Advice For My Tired Bookshelf Speaker -- With Pics

ted777ted

Member
2005-03-16 5:04 am
USA
Hi, I've been finally enjoying a small 14 watt per channel EL84 tube amp from the 50's that I restored. It's been a long ride! But I know very little about speakers, and I would like some advice...

I have a small living room (12 X 15), a pathetic budget, and I listen to mostly jazz, classic rock, and acoustic music. I have been using some 90db efficient JBL MR28 bookshelf speakers that I bought in '95. Okay, you can stop laughing now.

The funny thing is that I have been auditioning speakers from kind friends and surprisingly the old jbl's hold their own. It's hard to justify spending the money for the consumer level garbage from Circuit City that sounds no better. I did listen to a pair of PSB Image Towers through a solid state NAD and thought they sounded pretty sweet. Anyway, the JBL's seem to lack some detail and sometimes the highs can be shrill and squeak-like.

From the JBL website the MR28's:
• Dimentions 19.5 X 10 X 10
• High Frequency 14mm Ti Dome Transducer
• Nominal Impedance: 8 ohms
• Woofer: 8" nitrile polymer fiber cone
• Frequency Response: 50Hz - 20kHz
• Crossover Frequency(ies): 3.5kHz
• Sensitivity: 90dB (1 watt/1 meter)

I was thinking of laying down my credit card down at parts express for the caps, inductors, and resistors. I am trying to get a schematic for the crossover from JBL to figure out appropriate values to replace the inductors. What do you guys think? Should I spring for some new tweeters and drivers? If so, any recommendations? Even though the tranducer has that funny shaped plate, the hole in the cabinet is a 2 3/8" circle. I hope that some one can look at these pics and such and go, "Yeah, replace this and that...Or "Nahh, don't bother..."


HERE IS A LINK TO A PAGE WITH THE CROSS OVER & SPEAKER PICS:

http://members.cox.net/ted007ted/jbl/index.htm

Thanks!
 
JBL make very nice speakers, and I personally would hesitate before tampering with them.
However, I think the lack of detail you mension could be the fact that a 10" driver is not really suited to midrange coverage.
You may consider adding a dedicated midrange driver and adding another filter section to the crossover.
Also, the cabinets seem a little on the small side for a big 10" speaker, so a larger cabinet with a different alignment might bring deeper bass and better transient response.
Are the thiele/ small parameters for that 10" JBL driver available to you?
These are only personal opinions which you may not want to consider.
 

Jay

Banned
2003-02-11 9:02 am
Jakarta
I'm sorry that I have to say that that speaker is crappy. Big manufacturers
really excel in making products with a high quality-to-price ratio.
Many DIYers (even experienced ones) tend to have a system that has a
very low quality-to-price ratio. They think that by using expensive parts
things will get better! Replacing the inductors with Solens will lower
the Q/P haha!

It is not likely that you can get the drivers' thiele parameters from JBL,
and which DIYer will bother to measure such drivers??

Even tho the woofer is big in diameter, I believe the magnet is small so
basically it is a midrange. Adding a midrange for 3-way is a big don't!

IMO, not much improvement you will get from quality inductors and caps.
With Solen air-core inductors you will get cleaner and tighter bass, but
might not worth it. Solen MKP (caps) is expensive and I don't think the
highs will be improved (you may even prefer the current caps). Now the
resistor, that's the one you can replace :D

Why not just save the money for future speaker project? Most speakers are
not sensitive, and are intended for high power amplifier. May be that is
why you like your JBL with your 14W tube amp. So if you have to build a
DIY speaker, you must be careful to choose one that match your amp/needs.

Cmon, you're a tube guy now. You should buy a Fostex :D :D
 
ted777ted said:
I was thinking of laying down my credit card down at parts express for the caps, inductors, and resistors. I am trying to get a schematic for the crossover from JBL to figure out appropriate values to replace the inductors. What do you guys think? Should I spring for some new tweeters and drivers? If so, any recommendations? Even though the tranducer has that funny shaped plate, the hole in the cabinet is a 2 3/8" circle. I hope that some one can look at these pics and such and go, "Yeah, replace this and that...Or "Nahh, don't bother..."

As someone who spent an incredible amount of time and an embarrassing amount of money upgrading a pair of speakers that were better than yours to begin with, my advice is "Nahh, don't bother..."

If I were you, I'd begin by selling the speakers and start over again. There are a million DIY designs for 2-ways on the net that will give you better value than beating your dead horse JBL's. IMHO, there's nothing there worth saving.

PS, I doubt very much that JBL will give you a schematic of the crossover. That's normally considered proprietary.
 

ted777ted

Member
2005-03-16 5:04 am
USA
Thanks guys...I appreciate the advice. The seem to play well with my amp, so I might as well hang onto them for a little while longe. JBL posts some of their speaker crossover schematics if you go to their "pro" technical site. I guess they are going the open-source code route with their schematics.

By the way, my post take a few hours for the moderator to approve. It kind of feels like the security checkpoint delay at the airport. I guess they want to make sure that I am posting about speakers and not secret Taliban orders. Just don't mention WING ATTACK PLAN R. :clown:
 
True, you could sell the JBL's for a pittance and invest lots of time and money on some new DIY speakers that you might not like as much as your JBL's.
If you're reasonably happy with the overall tone of your current speakers and you'd like to 'have a go' and don't wish to spend too much, buy an aftermarket midrange (or scavange some old units from somewhere), a few capacitors and an inductor or two, and try my idea out.
If you don't like the results, stick the JBL drivers back in their original box and sell them.
You can keep the bits you buy for the future, but you might like the results.
That's DIY isn't it?;)
 

Jay

Banned
2003-02-11 9:02 am
Jakarta
bobhayes,

I doubt a builder with 5 years experience has the ability to improve the JBL by adding a midrange. I will bet then ted will have to sell the JBL and left himself with inductors and midrange he doesn't know what to do with. That's a normal DIY path I know :D
 
You like the sound of the speakers so don't muck around too much. Here's an easy job that will help the sound.

1. Change the electolytic cap to metallised polypropylene ones of the same value.

2. Change the iron core inductors to air core inductors of the same value (measure them with an LCR meter.... beg, borrow or steal).

3. Add some bracing inside the cabinet to break up the wall lengths and stiffen the box... you'll lose a small amount of volume and all that will do is make the port tuning go up a tad.

4. Add some gaskets behind the drivers (weather seal self adhesive closed cell foam).

5. While you are at it, put in some decent binding posts and get rid of the spring clips.

6. I can't tell if there is any damping material inside the cabinet, but if there is none, add some to the back, bottom and one side wall.

That's it.... not very expensive, keep the same drivers but do check on the woofer surrounds to make sure they are not perished (had JBL's in the past with foam surrounds but these look like rubber).

Reverse engineering of existing speakers are a pain but if you stick to the existing values in the crossover (looks like 2nd / 1st order combo) you can't go wrong.

Have fun with it.