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Thread: Is it sacrelege? Am I evil? (long) - 4343 xover mods

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    Senior nOOb 4343mod's Avatar
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    Question Is it sacrelege? Am I evil? (long)

    I have a pair of 4343's, #13602 & 16644.
    I bought these off of a friend who drove them with an Acoustat.
    (Jim Strickland Trans-Nova amp, before Hafler)
    This amp has a big DC pulse on power up. You could actually see the 15's flex outward for a moment! De-mag/re-mag service of the 15's at JBL in Northridge is needed to bring the sensitivity back to spec. Someday I will get to it.

    I initially used them with (2) 350W Jeff Rowland Model 7 monoblocks for a year or so, when a horrible thing happened. The cheese rotary switch on the left monitor started shorting at very high levels. The first 2 or 3 times, the Rowland would step into standby, like a thoughtful amp should. Then late one night when I was touching the face of god, inraptured in sonic nirvana, it happened again, and 1 output rail on the rowland went. (12) TO-3 transistors, toast. Rather send the 120-pound amp back to Colorado for repairs, I sold them both as-is to a local dealer. What a shame.

    That was the last straw. An incredible system, with almost limitless dynamics taken down by a $.50 part. Yes, these are rated at 150W, and saw 300+W regularly. Very convincing reproduction of concert levels! No, this disaster would never have happened with flea SE tube amps, I know. And I wouldn't need a towel next to the listening chair, either!

    It became evident to me that the design of drivers and cabinet were first rate, (obviously) and that JBL cut corners on hardware and internal componets. I, C, and R in the XO's were not up to what these speakers could and did deliver. the (2) 2 Ohm Ohmite series resistors in front of the 2420's were powdered grey from overheating, and that POS rotary switch in the binding post plate was blasted as if it saw arc from a MIG welder. All level controls were ALWAYS set to max; to turn them down, you might as well put cotton in your ears or listen with the flu.

    IMO, R is sonically destructive in a crossover. It should be used as little as possible, unless you like a veil between you and the music. Pots are the worst; fixed value parts should always be used when possible.

    Now, having seen the crossover schematic, and tanking up ideas with a friend, it became clear the XO slopes were more for withstanding abuse and protecting the horns from distorting, poor quality, low powered SS amps, and less for clean transmission of signal. The 2 big caps in each XO were 'Lytic cans! 25 year-old chemical capacitors are not what premium sound is about! What if I made purist 1st order XO's with beefy, quality (if not premium) parts?

    On to the debauchery!
    I removed the OEM crossovers, wire, binding posts & plates. I disconnected the pots, though they are still in the cabinets, acting as hole plugs. I made a couple of post plates to accomodate (4) pairs of posts each. These posts are wired directly to the drivers. I then made a couple prototype XO's with air core inductors, Solen metalized film caps, and Mills NI resistors. Some non-NI resistors have found their way into the circuit, as padding for the 2420's. I use a NOS Centralab rotary switch on each channel to select different I values across the 2420, ususally set to .15 or .20mH. All wiring is point2point with Wonder Solder. Wire is garden variety Belden 16 gauge, except the LF circuits, which are Belden 14 gauge.

    This arrangement allows for single, bi, tri or quad amping, as well as *gasp* active filtering with an outboard device.

    Using the (6) OEM T-nutted holes on the back of the cabinets, I put screws through from the inside, thereby giving me threaded studs projecting from the back. The XO boards are mounted on these studs. No cutting, drilling, or other permenent changes were made to the cabinets. Everything is reversible.

    On to the sound!
    There is not 1 area of reproduction that has not been improved with these changes. If you have heard 4343's in OEM form, all I can say is imagine upgrading all of your electronics and cables up a couple of levels. That approaches the magnitude of improvement wrought by this. The Bob Harley analogy of "looking through several dirty windows then through 1 clean one" applies here. Macro dynamics have not changed much, but the micro dynamics, the little sparkles and shimmers of detail are stunning. I can hear into the images so much farther and with greater accuity than ever before. The speed of attack and decay is astonishing! Those that have not heard a 2420 horn with a single cap and resistor in series are missing something.

    The bass response is more natural. Going from a 2nd order slope with an old, dried up 120uF 'lytic cap and tiny 20 gauge, 5uF inductor to 1st order with a 16 gauge, 9mH ERSE inductor is amazing. The boxiness is gone. The tonality and weight has taken shape like never before. The 15's play louder with less coloration. And the 15's are not even performing at spec sensitivity! Why would anyone use electrolytic caps in a high performance XO? And after 25-30 years, do you think the 120uF value is still stable? No way.

    All I can say is the OEM XO's are slow, veiled, garbled, soft JUNK.
    What a travisty to put this in front of such wonderful Alnico compression drivers.

    I welcome any JBL enthusiast in the Los Angeles area to drop by and have a listen. It may be a sin, but it sounds Soooo good.
    Any comments, slander or suggestions are welcome!

    Thanks for reading this, Andy Bacon

    System info:
    244VAC, 2-pole breaker out of a 100A load center into
    PowerStat 260V, 30A Variac, stepped down to 220VAC into
    Topaz/Square-D 5kVA Ultra-Isolator line noise supressor (transformer) lifted ground, ~120VAC into....

    Meridian 588 24-bit CDP, balanced out
    Harmonic Technology TruthLink, XLR
    Classe' CP-60 pre, balanced out
    Harmonic Technology TruthLink, XLR
    Crown Studio Reference1 or Krell KSA-200S
    Analysis Plus Oval9
    JBL 4343 Modded Macro Monitors!
    Attached Images Attached Images  
    Last edited by 4343mod; 10-17-2003 at 01:10 PM.

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