Cross over help for a newbe

Hi All,
I have just found some Jbl MR25 speakers,one with a missing woofer.I had some Skytronic 130mm woofers sitting around.After fitting them found the not very loud and no bass.Me thinks with the Jbl woofers being 4.6 ohms and the new speakers 8 ohms ,something is out of balance.Can anyone give some help designing a new crossover? Thanks Don


New woofers
http://www.surplustronics.co.nz/shop/product-902.420.html

Jbl Mr25
http://manuals.harman.com/JBL/HOM/Owner's Manual/MR25 om.pdf

Of course, crossover points changes when you change the impedance, but I don't know if this means "not very loud"... Acconding to specs, your new woofer is more sensitive than the MR25, so it could be that your actual woofer is attenuated in some way.

KEEP IN MIND THAT THERE ARE PHISICAL FACTORS ALSO, AND THE BOX VOLUME IS USUALLY CALCULATED TO A SPECIFIC WOOFER, but I think it can work acceptable good anyway...

Could you post a schematic of your actual crossover?

The specs says xover point is 4500Hz

The simplest crossover you can try is simply an inductor in series with your woofer: Using the nominal impedance it should be 0.28mH, but impedance use to be higher at higher frequencies, so you can calculate it with 11 ohm (0.38mH) or even 13 ohm (0.43 mH) (better: get a woofer tech sheet, and look at the impedance at 4500Hz, and use this impedance to calculate the inductor)
http://ccs.exl.info/calc_cr.html#first

You can do it more complex later.... add a Zobel network (to keep the impedance constant...), do a 2nd order xover, and so...

Again, check and post here the xover you have... It would be easy to improve it a bit to match better your woofer...

Thanks elnaldo-

Here is the specs on the Mr25.
Im not lookin for anything flash,just some more bass/loadness.
Any help is very much appreciated
Thanks Don


MR25 info
http://www.nodevice.com/manual/newmans/jblharman/MR25_tspdf/get47140.html


Thiele Small Parameters for the skytronics 902.420 woofers
http://www.commsandsound.com/images/css/manuals/thielesmall_902127.pdf

Hi,

looking at those specs, I see that there is already a Zobel network in the woofer ( a Capacitor in series with a resistor, both in parallel with the woofer).

I see a second order xover for the tweeter, + 1 resistance as attenuator (I guess, I'm not an expert)

Checking the woofers inductor value (0.82 mH) and the crossover point (4500 Hz), I can deduce (using on-line calculators like this: http://www.bcae1.com/xoorder.htm ) that the impedance considered for this low pass filter is 23 ohms.

You can check , using this same on-line calculator, that a 0.82 mH inductor, used in a 8ohm woofer, will give you a crossover point of 1550 Hz. But as I wrote before, impedance use to be higher than nominal at higher freqs (thats the reason why you need a freq response-impedance graph). Considering your woofer impedance raises to 15 ohm, that 0.82mH inductor would do a xover at 2900 Hz.

In both cases , you lack of a spectrum, from 1500/3000 Hz to 4500 Hz where the tweeter starts to sound... all in theory....

I don't know if your crossover is accesible, or if it's a sealed unit, but I would try 2 options: the simplest... connect the woofer without any crossover and listen. Since your woofer response goes to 10KHz, and the tweeter starts at 4500, you will get undesireable midrange...

Then, try the simplest 1st order crossover as I suggested: you can calculate it with 11 ohm (0.38mH), 13 ohm (0.43 mH) , 15 ohm ... (better: get a woofer tech sheet, and look at the impedance at 4500Hz, and use this impedance to calculate the inductor) or get a bigger inductor, and start to "unwind" it and try...

You could check also any speaker that uses your woofer, and deduce impedances from the xover

JBL_mr25_ts.pdf

The Skytronics has a Fs of 50 hz so the really low stuff is going to be absent, especially using a box, port, and crossover from another woofer. I bet you could snag a couple of replacement drivers for cheap. That might be easier than changing the network components.

Of course, there is always the "because it's there" school of thought. I would follow Elnaldo's suggestion and run the woofer full-range, outside the network. It may end up that you will need an attenuator for the top end because of efficiency mismatch. This can be as simple as adding another resistor. The port and volume of the cabinet can be modified using foam blocks and pipe. Depending on the behavior of the Skytronic, you may wish to block the port, entirely.

Good luck with your JBL's.

Thanks Elnaldo and Westend,I will have a look at inside at ways to bypass the xover this weekend.Don

Kiwidon said:

Thanks Elnaldo and Westend,I will have a look at inside at ways to bypass the xover this weekend.Don

Click to expand...

Hi Don,
Just thought I'd resurrect this thread as I too have MR25s with rotten surrounds and also have the same Skytronics drivers.

I'm curious, before I cut up my MR25 boxes to fit the Skytronics drivers; Did you ever get yours to work any good? (I've already 'upgraded' the JBL 12mm titanium tweeters for 25mm soft done tweets that I scavenged out of some old Wharfedale Delta 30.2 boxes. I'm not keen on metal tweets.)

I'm using them as the fronts on a 5.1 system, with a Jamo [2 x 8" driver] passive subwoofer wired in between the amp and the JBLs (as well as a separate powered sub) so I don't expect them to go down really low, in fact they won't be able to as the Jamo filters out the low bass. I'm just hoping to get some decent high bass, mids and low highs out of the Skytronics.

LOL, not asking for much huh?

Cheers,
Shaun.