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JBL 4343 - 4344 upgrade thread
JBL 4343 - 4344 upgrade thread
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Old 18th September 2005, 12:30 PM   #1
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Default JBL 4343 - 4344 upgrade thread

Earlier last month I started this htread over on the Lansing forums.

As a result of some interesting comparisons with the Mox crossover and my Hot Rod Aleph I propose to re post relevent aspects of the project here.

Attached below is the latest revision of the framework for this project. We've added sections on basic setup and speaker placement and a section on improvements that can be made to the cabinets. We've also added an open ended section at the end for members to describe their more exotic modifications to their 4343 based monitors' drivers, cabinets, crossovers, etc.

Upgrade Modifications to the JBL 4343/4343B Studio Monitors:

This thread will describe several different upgrades available to owners of 4343 and 4343B Studio Monitors. Although we are focusing primarily on upgrades to the 4343 series monitors, owners of other Studio Monitors, 4344, 4350, 4333, etc, will find that many of these upgrades could apply to them also. The upgrades will be broken down into various upgrades starting with relatively simple upgrades up through a full upgrade of the drivers and crossovers. The basic framework here is to offer modifications that improve the performance of the 4343/4343B monitors while preserving their vintage appearance. A requirement here is that the speakers continue to look original. Therefore we will avoid any modifications to the cabinets or grilles. In addition, it is not within the scope of this thread to discuss how to restore the cabinet or grills. There are numerous other threads on the LH Forum that cover these aspects in great detail.

Setup and placement

1. Amplifier power recommendations

2. Connections

3. Room acoustics and monitor placement

4. Slot Tweeter placement- inner vs outer

5. Removing the top baffle

6. Measuring performance.

Upgrade 1 System Maintenance:

1. Clean L-Pads, replace if necessary

2. Clean/improve terminations.

3. Testing condition of drivers and crossovers

4. Recones, refoams and new diaphragms.

Upgrade 2 Upgrade 4343 drivers to 4343B spec.

1. Replace 2231A with 2231H

2. Replace 2121 with 2121H

Upgrade 3 Bypassing the bi-amp switch.

Upgrade 4 Upgrade the stock crossovers.

1. Replacing capacitors.

2. Bypass Capacitors

3. Charge coupling

4. Upgrade crossover to 3145 spec.

Upgrade 5 Discussion on Bi-amping.

1. Pros and cons.

2. Active crossovers available on the market

Upgrade 6 Upgrade drivers to 4344 spec.

1. Replace or recone LF drivers to 2235H

2. Replace or recone MF drivers to 2122H

3. Replace or rediaphragm HF compression driver to 2425J.

Upgrade 7 Converting to Hi End Crossovers.

1. Internal vs External placement

2. Passive

3. Active

Upgrade 8 Cabinet Upgrades

1. Improve internal bracing.

2. Modifying the top 4343 baffle

3. Modifying the bottom 4343 baffle

4. Cabinet and grille repairs

Upgrade 9 Discussion on modern drivers.

1. Modern driver choices available today

2. Use of a subwoofer.

Upgrade 10 Other Upgrades.

1. External Horns

2. 4345

3. Other upgrades
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Old 18th September 2005, 12:32 PM   #2
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One of the things I am mindful of is the question "what can I expect from these changes" particularly in reference to steps 5, 6 and 7. And not everyone one will be keen to starting making updates without some knowledge of the gains or improvements.

This has not been an easy issue to provide specific answers on so we been doing some very interesting tests and comparisons.

First off, we need to look at the 4343 in terms of its have's and have not's and in order to see what is going on in the system this means operating it at its basic level. This means full passive mode.

So Ed and have been doing some listening comparisons to arrive at some conclusions and applying the results to the technical side.

Step 4 has not been fully explored however working on the premise that good terminations are vital any unnecessary switches should be removed from the signal path.

To test this I placed a heavy duty jumper lead with crocodile clips in series with the positive input to the crossover network and did some blind test comparisons.

The cable was pure OFC and terminated to a Cardas hi quality binding post.
Inserting the jumper lead in series with the crossover and the amps caused a loss of low level information and the sound stage collapsed and sounded flat and planar.

So, while cables are often the flavour of the month in audiophile chat rooms please be aware that it's the terminations that are the final arbitrator of conductivity. Hence, a switch or other brass contact, oxidised or otherwise will downgrade the purity of the signal. Think of the amp as your car battery and the speaker as the starter motor.....its a good analogy.

Good terminations...we will discuss this in more detail in step 4.

Step 5. Upgrading the stock crossovers. This is the most popular issue and also the most expensive and controversial update for anyone contemplating a speaker restoration or a new design.

Again, we have organised some comparisons and have come to some conclusions. Some of these comparisons were conducted with the 4343 system and others simulated using the reference system the 4345.

Specifically in the 4343 the 3143 crossover has the midrange, horn and slot signal routed via a 52 uf capacitor in the midrange bandpass filter.

This was done according to JBL to improve the protection to the compression driver and slot from amplifier turn on/off transients. In the early days this was a problem but with modern amplifiers it is far less of an issue.

In a recent comparison with the reference system I was able to determine that the arrangement of this capacitor does cause a loss on information to the compression driver and slot. Specifically a loss of ambience and very fine details....much like a more polite but otherwise muted sound that lacked snap and dynamics. The series capacitor was a Solen polypropolyne metalised film foil bypassed with a 0.1 Solen film and foil capacitor (not charge coupled). The amplifier used was a Passlabs Aleph design.

By removing the capacitor entirely and putting the system in bi amp mode using a very special, and hi quality active crossover we also observed marked improvements in midrange clarity, transparency and improved purity. This was very noticeable on vocal harmonies where individual voices could more easily be identified. The active crossover used in this comparison is a discrete class A unit with a Passlabs Aleph amplifier on the highs and a PSS 600 amplifier on the lows.

We also observed that the 4343 in full passive mode can perform better than in active mode and the determining factor here is the performance of the active crossover and the amplifiers. Again a loss of information lead to less realistic results in biamp mode while the bass was more detailed ahd more authority in bi amp mode. The crossover used in this comparison was the Ashly analogue unit using a Passlabs X 250 amplifier on the lows and Mac amplifier on the highs. A Passlabs X 250 amplifer used in full passive mode.

So we can see that by reviewing the crossover network, the manner in which it is arranged and the use of a hi quality active crossover will yield a net improvement in musical performance.

Details of various upgrades will be discussed in step 5.

Step 6 Biamping Pros and Cons

Refer to step 5. The pros are improved bass clarity and authority. The cons are loss of transparency, imaging and information. The extent of the later will depend on the active crossover in use. Analogue active crossovers a deemed better and more affordable for home use than pro audio digital active crossovers. However budget prices will only buy budget performance in both cases.

Crossovers available on the Market.

Well there are dozens from entry level units like the Ashly, the JBL 5235, the Bryston and the Passlabs. The last 2 are premium grade but aren't cheap. Please note 4343-4343 does however require a specific crossover characteristic in order that the system works correctly.

I may be enticed to assist with providing a customised crossover with appropriate performance to interested parties. Pm me for details.

Step 1, 2 and 6

Surprisingly drivers to deteriorate with age,particularly the compression driver diaphram. Have it check by an authorised JBL agent/reconer.

There in an improve driver/diaphragm used in the 4344...the 2425J and there is rare aluminium versions of the diaphram seen on Ebay.

While is had previously built a cloned 4343 its on on sight for direct comparison with updated driver. The 2425J is a superior a driver and the aluminium variant of the diaphragm sort after.

Many have trouble taming the horn in the 4343, I think this is one of the biggest issues with the 4343. The solution is a mix of driver, crossover upgrade and careful setup.

I have used both the 2231A and the 2231H and 2235H. There are differences in power handling and subtle differences in performance. The 2121 and the 2122H are different beasts. I have not A/B'd them so I can't comment. The 2122H does however require a different passive crossover.

We will discuss this in more detail later and perhaps other members can post there experiences with driver updates.


As mentioned in an earlier post it is important to view the loudspeaker as a system. That is the sum of all the parts of the system come together to create the desired level of performance.

In order to obtain the maximum potential from the loudspeaker we must therefore look critically at each part of the system and decide what updates are required and "how" to implement those updates.

From our investigations we have identified the 4343 system in its raw state is far from capable of producing the maximum potential of its performance. Care needs to be taken when introducing ancilary equipment like active crossovers or graphic equalisers, as they could well be hindering rather than providing a net gain to attaining the maximum potential performance from the system.

The purpose of this post was to give some degree of indication on the net effect of these updates.

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Old 18th September 2005, 12:34 PM   #3
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More experiments.....

Been listening to the stock JBL 5325 crossover today with 4345 cards that just arrived from Santa Rosa CA....thankyou SpeakerDave.

Well its been very interesting.

The good news is it's very clean sounding.

The bad news is the 5235 crossover makes every recording sound the same ....... .

A . By this I am referring to the presentation......there is a very strong sense of presence in the midrange, lots of drive.

If your JBLs sound in your face then this is why.....and ultimately fatiguing.

I think this is another long held characterisation and a false one at that of JBL's in particular the 4343 that is going to be sorted out in this project.

Basically the speakers are a hi end system and it these sort of issues that ear mark hifi from hi end.

B. I compared it to my own crossover and it by comparison sounds perhaps reserved, not in your face. Once I played different stuff the skill of the recording engineer becomes obvious. The music has soul and natural energy.

Removing the active crossover gives the same result as B with some loss of bass detail.

I will attempt the null dc offsets the discrete opamps and bypass the coupling capacitors (B) and see what happens.

I decided to completely bypass (short out) the audio coupling capacitors in a key area just to see what might happen.......

Well let me say its going to stay that way......

I will refer to this as the Earl revision to the design.

The very fine detail and nuance at frequency extremes has opened up while the mids now a sheer clarity that is difficult to describe. Individual tracks appear to take longer to play while at the same time I now feel compelled to focus on the characteristics of specific instruments.

This perhaps is a sign of one being at ease and accepting the sound as correct.... I think so.

I now propose to charge couple the polystyrene capacitors in the actual crossover filters. This will be referred to as the Giskard revision to the design.

What amazes me is that one easily can pick and hear all these refinements up on a 20+ year old loudspeaker design.

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Old 18th September 2005, 12:36 PM   #4
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Here is the rear panel of The High Low Pass
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Old 18th September 2005, 12:39 PM   #5
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Here is a nice interior shot.

My special thanks to Milan and Nelson for helping make a dream come true.

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Old 18th September 2005, 12:40 PM   #6
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After evaluating the 5235 Crossover and the earlier comparisons Ed did b/n full passive an the Ashly active crossovers I am convinced active crossovers can be the route of all Evil in attaining hi end sound quality.

The driver upgraded 4344 is quite capable of hi end sound reproduction if
only given a chance.

Sure, nice passive crossover parts refine and improve transparency but the effect of the Pro PA active crossover cannot be undone..ever....

My tests and modifications which I am yet to finalise on my own class A
active crossover confirm the level of subjective sound quality the 4344 can

Done right, an active crossover can provided pin sharp bass transients and
mid range clarity unattainable with a full passive crossover.

Done wrong and its all a pointless exercise...unless of course you prefer listening to Mid Fi.

Yes this does sound like a rant and maybe some of you already appreciate the above. I just find it very annoying that JBL never put out a better unit for these vintage monitors......its not as though they were'nt selling in Japan.

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Old 18th September 2005, 12:47 PM   #7
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Upgrade 1 System Maintenance:

Clean L-Pads, replace if necessary

Clean/improve terminations.

Testing condition of drivers and crossovers

Recones, refoams and new diaphragms.

Okay lets get on with project..its dragging on a bit....

Cleaning L Pads

Remember we are following a total system philosophy here, so there is no point just focusing on say cleaning the L Pads without addressing all the issues in each topic.

Before proceeding to embark on the above consider if you want to do upgrade No 6 and 7 as these upgrades will require change out of the Mid and HF L Pads in the original 4343.

If however you are like me and prefer to build an external outboard crossover with audiophile parts the original L pads can be bypassed and left in place by using external L Pads contained within the outboard crossover. This approach will allow you to preserve the 4343 in vintage state should you decide to sell your 4343's a some stage. (we will discuss details of external crossover implementation shortly)

At any rate give the L Pads a turn and listen if they crackle. If they do then you will need to either replace or clean them. I would tend to replace them if they are stock due to age..they are not expensive. They are a 50 watt 16 ohm version for the mid and HF Levels and 8 ohm version = for the Uhf Level. These parts can be found on the Parts Express on line catalogue.

I personally am not familiar with the proceedure for removal of the L pads in the 43xx monitors but you will need a hot air gun or hair dryer to carefully remove the Foil Cal.

Perhap someone can come in with the fine detail here......



Now the terminations.

Unfortunately, teminations tend to oxidize over time and should be cleaned with a contact clean..ProGold or similar.

The stock terminations as I recall were not the best by todays standards so you might also want to consider replacing them with a set of quality binding posts.... I highly recommend the Cardas and the Eichmann range.



Do Not use cheap brass binding posts, they have poor conductivity and tend to be of poor quality construction.

The speaker wire should be stripped and the conductors cleaned with a sharp knife until you see bright copper. The bare wire can either to secured firmly to the binding post or you can use a spade.

Again, Do Not use cheap Brass Spades, use only pure Copper Spades.

You can see in the picture below how someone has replaced the binding posts to the rear terminal panel of the 4343.(C/ - Jean)

Testing condition of drivers and crossovers.

Have a look at the crossover for signs of heat stress or burns. If any of the resisters are blackened they should be replaced. Consult a qulaified technician unless you have the tools and skills to perform the repairs. Some experienced members may also be willing to assist.

Now the drivers, as per the hundreds of posts on the forums check if the woofer and mid cone have damage to cones or foam rot. I do not recommend attempting to refoam a driver. Again there are threads on the forums that explain the reasons for this.

Send your drivers to an authorised JBL Service Agent, they will come back as good as new.

The same applies to the compression horn driver and slot radiator for diaphragm replacement, its just not worth screwing around with at home.

However if you must look inside the compression driver go here:

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Old 18th September 2005, 12:52 PM   #8
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Upgrade 3 Bypassing the bi-amp switch

Basically bypass this shocking rotary switch

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Old 18th September 2005, 12:54 PM   #9
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Here the actual switch
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Old 18th September 2005, 01:00 PM   #10
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Also have in the wings something I have waiting to complete for several months for this project.

Anyway this device called The Class A Plate amp. ..aka The Aleph Hot Rod/ X Aleph

Most of use know a Plate amp as a bolt on amp that is generally used for sub woofers..they are sold by vendors like Parts Express.

This however is a Plate amp with a difference. Its custom designed for the 4343-4344 to drive the mid and Hf sections.(75 watts rms)

Its is proposed that a simplified Mox crososver be used to provided the correct 4343 crossover transition.

The Class A Plate amp will offer the benefits of bi amping..without the negatives.

Lots of wires and amps lying around the listening room. Basically a mess you and your wifey don't need. Plus the active crossover will enhance the performance rather than smear the sound as we have found in recent tests.
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