View Full Version : A List Of Successful Mods For JBL L150a's

02-15-2004, 10:51 AM
Here is a list of successful mods that I have done to my L150a's.

1. Replaced the internal wiring with Tara Labs RSC Prime 500.

The internal wiring should only be replaced if you are willing to replace all caps and resistors. If you change the wire before the caps and resistors as I did, things will get harsh in the upper midrange and high frequency.

The wire from the circuit board to the L-pads is very important and must be replaced with the rest of the wiring. Use a damp rag along with a iron set to low to remove the Inspection plate to expose the screws that secure the l-pad to the front baffle. This is the hardest part of the modification but must be done!

I terminated the legs of the l-pad with female quick disconnects to allow the use of 10awg Tara wire without trimming away the conductors.

On my circuit board there is a jumper wire from the .75mh inductor to the 2.4ohm resistor in the midrange signal path. I played with various brands of wire in this position and settled on 99.999 solid core pure silver. The silver, all though a bit brighter, eliminated the grain.

2. Replaced the mylar capacitors.

Capacitor C1. With cost being an object I used: 1. 12uf solen and a 1.5uf solen in parallel to make up the 13.5uf original value. The solen caps on their own sound very thin and bright in this position.

Capacitor C1a. I added a 0.01uf Hovland misicap as the bypass to the solen and this resulted in a bigger, smoother, and warmer sounding midrange.

Capacitor C2. I used 1., 6uf Hovland musicap. This capacitor smooths the roll off of the upper midrange to the high frequency. Since the day I bought these speakers I hated the harsh, screachy upper midrange vocals. The hovland cap eliminated this. The transition between the mid and high frequency is now seamless.

Capacitor C2a. Not used.

Capacitor C3. I used 1., 4uf Hovland musicap. The hovland cap in this position is a must have. With the original mylar cap the metal dome 044 sounded very brassy and the tone of a symbal seemed to carry on to long. The sound was cold and exagerated IMHO. The hovland is much faster, the sound is smoother, fuller and much warmer. There is more hf but it presents itself in a delicate way.

Capacitor C3a. Not used.

For capacitors C2 and C3 I had to drill the pads on the circuit board to accomodate the heavier awg leads of the Hovlands. I also installed larger, heavier solder pads in this area.

3. Resistor Upgrade

I replaced the 2.4ohm resistor in the midrange signal path with a 2.5 ohm mills non inductive ww.

4. Replaced the dampening foam under the Printed Circuit Board.

5. Replaced the Insulation inside the cabinets.

6. Binding Posts
WBT topline posts. I would have never upgraded to these expensive posts but I got a great deal at a fraction of the retail price.

Modifications Not recommended:
Do not mess with the disks on the rear of the PR300 passive. I removed weight from the pr300 and believe it led to the demise of one of my 128h drivers.

I am 100% happy with the sound of my modified l150a's as is and see no need for further modification. But curiosity and a good deal on ALPHA-CORE inductors brings me to the following:

7. Pending Modifications:

1. Replace inductors L1 2.5mh and L2 .75mh.
2. Bypass the l-pads using fixed resistors.

I am not sure if it is a good idea to mess with the inductors but we will soon find out!

My System:

JBL L150a front left/right, JBL L112 rear left/right and center channel.

Modified Bryston 4B (all wiring, transistors and capacitors have been upgraded)

Pre Amp / Processor:
Marantz SR7000 AV Reciever.

Cd / DVD Player:
Marantz DV4300

Thorens TD 125 MK2 with Rega RB300 Tone Arm

Tara Labs RSC Digital 75, Tara Labs RSC Reference Generation 2 Interconnects, Tara Labs RSC Reference Generation 2 Speaker Cable.

Crossover Mod Pictures (http://www.geocities.com/atticoftreasures1/marantz.html)


02-18-2004, 11:25 PM

One thing:

Originally posted by SteveC
Do not mess with the disks on the rear of the PR300 passive. I removed weight from the pr300 and believe it led to the demise of one of my 128h drivers.

Gosh golly, I sure messed with them -- they "tune" the radiator for lower bass (more mass) or more efficiency (less mass). While the bass is incredible no matter what, with all of the disks in place, my L150A's just plain weren't in balance -- not enough bass to balance the mid/high end. So, I took 'em all out except for the one that's essentially "glued on," and it just made all the difference in the world. The speakers were finally in balance, and they were wonderful.

You might go into detail in regards to your experience with your 128H driver -- I've had this modification in place for well more than a year, and I sure don't see anything amiss. Though, I'm not exactly playing at "knock the walls down" volume levels, either...



02-18-2004, 11:27 PM
Hehehe -- how about a group hug photo?



02-19-2004, 08:44 AM
Hi Tom

That is one mint group of speakers you have there! Looks alot like my collection!

At the moment I have:

L212 system, L150a, 2. sets of L112's, L100, 4311, 4312b, L200, you know the ones that almost ruined JBL's rep in the UK. decade 26 - 36.

My experience with the passive mod:

I was listening at fairly loud volumes, nothing out of the ordinary. My woofers "both" simply bottomed out. I believe that removing the weight allowed the cone of the woofer to move "without resistance" thus causing the 128h to over extend. I have never had this happen before nor since putting the weight back on.

My old bryston 4b amp benches at a clean 310 wpc at clip so it is not related to lack of control or pushing the amp too far. The woofers start then stop on a dime.

In your instance, listening at low to moderate volume you may never have a problem. But be very carefull if you ever decide to crank it up!


02-19-2004, 09:36 AM
Hi Tom
Try replacing the wire from the binding posts to the xover and also from the Xover to the 128h. Use wire between 8awg and 10awg. You will hear "bass weight" that you never thought possible from a 12" driver. What this does is gets the original signal from the amp to the lf driver with almost no signal loss. This mod is easy to reverse if you dis-like the result and it won't mess with the mid/hf response!