com/pdf/Network%20Schematics/3143%20Network.pdf and understand that in bi-amp mode, all the "LF" section is bypassed
like the 52F capacitor in the "MF" section (actually the low cut off frequency of the high pass section in the midbass frequency range) ;)

Ratitifb, Thank you for your response. What you are saying is my understanding in looking at the schematic. That is, the LF basically goes straight to the active crossover. However, doesn't everything above the active crossover setting of 300HZ still filter through the passive crossover including the 52uf cap? If so, isn't there a benefit to recapping the passive crossover?

ratitifb
03-10-2011, 01:25 PM
doesn't everything above the active crossover setting of 300HZ still filter through the passive crossover including the 52uf cap? If so, isn't there a benefit to recapping the passive crossover?the 52F cap is bypassed in biamp mode (S1A switch) so there is no benefit to recap it in that mode excepted if you want to use a day the full passive mode of the 3143 X-over. For sure and if needed there'll be a benefit to recap all the other caps in which the audio signal go through ;)

Rolf
03-11-2011, 04:04 AM
Replace the original network(s) with CC network, and you will get an "WOW" experience.:)

pen_man
03-11-2011, 10:55 AM
the 52F cap is bypassed in biamp mode (S1A switch) so there is no benefit to recap it in that mode excepted if you want to use a day the full passive mode of the 3143 X-over. For sure and if needed there'll be a benefit to recap all the other caps in which the audio signal go through ;)

Just to confirm, I can see that the 52mf cap is out of the LF circuit; with a flip of S1A the Red LF terminal goes to the green wire on the LF driver and the BLK LF terminal goes to the black wire on the LF driver. However, that still leaves the MF, HF and UHF. From my study of the schematic, it appears that the above 300hz signal generally goes through the remainder of the network including the 52uf cap. Is this correct? Hence, would it not be a good idea to recap the entire network even if in bi-amp mode? After all the above 300hz signal is still going through several caps and resistors.

pen_man
03-11-2011, 10:59 AM
Replace the original network(s) with CC network, and you will get an "WOW" experience.:)

Rolf, I saw several threads on the Charge Coupled network and was very interested in the idea of a CC network. Would one use this with Bi-amp? Is someone making these and offering them for sale as I never really saw a step by step on how to do it. Are you using a CC network? If so, please tell me a little about the difference it made.

DavidF
03-11-2011, 11:40 AM
Just to confirm, I can see that the 52mf cap is out of the LF circuit; with a flip of S1A the Red LF terminal goes to the green wire on the LF driver and the BLK LF terminal goes to the black wire on the LF driver. However, that still leaves the MF, HF and UHF. From my study of the schematic, it appears that the above 300hz signal generally goes through the remainder of the network including the 52uf cap. Is this correct? Hence, would it not be a good idea to recap the entire network even if in bi-amp mode? After all the above 300hz signal is still going through several caps and resistors.

It appears to me, also, that the roll-on cap for the lower mid (52uF) is taken out of the circuit under bi-amp mode. In biamp mode this leg of the circuit relies upon the signal modification of the outboard crossover to form the roll-on for the lower mid. The signal comes back around the 52uF cap and extends on to the high and ultra high circuits.

In biamp mode a blocking cap for the lower mid sure should be a consideration if you have a DC amp feeding the lower mid driver.

Rolf
03-12-2011, 05:21 AM
You got a PM.


Rolf, I saw several threads on the Charge Coupled network and was very interested in the idea of a CC network. Would one use this with Bi-amp? Is someone making these and offering them for sale as I never really saw a step by step on how to do it. Are you using a CC network? If so, please tell me a little about the difference it made.

Carl
11-23-2011, 01:39 PM
Before I get to my question, please allow me to provide a little bit of extraneous background information. I have owned a pair of JBL 4343 Studio Monitors for about 4-5 years now. I got them from the original owner and they even came with the original boxes. They are the grey utility cabinets and are all original, though I wish they were in wood veneer . Because they needed foam surrounds on the cone drivers, I never got around to hooking them up; instead I tweeked my Klipsch Khorns and continued to build around the Khorns. Out of the blue, I decided to refoam the drivers and give them a listen. I substituted the 4343's for the Khorns in my biamp setup where I use a Crown DC300 on the LF and a pair of Marantz 9's on the HF's along with a pair of Heathkit XO-1's for the crossovers and a pair of Dukane Ionovacs to supplement the HF's. Though I loved my Khorns, hearing the 4343's sealed the deal for me as to which pair of speakers I decided to go with in the future. I was amazed and could not believe the reveling nature and detail combined with the smoothness of the 4343's when combined with my other gear.

Now that that is out of the way, here is my quick question. Based on my reading of some threads in connection with the 3143 crossover network, the threads seem to say that in bi-amp mode the crossover caps are eliminated from the network. Is this a correct reading? Though this seems counter to my understanding of the schematic, I readily admit I am a novice at these sort of things. If this is indeed correct, I see no sonic benefit to replacing the caps if one intends to bi-amp. Is this correct logic or I am I missing something (as is most likely the case).

i too have a pair of 4343's and have recently had the 10" units re-built. The shop said the original voice-coils were burned probably due to my xover caps may be bad, since they are still original, dating from the late 70's.

my ? is, should i re-build the xovers or invest in a bi-amp package? if bi-amp, can u suggest a list of amps & active xover?

in the re-build of the existing xover, where can i get the caps and shouldn't the inductors be OK since they're merely just wire?

Carl

ivica
11-24-2011, 12:49 AM
Just to confirm, I can see that the 52mf cap is out of the LF circuit; with a flip of S1A the Red LF terminal goes to the green wire on the LF driver and the BLK LF terminal goes to the black wire on the LF driver. However, that still leaves the MF, HF and UHF. From my study of the schematic, it appears that the above 300hz signal generally goes through the remainder of the network including the 52uf cap. Is this correct? Hence, would it not be a good idea to recap the entire network even if in bi-amp mode? After all the above 300hz signal is still going through several caps and resistors.

Concerning the "fragile" nature of the coil of the 2121 driver (repairs almost impossible to be done, or recone),
my suggestion would be to use larger capaciotr (then 52uF) in-series to the rest of MF network section

Robh3606
11-24-2011, 10:33 AM
Why would you use a larger capacitor?? It won't add any additional protection for the driver.

Rob:)

cohearent
11-24-2011, 11:10 AM
Caps in parallel add. A cap in series REDUCES the capacitance, which would raise the cut off frequency. -Less bass through the driver. May or may not help the problem. It will reduce the excursion some, but won't reduce the power through the driver much.

ivica
11-24-2011, 12:34 PM
Why would you use a larger capacitor?? It won't add any additional protection for the driver.

Rob:)

To be clear, instead of 52uF and 2.9mH I would suggest larger capacitor (say low impedance for frequency of interest in driving 2121) just to protect 2121 from LF frequency. So good would be just to disconnect 2.9mH, and add some larger capacitor parallel with 52uF, so frequency for 2121 would be mainly influenced by "front end" electronic network, but DC and low frequency would not burn the coil of 2121.

as: http://www.audioheritage.org/vbulletin/showthread.php?30651-Short-Intro-and-Quick-Question-on-JBL-4343-Crossover-Design&p=323836&viewfull=1#post323836

Robh3606
11-24-2011, 08:23 PM
Hello ivica

Here is the voltage drive for a series capacitor with an 8 ohm load. One is 52uF the other 100uF. It's clear any increase in capacitance will have no additional benefit for the LF protection of the 2121. If anything it actually would make matters worse as it would allow even more LF drive into the 2121. As shunts the value change has no effect.

Rob:)

Allanvh5150
11-24-2011, 10:38 PM
Wont a larger capacitor in parallel with the 52uf make something somewhat smaller than 52uf?

Allan.

ivica
11-25-2011, 05:31 AM
Hello ivica Here is the voltage drive for a series capacitor with an 8 ohm load. One is 52uF the other 100uF. It's clear any increase in capacitance will have no additional benefit for the LF protection of the 2121. If anything it actually would make matters worse as it would allow even more LF drive into the 2121. As shunts the value change has no effect. Rob:) I can agree with you. My intention was to "protect" 2121 as much as possible, with as less as possible the influence of passive network between bass and mid-bass, so on the figure the influence is visible. On 4343 L-pad to 2121 I think is 16 Ohm (expected 2121J - 16 Ohms, may be)

May be interesting:
http://www.audioheritage.org/vbulletin/showthread.php?15666-Homebrew-4343&p=160573&viewfull=1#post160573
or
http://www.audioheritage.org/vbulletin/showthread.php?774-4355-3155-clones&p=6343&viewfull=1#post6343
or
http://www.audioheritage.org/vbulletin/showthread.php?15666-Homebrew-4343&p=162016&viewfull=1#post162016

4313B
11-30-2011, 02:18 AM
There is nothing wrong with the 2121 (other than it has undesirable breakup, hence the development of the 2122). I do like the looks of that 4-inch inverted dust dome though. It doesn't need any "protection".

Just bi-amp the whole mess and use this network. It's the bi-amp only network for the 4340, 4341 and 4343.

Soloford
04-08-2012, 03:31 PM
There is nothing wrong with the 2121 (other than it has undesirable breakup, hence the development of the 2122). I do like the looks of that 4-inch inverted dust dome though. It doesn't need any "protection".

Just bi-amp the whole mess and use this network. It's the bi-amp only network for the 4340, 4341 and 4343.

I just came across this thread...I hope I'm not butting in here, but this thinking fit's in exactly with my thinking on my own 4343B Project. As I've started going thru the Parts Express catalog I have a couple of questions...

First, is there a sonic difference between the alnico drivers from the 4340 and the ferrite drivers in my 4343B's that has to be compensated for in the replacement network?

Second, can/should I re-use the L-pads from the original 4343B's, or should I replace them with new pots? I'm wanting to replace them, so that I do less to the original network. If I go the replacement route, any recommendations for which pots to use?

I originally thought of using no pots, but looking at the efficiency of the 2121h versus the two horns makes me think I will need something to dial back the horns.

Thanks for any comments.