Passive Radiator weights

[Cliff A.]

I see that a few people have taken off some or all of their PR weights in the L150 to get better bass. It makes sense to reduce the air resistance in the cab by doing this, but has anyone experienced an increase in unwanted 20 Hz resonance effects with the active driver?

[Chris Brown]

I took the passive radiators off of my JBL L150’s today and there were 4 cardboard weights attached to the back held on by a single screw in the center. I removed 3 from each as well as the screw and just left the single cardboard weight which looks glued to the rear of the assembly. I then re-installed the passive radiators.

I had read a lot of comments about how removing the weights would “unleash the hidden bass energy from the speaker” and stuff along those lines so I had to try it.

My first impression is that the bass output is definitely increased. You can see the passive radiator moving a lot more than it did before. It also seems like when the bass hits that you feel it more, but it is “softer” than it was before. It also seems that while the overall bass has been increased, that it rolls off at a very slightly higher frequency now. I have not noticed any strange resonance at 20hz, but my listening room is fairly small.

I’m still going to take more time to get a feel for the changes but I’m curious to hear what others who have adjusted their passive radiators think about the changes.

[Zilch]

You have sacrificed extended bass in exchange for unleashing the artificial disco boom hidden within....

[Cliff A.]

It might be interesting to see a response curve for the speakers without the PR300 weights and compare it to the standard version. I imagine that the original designers have done all that back in the 70s and came up with the best version for extended low response. It makes sense that the bass will increase over a certain frequency range with a reduction in the weights on the passive radiators, and also that the slower bass excursions will be reduced, so roll off comes at a higher point.
The resonance in my speakers had a different source all together - I found my voice coil/s were cooked and one active bass driver was producing unwanted sounds at various frequencies (see seperate thread)
I imagine that effective cone mass will also influence the sound in a similar way to the PR weights to some extent, so I hope that the replacement cones currently available for the 128Hs do not degrade the classic sound of my L150s (should I go that route.)
Cliff A.