Hi all. I've recently inherited a pair of JBl 2231a woofers, in need of refoaming, plus 2420 horn drivers, 2312 horns and 2405 slot tweeters. Good for 4333 build, or something close. Id like to build cabinets that get the horns to seated ear height without needing stands, for one thing, possibly positioning the horns on top of the cabinets, bracketed somehow.
Anyhow, my questions really are around crossovers. I'd like to follow Nelson P's design, simplified to exclude l,pads. I see there is a seller on ebay that sells PCBs for a version of this, that features a charge coupling circuit.
Curious if anyone has heard either of these operational... how do they sound?
Also looking for general comment.. I am thinking on First Watt SIT5 to drive these... enough power? I have never had anything to do with JBls before... please excuse my ignorance. Also have MC275 mk6, FirstWatt VFet amps, and a bryston kicking about as alternatives. But really hoping for an expansive, holographic soundstage with the SIT5 for electronic music... not necessarily loud, just lots at low volume. Thanks for any advice here..
Anyhow, my questions really are around crossovers. I'd like to follow Nelson P's design, simplified to exclude l,pads. I see there is a seller on ebay that sells PCBs for a version of this, that features a charge coupling circuit.
Curious if anyone has heard either of these operational... how do they sound?
Also looking for general comment.. I am thinking on First Watt SIT5 to drive these... enough power? I have never had anything to do with JBls before... please excuse my ignorance. Also have MC275 mk6, FirstWatt VFet amps, and a bryston kicking about as alternatives. But really hoping for an expansive, holographic soundstage with the SIT5 for electronic music... not necessarily loud, just lots at low volume. Thanks for any advice here..
Some of the eBay vendors offer a crossover modelled on the Pass design with added charge-coupling. Feedback from DIYers suggests they are well-made, though you should verify the PCB layout is correct and has appropriate trace widths and spacing.
If your 2311 horns lack the 2308 slant plate lenses, their response will be very beamy (narrow, only about 40 degree dispersion) compared to the 2405's 140 degree horizontal dispersion in the HF crossover range.
With or without the 2308, the horns should be on a wide and tall baffle for their response to be similar to the cabinets and crossovers you wish to emulate.
The cabinet could be built with the same internal volume with the horns at seated ear height without needing stands. That said, since you are looking at low volume, low power, increasing the cabinet volume and tuning a bit lower than the 4333's ~35Hz Fb may be a better choice considering how much content below that is common with electronic music.
The JBl 2231a are about 93dB 1watt/1meter, the 2405 is ~105dB 1w/1m.
The 2420 on the 2312, ~112dB, or ~107 with the 2308 lenses.
Without L-Pads, the mid/high above 700Hz would be ~+10dB louder than the bass, the speakers would sound honky and tinny, and the the crossover frequencies are based on the L-pads in circuit.
https://www.firstwatt.com/wp-content/uploads/2024/01/art_l300.pdf
I have not compared the charge coupling circuit to a standard crossover, but from what I've read about what they do, I doubt I could have heard the difference when my hearing was good, and the drivers were new.
The SIT5 puts out 35 watts @ 1% THD, so the pair of cabinets should do around 114dB at 1 meter with no room gain.
That's fairly loud in a domestic setting.
The MC275 mk6 is rated for 75watts, about +3dB more output.
150 watts, another +3dB.
Past 150 watts, the low end won't get much louder, but will distort more, which sounds louder.
The 2405 coils can be vaporized with fairly short peaks, and only aftermarket diaphragms are available now.
At low volume, you probably will want to turn the mid range L-pads down from "flat" for an appropriate "loudness contour".
Art
You lucky, lucky bastard. Oh, to inherit a pair of JBl 2231a woofers, 2420 horn drivers, 2312 horns and 2405 slot tweeters. Doomed to play them softly!
Sorry my Monty Python was hanging out.
Sounds like a great project.
Sorry my Monty Python was hanging out.
Sounds like a great project.
@weltersy- thanks for this marvelous response. I'm hugely appreciative.
I suspect the compression drivers have radian diaphragms in them - i must check on that sometime. At any rate, they measure 8 ish rather than 16ish ohms.
I suspect the compression drivers have radian diaphragms in them - i must check on that sometime. At any rate, they measure 8 ish rather than 16ish ohms.
The nominal "8 ohm" 2405H has a DC Resistance of 6.25 ohms, the "16ohm" 2405 DCR is ~8ohms.
The 2420's DCR will be under half of the rating, ~6-7ohms for the "16ohm", 3-4 for the "8ohm".
The actual impedance curve is dependent on the horn used.
It is because a very long time ago, I owned a set of 4343 speakers and I still miss them. I won't say that the sound is for everyone, but there is no doubt about these models being highly sought after still after all of these years. If I were you, I would be very inclined to build at least the cabinets to what was original. I am sure that you not building these to sell, yet I am stating that this is a product that you don't want to short change or cut corners on. You have inherited a coveted product, and not someone else's crap. If done with care, these would be a desirable speaker for years to come. I would advise having adjustments for the drivers from the cross over though.
I haven't seen such a clean 2420 since 1979...
The DC resistance is correct for a "16 ohm" diaphragm.
As far as what manufacture made the diaphragm, not sure, but I don't think JBL ever used the "35C" designation.
Radian puts their name on the diaphragm.
At any rate, the gap cleaning and diaphragm alignment takes some skill, do a low level sine wave sweep test from ~200-16kHz (.5 volt or so) to make sure there is no buzzing.
@weltersys some of finds. The 2231a cones are original, but the foam surrounds have been replaced, and poorly. They will have to come off and be redone.
Attachments
The surround install looks decent in the photo, what do you find wrong with them?
The foam surrounds have been succesfully glued to the basket, however not so to the cones. There are open gaps where the bonding has not taken place. It is a shame it was not done correctly, but it is what it is. I am considering getting the job done by a professional - I am sure what has happened here is an amateur attempt - and I am not certain I would do better.
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That is a JBL diaphragm. The JBF stickers were on tons of them. I don’t know what the 35C means but possibly denotes a production run as these are not constant like the JBF was.
If it has a half roll surround formed into the diaphragm is is a 2420. If it has diamonds like your picture in the white foam and is aluminum it is a 2421. If it is Ti it is a 2425.
The diaphragm on the white foam is also a real JBL.
Barry.
Although some JBL ring radiator tweeters had 8 Ohm labels, the coils are all very nearly 12 Ohm AC impedance. There are three diaphragm variations but the coils are all the same.
Nice collection you have there.
Barry.
No, the aluminum 2410/2420 had a spiral diamond pleat, the phenolic 2461/2470 had a half roll.
The OEM diaphragms used to come with this card warning "This Part Is NOT Self-Centering":
With out proper centering, the voice coil/former can drag and buzz, sounding terrible even before the coil would burn from arcing in the gap.
Correct.
After decades with no change in the LE85/2420, the 2421 was only made for months before the transition from Alnico ferrite slab magnets and Aluminum to Titanium, with increased power handling.
For PA use, it was the difference between replacing shattered aluminum diaphragms on a near seasonal basis to titanium lasting over a decade.
Art