JBL SF25 rebuild

[spacesun]

Hello diy´ists

ive own the old SF25 from JBL
but the tweeters are broken...
the M115-8A are still working fine

so now ordered two Eminence PSD:2002 1" drivers and put them in the 338800-001 JBL Horn

now the funny part of calculating the crossover network

i found the free Boxsim Programm and i did some calculations

is the frequency course really that bad on this speaker? or i did something wrong withe the parameters in Boxsim?

i intend to set the crossover frequency on 1200-2200 ( 2200 Jbl use on theyr actual JRX Models )

do you have any suggestions for the crossoverfrequency?
im not shure how much amplification the Horn 338800-001 will give me

i will soon measure it with ARTA ( LIMP )

i hope someone can give me some advices to make the courve more smooth


thx

 

[spacesun]

pics

atteched pics from Boxsim

 

[EarlK]

Hi (name?),

You'll need to acquire ( ie; create & then post here ) some actual frd and zma files ( for that Eminence driver on that JBL waveguide ) before any meaningful help can be offered ( for a SF25 network redesign ).

I suggest you download REW and get yourself a calibrated test mic from miniDSP .

Regarding your BoxSim question;
- You need to clarify what steps you have taken ( in it ) to arrive at what we see in the posted pic.

 

[spacesun]

Hi EarlK

thx for thew answear

so the +db of the exponetial horn isnt calculable?
+ the thiele and small parameters of tweeters are not such important?

in the meantime im imported the dB and resistance cource from the graphik via FPGraphTracer

i currently making the setup for measuring dB phase ecc with ARTA...
i will use the shure SM58 as mic ( i borrowed it from a friend )

oh i see noe that this forum dosent allow to upload such BPJ files sorry i thought is was attached...

how do i calculate the resonance frequency of my box?

i read somewhere 343/2/hight of the box?
but how you do such with double woofers? the you have also to half the volume?

oh this REW is nice thx maybe i try it when i do the measurements

 

[system7]

Aren't you getting a bit bogged down in complexity here?

This replacement compression driver really can't be that hard!

What is your existing crossover? Surely you have looked. The bass section should need no work whatsoever. Maybe adjust attenuation on the CD tweeter. Which is a matter of adjusting the resistors in the tweeter section.

Eminence do some off-the-shelf filters that seem to work with single 8 ohm drivers. Like this coaxial approach, but it's still a CD in a horn.
http://www.eminence.com/great-uses-for-coaxial-products/
Crossovers | Eminence Speaker

The glass thing is a fuse.

 

[spacesun]

Hi Steve

yes i know i got also a bit overhlemed by all this work for just replacing a driver but

The original crossover is broken too i dont have it
and buying the original one for modifying it is ..

so i checked first the original one and found out that this woofer M118-8A is pretty common in JBL speakers so i found also the newer model JRX225 it has much higher crossover frequencys the old SF25

SF25
woofer 1 574 Hz
woofer 2 1083Hz
1" Compression driver 6,2kHz

JRX225
Woofer 1+2 968Hz
1" Compression driver 4kHz

i hope my quick calculations where not way off

On the eminence 1" the datasheet tells me to let them play already at 1,2kHz but i think thats far to low...

are such strange crossover frequencys common in PA Systems?
such huge holes in midrange? i dont want such..

so i decidet to make something decent out of them and now there are many software tools that are helping this calculations and i dont have to do it manually

could you give me some advices where i should put the cut frequencys?
how it would be possible here to share a Boxsim Project?
or someone could recommend me another programm for such calculations?

best regards
Martin

 

[system7]

I think you are making good progress here. Seems to be a pair of JBL M115-8A (not M118...) woofers wired in parallel as your above schematic for the SF25 shows:
http://www.jblproservice.com/pdf/Sound%20Factor/SF25.pdf

I see the Eminence Delta 15A is considered a competent replacement and I include its frequency response below:
Speaker Detail | Eminence Speaker

I am slightly alarmed at the possible low impedance that might result, but it's around 4 ohms or less I'd guess at the bass end. The DCR of the JBL woofer was about 5.1 ohms when I looked. Be worth measuring that.

It's a characteristic of horn loaded tweeters that you use smaller values on the tweeter crossover than you might expect. This is because they have a rise in output at low frequencies. I think you might need some attenuation resistors here with a different CD. But I really don't know.

Bit of a task to model it. But you can get a good idea of the electrical slopes and impedance characteristics with any similar woofer. I would guess we want one with lowish inductance. Usually around 0.6mH for PA speakers. I might have a horn loaded tweeter to compare somewhere too. Have to delve a bit in old projects. Somewhere between 2 and 3kHz crossover we'd guess. The SF25 is a glorified 2.5 way really. The bottom woofer reinforcing the bass.

 

[spacesun]

oh ok yes of course M118-8A sorry

yes i found the Eminence Woofers too ( i think they would be even better then original ones ) but i have to stick to the M118-8A so...

yes i see that too but im shure that my old wonderful Crest Audio LA 901 can drive them but i will measure the DC resistance

in the pics i put the crossover frequency to 2200Hz with a attenuation of -3 dB to the Horn ( i still dont know how loud is really ) so thats just a guess until i get real data

so you think i should get smaller with the crossover frequency? even near the resonance frequency of the CD? ( 540Hz ) eminence reccomend to use this driver from 1200Hz on!

the modelling of the crossover network and the final optimizzation is done by the boxsim programm ( after initial setup )

but i still struggle with the mecchanical parameters of the cabinet iself

how do i calculate or measure the resonance frequency of the cabinet with two woofers and how about the resonance volume each speaker has?


i dont think this spike at 220Hz can be real ( and in case it is i have to remove it.. )

 

[SpeakerBob]

The JRX 125 speaker has the twin M115 8A speakers running as a 2.5 way with a crossover to horn at 2k whereas the JRX 225 seems to be running the woofers in parallel with an xo at 2.2k.

(I own 4 of the woofers so looked up the crossovers when I was planning a 2.5 way project)

 

[spacesun]

Hi RobWells

Yes the JRX are the sucessors of the SF series

you have this woofers too? do you have also this odd spike at ~200Hz
its seems that is comming from the TS Parameters of the Woofers..

is someone here that has experience with Boxsim and could review my project and maybe find my design faults? i can not upload the project here so via PN?

thx

 

[EarlK]

spacesun,

What you need to do within BoxSim is to recreate the newer JRX225 network.

What you've currently constructed within Boxsim makes little electrical sense.

 

[system7]

To convert the TMM 2.5 design to an equivalent MTM or TMM two way:

Wire the two woofers in parallel, and give them a 1.2mH/22uF bass filter. Simple as that.

 

[SpeakerBob]

Yes the JRX are the sucessors of the SF series

you have this woofers too? do you have also this odd spike at ~200Hz
its seems that is comming from the TS Parameters of the Woofers..

is someone here that has experience with Boxsim and could review my project and maybe find my design faults? i can not upload the project here so via PN?

thx

Yes I am running them under JBL 2385A horns crossed at 800Hz. There is no spike at 200Hz. I do not have any measurements saved to post here but can assure you they perform well. If I take any in the near future I will post them here for you.

Rob.