4311B woofer constantly "flapping"

[eso]

http://www.youtube.com/watch?v=2L0nG...eature=related
This test at the low frequencies 5~10Hz reminds of what my D140F looked like on that one amp. The cone would just move back and fort at a very low frequency. Look at the motion about 20sec into the video.

That's a cool video.

When I first powered up my bass horns I borrowed a disc "Bass Mechanics" from an auto sound guy I knew. It does a slow sweep from 0-100Hz or something like that. Car guys use it to locate rattles. I had to reseal my windows and secure a few things after doing that sweep a few times, but the room is quiet now...

Some serious over excursion on that woofer in the video.

eso

[Rolf]

I had a similar problem with the 2231A woofer some years ago. When the Alnico magnets was re-charged, the problem was gone.

Thanks for all the excellent suggestions, but this one has got me really stumped. Here's a run-down:

1. With amp volume on zero, one of the JBL 4311Bs flaps a lot, the other flaps a little. i.e. it's not turntable or other rumble (I have no turntable) so a high pass filter at the input would make no difference. Indeed, if I disconnect the preamps the flapping continues.

2. No matter which channel the speakers are connected to, the speaker that flaps a lot continues to flap a lot! So it's not one amp channel "flapping" or motorboating more than the other, it's one speaker "flapping" more than the other.

3. Plugging the 4311B's port stops the flapping

4. Changed out the 2 x 2200uf power supply capacitors - no difference.

5. No other speakers hooked up to the amp show the least signs of motorboating or flapping.

It just must be the way that that particular 4311B interacts with the amp. I'm gonna try webmonkey's idea of the 20Ohm/10Watt resistor - his/her explanation of an impedance peak at low frequencies sounds most likely.