L20T tower

[duaneage]

The venerable L20T is a popular system. Containing a 115H-1 driver, JBL has made 4 variants of this system as a 18Ti, 4406, L20T and L20T3. Others have created biased networks for this little system to rave reviews but always using the same cabinet volume.

The 115H-1 is a high Qts driver. In keeping with JBL's tradition of using High Qts drivers in ported boxes the final response of the system has a hump in the midbass region, just like the L100, Control 10, and other systems. I wanted to see what this driver could do in larger cabinet. I also wanted some newer looking thin towers for HT use and higher WAF.

Comparing the original design to an optimum we see greater bass extension with higher delay times for the driver. Going from 14 ms to 19ms is a shift but a proper LF cutoff filter would protect the driver from subsonics.

[duaneage]

I built new cabinets from furniture grade plywood with a MDF baffle board. I like to extend the baffle board to the edge of the cabinet, it's a style I have used for many years. Since so many have complained about the full veneer look it offers a different perspective. JBL blue was considered but rejected as having a low WAF. I used the same driver layout and offset spacing as the L20T. THe grills stand off from the cabinet about 3/4 inch and are copied from the L20T as well. The crossover network is the same except for a Zobel of 5.6 ohms with 15uF to tame the woofer impedance rise. In testing the midrange was better for it.

The 115H-1 has a very high output in the 2500 - 4000 hz range so JBL went with a 1.5 mH / 9uF second order network to counter it. I experimented with different slopes but came back to the original value.

The rear is modeled after the L100T. The rear facing port works better and unclutters the front of the speaker IMHO. I finished the cabinet with a traditional red oak stain, I have to apply polyurethane yet,

[duaneage]

The first thing noticed is the midbass boom is gone. The highs and midrange are much clearer than before. Snare drums are much better as well. The larger cabinet provides solid bass to 50Hz whereas the smaller box cuts out in the 75Hz range. You can't listen to the L20T next to this system, the l20T bass performance is weak compared to the tower. What is hard to measure is the transient performance. The L20T is way overdamped, the woofer is tightly controlled by the small box and high tuning frequency. This worsens the decay time. The transient hits and falls away too fast. The L20T has abnormal harmonics as a result. Increasing the box size and lowering the tuning allows the driver to accurately reproduce transients.

I did some testing with my SPL meter to confirm the results, although testing in my garage is not the best environment. The Zobel network reduced output slightly in the midrange. Room testing showed a preference for the middle of a wall, out of the corners.

I have yet to mount a base I created to the bottom. It seems they are a little top heavy so I made a base 1 inch thick from solid oak that extends an inch all around, providing support and a little more weight on the bottom.

The difference is dramatic enough it makes me wonder why JBL didn't do this before.

[4313B]

Nice!

The difference is dramatic enough it makes me wonder why JBL didn't do this before.

Price Points.

[bigredplane]

So what cu. ft. did the cabinet end up being?
PS. I love the idea and looks even better

[duaneage]

The box went from 720 cu inches to 1434, about double. WAF is so high my spouse wants to use them alongside the TV. The port ended up being 2 inches in diameter, an improvement over the 1 1/2 inch ports which were probably too small. Length was 2 1/16" tuned to 53 Hz

I also did a front to back brace on the enclosure and used oak strips for the corners to increase strength. Fill used was polyester, I don't like using fiberglass because of potential shedding problems.

I got the woofers from you, Mr. BRP, so they shold look familiar.

[Zilch]

I found 115H-1 to be a mighty woofer testing them for Minis.

They must really pump in those beautiful towers.

[bigredplane]

I'm working on a little project myself. It's using 115h-1's and 034's. It my involve new larger cabinets also. I tried 115h's they seems to have a better bass but not near as good of mids to me as the 115H-1s. I know I could of used 035ti's, but its more of challenge to make the 034's sound good. I'm packing the crossover full of the high dollar caps to help out the 034's. I have no test equipment or computer programs, I'm do it all be ear. It's I lot of trial and error, but we will get there. Thanks for your post on you speakers there alot of good info there for me. I like your speakers, wish I could hear them.

[duaneage]

What really helps in crossover design for me is my crossover breakout box. It allows rapid changes and A-B comparisons of different parts without wasting time soldering or unhooking everything. It was not cheap but worth it.

The 115H is a tough bugger to crossover from. It does have a rise in output, probably due to the plastic cone. Paper woofers damp better and aquaplas coated woofers damp even better yet.

I might try a small sealed enclosure , I do have a pair of L20T's that don't sound as good as my design now.

Here is an interesting tidbit: The C1003 woofer used in the control 1 has the same electical and mechanical Q qualities as the 115H. The main difference is the smaller cone results in half the Vas and lower efficiency. When I ran both through similations the C1003 had the same response curve as the 115, albeit lower in output, in 720 cu inches.

The C1003 is an inexpensive driver still available from JBL. While some may scuff at it, it IS a good sounding speaker that can take a lot of abuse. If you have access to a pair of control1 speakers build a 720 inch test box, like a tower, and tun it to 53 Hz. you may be surprised at the result.