JBL L20T3 Crossover Upgrade (4406?)

[rodabod]

Hi there, I'm new to this forum.

I own a set of JBL L20T3 speakers which I use as nearfield monitors for recording. My understanding was that they were based on the JBL 4066 monitors.

In the interest of getting as accurate results as possible, I was considering "upgrading" the crossover in my L20T3s to that of the 4066 which people seem to respect. The cabs and drivers are the same, the only difference being the bass port which is rear-firing on the L20T3. I'm not sure why they moved it to the rear. I could measure its length and compare...

If I were to change the crossover, I'd probably omit the L-pad, or maybe just ad a -3dB switched pad. Which brings me to my main question, apart from the 4406 crossover using film capacitors throughout, what benefits might it provide?

From looking at the following PDFs of each of the crossover circuits, the only difference I can see is that they use different components values (4406's L's and C's are smaller), and the 4406 has a 39R resistor loading the woofer output (perhaps this is just here to compensate for the effect of the L-pad on the tweeter though?).

http://manuals.harman.com/JBL/HOM/Te...L20t3%20ts.pdf

http://www.jblproservice.com/pdf/Net...%20Network.pdf

Has anyone been able to measure any differences between these two crossover designs, or otherwise could offer any insight?

Regards,

Roddy

[4313B]

I posted all the voltage drives years ago. Having owned them all I found the 4406 network to be my personal favorite. I liked it so much that I built some charge coupled versions. The results were stunning.

[duaneage]

I incorporated the low pass from the 4406 network with a high pass section from the L20T and found that worked pretty good.

Best thing to do is build versions and test it to see for yourself.

[rodabod]

Thanks for the responses guys.

I've attempted to plot the responses from the two crossovers in Spice to look at them visually. These are linked below. From what I can see, the L20T crosses over at closer to 2KHz, whereas the 4406 looks to be more like 3KHz. Also, the resistors (and 8-Ohm L-pad in max position) in the HF section attenuate by differing amounts it seems; the L20T around -6dB, whereas the 4406 around -5dB.

Obviously I have not taken the reactance of the drivers into account as this is beyond my scope when using Spice since this is the first time I've used it!

I'm still trying to figure out why they revised the crossover design in the first place (apart from the L-pad of course) and whether or not to give this a shot myself. A/B comparisons are tricky with only one speaker at a time.

Regards,

Roddy

L20T3: http://img220.imageshack.us/my.php?image=spicel20t3.png

4406: http://img220.imageshack.us/my.php?image=spice4406.png

[4313B]

I'm still trying to figure out why they revised the crossover design in the first place

Since JBL tuned each model empirically it isn't too far fetched that the 18Ti, L20, L20t3, and 4406 ended up with different networks. I once asked the designer of them all why he did different networks for each and his response was "I thought they were all the same." (To be fair, he's done hundreds and hundreds of designs so expecting him to remember the details of each one is probably absurd.)

The only systems that come to mind that used verbatim networks were the L96, L112 and L150A. There could have been others but I don't remember them off hand.

[duaneage]

It's interesting that even though a network can have different component values sometimes they perform the same. There are quite a few variables at work.

Sometimes it is a matter of what components are easily available or cost effective. Ordering up unusual values of capacitors or inductors costs more so designers are more likely to go with what is available off the shelf.

[hjames]

Interesting ...
Just got a pair of L20T3s today in DC ... initially hesitant, once I hooked them up for the Aja Listening test, the difference is amazing!
Not sure I could ever listen to the L20Ts standalone again - so much more shrill and actually kind of mushy in the high end!

The cabinets are a bit differently sized, and front port vs rear port on the T3 - but its the same doggone drivers!

I was just planning to use these as surrounds but - seems like a waste!

[4313B]

its the same doggone drivers

Aren't box sizes, tunings and crossover networks fun?