JBL S38 crossover mod (1 Viewer)

[Chris Tsutsui]

Inspired by Dean_G and Lemarcus's successful S26 modification I decided to do the same to my S38s.

I took apart my JBL S38s and I recorded the crossover into a schematic for easy readying.

Each 3-way Crossover uses 5 resistors, 3 capacitors (All bypassed by 0.01k 250v), and 3 inductors.

So does this mean that I use a 0.01uf bypass cap as a replacement for the bypass components?

Another question is the capacitors are the following numbers: 4uf for tweeter, 24uf for mid, and 60uf for the woofer. Because the 60uf is hard to find in an exact value and they also don't run cheap, I'm thinking of just keeping it in there because I'm pretty happy with the bass. Lemarcus didn't notice a big change in bass anyways, and I believe the biggest gain to be had is in the tweeter and midrange.

It costs an additional $26 for the axon true 61uf capacitor, do you think it's worth it? It's $16 for the rest of the caps, and the resistors add up to about $30 or so making this roughly a $50 total upgrade per pair.

If I use a Sonicap 4uf cap for the tweeter, that adds about $16 to the upgrade. I don't think it'll be worth it though. I definitely do not want to go more expensive than Sonicaps either.

I'm thinking of saving a bit of money and going with Dayton 10W non-inductive resisitors. Do you think they will perform on par with Mills despite the brick look and 2% tolerance?

Another thing that's interesting is that the midrange driver appears to be wired out of phase with the tweeter and woofer. Does this make sense to those of you familiar with 3-way designs? I know that the tweeter is commonly put out of phase in 2-way designs, but why is the midrange done in this case?

Another question I have is that the drivers are shielded. The 8" woofer has a cancellation magnet glued to the back, while the midrange driver has a large tin can over it. Do you think it would be wise to apply a dampening sheet on the "tin" can shielding the midrange driver to perhaps reduce some vibrations. I noticed that tweeters with tin can shieldings have frequency responses that extend deeper into the frequencies and don't cut off as sharply as speakers without shielding. Is the fast cut off a good thing in a tweeter? or should I keep the can as it is so it resonates which lets the tweeter play deeper.

Another thing I'd like to try is that the EOS waveguide is a plastic horn that's rather thin but is braced. Do you think it'll be wise to apply dampening material to the plastic to reduce vibrations? I don't know if the vibrations of these plastic and tin components were taken into consideration in the design and will cause a dip in the response if meddled with. I know that it's likely minimal but I'd like to tweak everything.

The wires in the S38 are generic 16awg with female disconnects. I was considering using 16gauge 99.99% copper wire and use direct 4% silver soldering during the rebuild. Any reason not to do this other than extra work?

This is my first speaker alteration so any feedback appreciated.

Boy the JBL 8" woofer looks very pretty with it's matte aluminum cast frame and bright white cone. Everything has foam gaskets, and the midwoofer is sealed inside a plastic separate enclosure. What if I dampened that plastic enclosure so it doesn't resonate? Anyways, there's a 1/2" thick gray/black fiberglass batting lining the walls of the MDF box. I was suprised to see the A-grade MDF used in the box construction, and the precision of the CNC routering.

If anyone's interested in some pics I'll post them, but only if someone requests it because it does take time.

 

[Brian Tatnall]

I would be insterested in some pics and know if the upgrade is worth it since I should be purchasing these speaekers soon.

Thanks

 

[Brett DiMichele]

Chris,

I think proper damping will be as crucial if not more so to
the performance of any speaker as replacing caps and resistors.

My AR9's are built from heavily braced 3/4" CNC Routed MDF
and the braces run along each wall and are placed about
every 7" up through the 32" tall enclosures.

The Midranges have thier own seperate sealed enclosures
within the main enclosure. The Midrange enclosures are made
out of thick cardboard tuning (like Sonotube) with a piece
of 3/4" thick MDF in the back.

I plan on spraying the whole inside of the enclosure with
either generic Asphalt Undercoating or Rockford Fosgate Noise
Blocker and then I am going to wrap the mid enclosures with
Dynamat on the outside of the enclosures and run a deflex
pad on the inside.

Along with rebuilding the XO's I think this will make a
fantastic speaker even better. The biggest issue in my
situation is that I have those 10" subs pounding in the
main enclosure and they could transfer thier vibrations into
the Mid enclosures which *could* color the mids at high
volumes.

 

[Brett DiMichele]

Ohh a couple other things I forgot to mention.

My stock XO's were wired with regular tinned 20Awg and I
removed all of the original wiring and had to drill out the
holes in the XO PCB's and I used 12Awg 99.99% OFC to all the
speakers and used 4% Silver Bearing Solder on all connections
even to the speakers.

Plus I went to the local dollar bargain and bought a pair
of cheap pollyfill pillows and ripped em apart and stuffed
the main cavity with 50% Polly and about 70% Polly in the
mid enclosures.

 

[Chris Tsutsui]

Thanks Brett.

I'll post pics of the mods once I get it all going as well as one of my exceptionally long reviews comparing the old and new jbls.

 

[Chris Tsutsui]

I modified the crossover with the following:

Solen Fast caps to replace the tweeter's and mid range capacitor.

Dayton Non inductive audio grade resistors to replace all the resistors.

Total cost came up to be about $30. I did not replace the 60uF cap on the woofer.

Turns out the speaker had a dramatic transformation. The mids changed from muffled, to clear instantly. And the highs had more life and detail. The speaker had an increase in dynamics, and now sound a lot easier on the ears. The speaker is far less fatiguing and can be much louder. The sensitivity increased about 1-2db, but the speaker appears to be quiet because it's so much easier on the ears. I couldn't tell that the new ones were louder until I put a movie on and noticed that the modified S38s were now slightly louder than the other speakers. (They used to be level matched)

The PCB board was able to fit all my components nicely and screw back into the box just like it was before. I'll take pics once I go back into the speaker to replace the tweeter.

Why am I replacing the tweeter? because I was a gimp and pulled too hard on one of the disconnects attached to the lead. The lead broke and now I'm left with a small dome paperweight. Then when I tried to fix it, I pushed in the dome, and after using tape to pull the dome out, it still has creases and imperfections on the surface.

What really irritates me is the fact that Harman International charges $56 each for those tweeters, and I have to pay $61 total because I get charged CA sales tax.

So this ended up being a $91 total upgrade to the S38s, but suprisingly enough, they're worth it with the new sound I get from them.

Just a lesson to anyone out there considering this.... Be VERY careful with the driver leads.. one mistake and it can cost you $60.

 

[Mark Hedges]

Has anybody else tried this?

Mark

 

[John Garcia]

I was considering using 16gauge 99.99% copper wire and use direct 4% silver soldering during the rebuild. Any reason not to do this other than extra work?

If you don't have the silver solder already, it is not cheap. This will probably be a small improvement, and if you have the time, would be worth it, IMO. It may not translate into a dramatic improvement, but it isn't going to make anything worse while you're in there.

I wouldn't touch the waveguide. It's hard to say if damping the plastic midrange chamber will help anything, and I'd be leaning towards no. This chamber is not to tune the midrange, it is to prevent the 8" driver from interacting with it.

I hate to say it, regarding the x-over components, but your only way to know how it is going to sound (improvement or not) is to try them out. Best guess doesn't guarantee good sound.

 

[Brett DiMichele]

John,

It's called a "Bucking" Magnet.. It cancels out, outward
radiation of magnetic waves so the speaker can be used next
to a CRT without damage to the CRT.

 

[KyleGS]

Could you please post some pics. I'm very interested in trying this out on my S-38's.

Also- does anyone have the link to the S26 mod? I couldn't find it in the search funct. Thanks

 

[John Garcia]

I have heard of bucking magnets, but I was not aware that this is what they were for. I saw one on a display Velodyne that had a clear acrylic enclosure.

 

[Brett DiMichele]

John,

Yep that is the sole purpose of a Bucking Magnet. When you
hear about a speaker being "Magneticaly Sheilded" this is
what they are reffering to.

They usually take a magnet the same diameter as the motor
magnet and they glue it to the back of the back plate of
the motor assembly in reverse magnetic polarity and then
they put a metal "cup" over the entire motor assembly.

This method effectively cancels out the stray magnetic
fields that would otherwise reak havok on a CRT Display.

 

[Mark Hedges]

Another bump to see if anyone else has tried this...

Mark

 

[John Tami]

I also second a link to the S26 mod. I have S26II's for my front stage, love them. Always interested in a Mods though....

Edit..found it: http://www.hometheaterforum.com/htfo...hreadid=140533

 

[Mark Hedges]

A few questions for Chris -

1) How did you get the case open? Is there a way to take the front panel off, or do you just have to remove the drivers and work thru the holes?

2) Where did you get the parts to do this?

3) How carefull do I have to be when working with these capacitors? Do i need to short them to drain any charge, etc?

4) Did you replace the bypass caps?

Thanks! I am really considering trying this, as I think my speakers are the weak link (relatively) in my system. I would just upgrade but my setup really lends itself to a horizontal speaker and I do not know of any that are better than the S38.

Mark

 

[John Tami]

I also wonder, what differences there could be from the S26 used in the mod to my newer S26II's, if anything in the x-over's.

 

[MuneebM]

I'm very curious about this mod. I assume it can also effectively be done to a pair of S310IIs. How about the S-CenterII and S36II? Anybody modded those?

Is there really a significant improvement in sound quality after this mod?

 

[Mark Hedges]

Well I decided "What the hell" and am going to give this a shot. I took my S38II's apart yesterday and recorded the crossover info. Curiously, on mine the mids have a 15 µF cap instead of the 24 µF cap that Chris found.

I ordered Audiocaps for the bypass caps, Solens for the tweeter & mids, and Mills non-inductive resistors. Like Chris I will leave the woofer cap in (but change the bypass for it). I also will try the deflex panel mod and will spray the case with stiffener.

I will post the results (probably after Easter).

Mark

 

[SteveLeach]

Hey Mark.
I'm very interested in how this turns out. I have S38II's all around. So if it turns out I'll have 6 of them to do.
Care to post all of the values of the components you found?
We may find that there is a difference between manufacturing dates/runs. Which would be kind of interesting in it's self.

Were did you order your parts from?

 

[Joshua H]

I'd imagine the difference is because he has s38ii's and Chris has the original s38's. I happen to have a pair of each, and am also curious if this upgrade improves the sound enough to be worth the cost and effort.