Changing the driver in 212 sub

[Freiberg Baard]

Hi, I recently aquired a beautiful set of the L212. I really liked the sound, funny how one can hear the difference between all the different old JBL models like that. Not that I have heard that many, but quite a few, yes. Regarding the sub in this set, I had heard and read that it was weak, and I tend to agree. Then I did this: I replaced the 121A driver in the 212 sub with a 12" driver from the JBL PB12. I could hear the difference imediately, the bass was more "detectable", louder. But then I came to think about the amp in the sub, could this change in drivers be harmfull? So I put the 121A back in. But the question still bothers me, was it harmfull for the amp or not?

[HCSGuy]

As the driver in the PB12 is not available separately, JBL does not publish impedance specifications for it. You should use a multimeter to at least check the DC resistance and make sure they are in the same range. If they are, you will be safe using the new driver, but the subwoofer is the weakest link in that system - 80 watts, right? You can build or buy many better subwoofers, though the sats are fine left as is.

[Rolf]

Hi. I am not technical, but if it sounds ok, then it is ok.

Hi, I recently aquired a beautiful set of the L212. I really liked the sound, funny how one can hear the difference between all the different old JBL models like that. Not that I have heard that many, but quite a few, yes. Regarding the sub in this set, I had heard and read that it was weak, and I tend to agree. Then I did this: I replaced the 121A driver in the 212 sub with a 12" driver from the JBL PB12. I could hear the difference imediately, the bass was more "detectable", louder. But then I came to think about the amp in the sub, could this change in drivers be harmfull? So I put the 121A back in. But the question still bothers me, was it harmfull for the amp or not?

[HCSGuy]

The reason I recommended checking the DC resistance is that in modern powered subwoofers, the manufacturers can have the woofer and amplifier built non-standard, but built for each other. Some use lower impedance woofers to get more power out of a given wattage design - this would not be could for the bass module's amplifier, and finding someone with a schematic for that amp would be tough. OTOH, I repaired a Def Tech sub a few weeks ago that used a woofer who's DCR was about 60 ohm. So, it may sound good at low volume, but may blow the amplifier at high volume. Be safe; it will take 5 minutes to check.

[Mr. Widget]

O

Regarding the sub in this set, I had heard and read that it was weak, and I tend to agree. Then I did this: I replaced the 121A driver in the 212 sub with a 12" driver from the JBL PB12. I could hear the difference imediately, the bass was more "detectable", louder. But then I came to think about the amp in the sub, could this change in drivers be harmfull? So I put the 121A back in. But the question still bothers me, was it harmfull for the amp or not?

Short of pushing the system to it's limits, I doubt you could damage either the amp or the driver doing this... as for more bass, the 121A wasn't a sub by today's standards and will never produce the uber deep sounds that a modern sub is capable of. That said, I imagine the upper notes reproduced by the 121A will be significantly more musical than the newer woofer.


Widget

[JuniorJBL]

I will take the 121 off yer hands if ya like!

[4313B]

I will take the 121 off yer hands if ya like!

If it was a 121H I'd take it too! Killer subwoofer transducer. You just need to know how to use them.

The amp in the B212 has no current capability to control that massive four-inch coil so it's pretty much useless.

I just recently built four brand new 121H's for some custom subwoofers to be used with Tannoy DC6's. They're simply outstanding. Be forwarned that all the 121 kits from JBL now come with thoroughly rotted surrounds so order surrounds from Rick Cobb.

[Titanium Dome]

They're simply outstanding. Be forwarned that all the 121 kits from JBL now come with thoroughly rotted surrounds so order surrounds from Rick Cobb.

Of course, JBL gave you a discount and/or partial refund for that, right?

[Freiberg Baard]

Well, now the PB12 woofer is back in, and it sure sounds a lot better. Still, it's to bad that the amp in the 212 can't move the 121A a little more. How about replacing the old amp with a better one?

[HCSGuy]

I think that what 4313B is saying is that the 121A is an excellent sounding subwoofer within its linear output range, but this range is limited, so if you want high output, it must be used in multiples. Also, that the provided amplifier (80w, right?) does not have the current capability to control the 121A woofer. Therefore, if you want to improve the sound, you could:
1. Get a much better quality amplifier/crossover and enjoy your single woofer at limited output.
2. Use your PB12 woofer, get a better quality amplifier/crossover and enjoy the sound quality you get (which is unknown to us) at up to whatever maximum output the PB12 woofer has.
3. Get more 212 bass modules and more amplifiers/crossovers, and make a statement.

I'm not a high volume listener, so I'd go with option 1 just to see how good it can sound. Keep us updated on what changes you decide to make and how they sound!

[4313B]

Right.

It's xMax limitation stems from the basic design of the driver core. The spider pad is too close to the top plate. When you smack the mass ring you are also hitting the spider at the point it connects to the coil former. These days I simply put the drivers in half cubic foot smaller enclosures and drive them with a decent external amplifier. The driver has outstanding tansient response. But it is xMax limited...

Two of them have always seemed perfectly adequate for the L212 side panels. JBL made the B212 available separately so one could buy a second if desired. The first amp I used with them was a GFA-535 which totally killed the stock E212.

I've used them several times with Tannoy mains because they go so well together.

Very recently Saeman put a pair in some custom 4315B's he built and he was astounded. He made the 121 subenclosures just under 1.5 cubic feet, sealed, and was floored at the result. The 4315 used the same 8-inch and 5-inch that the L212 used. The difference between the 121 and the 124/2203 is the 121 uses a 95g mass ring and the 124/2203 uses the ubiquitous 35g mass ring (that was also used in the 136/2231 and 2235H). They both use the same coil, cone and surround. The 121 has a stiffer spider. The 121 likes a small seal box and the 124/2203 likes a slightly larger vented box. The stock 4315 box was really at the very upper limit for the 124/2203, it works better in a 2.0 to 2.5 cubic foot vented box, but JBL's requirement was that it maintain the same bandwidth as the larger monitors, which it did, hence the 3.0 cubic foot 4315 box.

[doyall]

... These days I simply put the drivers in half cubic foot smaller enclosures ...

... He made the 121 subenclosures just under 1.5 cubic feet, sealed, and was floored at the result. ...

Would it then be a good idea to add ballast (reduce volume) to the original 212E box (with a 121H transducer), as it is almost 3 cu.ft. (if my calculations are correct)? What is the ideal sealed box displacement for the 121H when used strictly for music.

[4313B]

I believe the B212 box was roughly 2.0 cubic feet net.

[macaroonie]

Right.

It's xMax limitation stems from the basic design of the driver core. The spider pad is too close to the top plate. When you smack the mass ring you are also hitting the spider at the point it connects to the coil former. These days I simply put the drivers in half cubic foot smaller enclosures and drive them with a decent external amplifier. The driver has outstanding tansient response. But it is xMax limited...

Two of them have always seemed perfectly adequate for the L212 side panels. JBL made the B212 available separately so one could buy a second if desired. The first amp I used with them was a GFA-535 which totally killed the stock E212.

I've used them several times with Tannoy mains because they go so well together.

Very recently Saeman put a pair in some custom 4315B's he built and he was astounded. He made the 121 subenclosures just under 1.5 cubic feet, sealed, and was floored at the result. The 4315 used the same 8-inch and 5-inch that the L212 used. The difference between the 121 and the 124/2203 is the 121 uses a 95g mass ring and the 124/2203 uses the ubiquitous 35g mass ring (that was also used in the 136/2231 and 2235H). They both use the same coil, cone and surround. The 121 has a stiffer spider. The 121 likes a small seal box and the 124/2203 likes a slightly larger vented box. The stock 4315 box was really at the very upper limit for the 124/2203, it works better in a 2.0 to 2.5 cubic foot vented box, but JBL's requirement was that it maintain the same bandwidth as the larger monitors, which it did, hence the 3.0 cubic foot 4315 box.

Thanks for that thorough explanation .