It's part of the 4365 network. I took a quick screen grab from the Engineering Design Specification. It was all I had handy at the moment.
4365 schematics. Post 25
Diodes are cheaper.
And to answer your other question, the L212 system probably takes the cake for extended low frequency response.
All the other systems you mention are quasi-third order (without the active EQ) or quasi-fifth order (with the active EQ) so they all roll off much faster below 25 Hz.
Room gain is typically a second order rise below a certain frequency determined by room dimensions. It's easy to see why sealed box systems can potentially yield the lowest response. Unfortunately for the B212, it blows up fairly easily when driven real hard. Given this information Jerry seriously reinforced the 1500SUB and W1500H drivers so they could withstand severe abuse in small sealed enclosures. These particular drivers may go deeper in sealed boxes but alot of people prefer the greater efficiency a vented box affords and are willing to give up a bit of bottom end to get it.
No I have a pair of the 121A drivers in sealed 2 cubic Ft boxes. The quality is great the issue is output. They have limited X-Max and sealed you are depending on room gain to fill in the last octave.Was the L212 really the be-all end-all bass monster of the JBL family tree ?
"I could be arguing in my spare time"
Well Yes and No about being way off topic. I had not considered that they had EQ in the bass module. That doesn't change the limited X-Max and power handling. The EQ steals gobs of headroom from them so in some ways it actually makes matters worse. If you drop them into 1.5ft sealed box you need about 10db of EQ to get them flat at 32Hz. Unless I am missing something that gives them what about 10db headroom based on the amps power output?? If that assumption is correct that's not too much to work with and clearly why you needed more than 1.But that is the driver... the L212 had an eq built in so it did dig down deep. Same is true of the Sub1500... in it's initial guise as the original Revel Ultima sub with equalized ML electronics, it was a formidable sub. Though like it's vintage counterpart, it too blew up under high SPL demands. Using multiples helps a lot in both cases.
Not saying they were not great in their day just that it has past.
"I could be arguing in my spare time"
The B212 was -3 dB at 25 Hz and 60 Hz. Below 25 Hz it rolled off at ~ 12 dB per octave. Vented subwoofers roll off at anywhere from 18 dB/octave (quasi-third order which low Q JBL transducers excel at) to 36 dB/octave (6th order assisted as per Keele). There was no EQ bump with the B212, it was done via bandwidth limiting, aka low pass cut filter. And yes, that is pretty inefficient. It is more fun to do it with something like a 99 dB SPL 2242H that is really hard to blow up under any kind of home hi-fi conditions.
JBL has never made a sub capable of its extended low frequency performance, quite frankly, they couldn't afford the warranty claims against such a subwoofer. Modern source material (the digital compact disc) revealed the shortcomings of a transducer having such limited xMech.
The 121 wasn't the only one to blow up under pressure. The 124/2203, LE14, 136/2231 and 2235 did as well. All of them had the exact same limited xMech. The coil formers would rip away from the cone assembly. The slightly more robust 2245 did better but even that 4645 system had to be tuned back up from 25 Hz to 30 Hz when people started putting them in IMAX theaters and they'd go poof. When JBL dreamt up the 2242H, the 4645 could be tuned back down to the traditional 25 Hz without fear.
What really floors me is some people were actually able to blow up B460's in their homes even with the extra protection of the BX63. I find that truly staggering. But you know what they say, you can take the <> out of the trailer park but you can't take the trailer park out of the <>.
That's where a sub such as the S2S comes in. Clamp the crap out of the fifteen with a spider so stiff the free air is back up in the thirties and tune it so it yields a ton of thump. Don't even let the thing get down into the twenties.
Before I got a sub for the home theater, I ran a 5.0 4430 system that did not have any subsonic protection/filtering.
I ran that system pretty loud at times, on the edge of uncomfortable for my hearing, and I don't recall hearing mass rings smash into things or other signs of stress. At the time, I wasn't running any room correction so I had a massive room contribution to the bass. This was a fairly small room < 1500cu.ft room.
At least for me asking for more volume would have been system/auditory abuse to see "what it could do".
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