Jbl 4425 pb with potentiometers ?

[davidpou]

I have been doing some measurements with a newly acquired pair of 4425, and this afternoon I was testing the effects of the potentiometers on the FRC.
To my suprise, going from 0 to - 8dB on the MIDS didn't produce any effects: the response curve is the same than the one with both potentiometers at 0dB, and this on both speakers.
On the other hand, when I put the HIGH at -7 dB the response curve goes down but from 800 Hz and not the 12kHz specified on the speaker.
None of the potentiometers does crrrrrrrr when moved...

Too draw a total picture: Impedances on both speakers (speaker terminal) and all four drivers are fine. The crossovers look as stock to me.
But an odd thing is that the horns are glued to the case, so even if I unscreew It won't go off...

What would you say: is a pb with the crossover ? the potentiometers ?

David70813

[mech986]

In my opinion, it seems like something is wrong with the wiring, as if the mid controls is disconnected and the compression driver connected to the midrange control, giving a freq adjustment which is much too low.

you might want to check if the wiring and color coding to and from the crossover to controls and drivers is correct, that the L-pads are correctly oriented on the mounting panel and front panel (not upside down so that the HF is in the MF hole), and that the L-Pad controls are the correct OEM JBL units, not replacements. Of course, a good look at all solder joints should reveal if any joints are suspect.

if the horn has been sealed in, this is evidence someone has messed with the speaker and all bets are off that it may be still in an OEM configuration, so careful inspection and verification is important. I may also be particularly concerned that the compression driver diaphragm is not JBL but an aftermarket diaphragm.

[davidpou]

yes that is what I told my self but close examination of the wiring shows that every thing is stock... at least on the potentiometers: JBL wires colors with the 1,2,3... I haven't checked on the crossover itself but everything is original there aswell. :dont-know:

[mech986]

Did you check each pot for resistance and wiper contact for correct tracking? The pots could be open.

[davidpou]

sorry for late answer ...
No.... They are difficult to access if one cannot remove the horns and the compression... Plus I had so much trouble with so called tech here in France being widely raped last time, two times, that I decided to buy a mini DSP 2x4HD and try to learn insted of being roped... So I started a week ago didn't have to much time and still doing some things wrong but still I hade the 4425 playing in bi amp with a Crown XLI800 for the boomer and a MC275 CE for the compressions.... cut the boomer at 7 or 800 Hz with a 24dB slope in LR and cut the compression at 1200 Hz with a LR 12dB slope.... this is the first attempt.... cannot wait till I masterize more REW cause I aml importing correction from REW into the mini DSP, which does only the gain and the crossover.... so far.
STILL TO WORK ON TIME ALIGNMENT AND PHASE.....71573
everything here is measured at one meter in the middle of my room to avoid or minimize rather wall reflections (midlle in height and from every walls, it is not squared)
so one placed back to the back wall it will reached 30 Hz at 0dB I will even have to attenuate the bass response I guess.... The only dispointment is the compression really....

[pos]

But an odd thing is that the horns are glued to the case, so even if I unscreew It won't go off...

Are you sure it is glued?
The gasket tends to stick after a while, and a lot of force is required to pull them off the baffle board.
A spatula can help, but you may destroy the gasket in the process (not that easy to make though, using some thick paper and a pair of scissors...).
All in all it is better to leave the horn in place :D

[pos]

STILL TO WORK ON TIME ALIGNMENT AND PHASE.....
The drivers are supposed to be geometrically aligned, so if you shoot for a symmetrical LR acoustical slope you should not need too much delay adjustment.
You can try the reverse polarity trick (reverse polarity on one driver and adjust settings to get the deepest null at the crossover point).

[davidpou]

hi thomas,

I was thinking of getting in touch with you for this time aligment story cause you are a star quoted in minidsp site for what you did in this domain...I did intensive reading and REW and mini DSP are not to coplicated if you have the will (starting from scratch for me) but a little lesson of two hours with a master of the art would save me a lot of time and some mistakes... do you know anyone near Paris who would take some € teaching me... ?
I will have a look at what you did but time domain is more complicated so it seems at firs glance...

by the way one related (naive) question: what is the use of sticking to JBL DSP for the M2 if:
1 you can find back some results by mesuring yourself
2 one can taillor the sond of the M2 to it s taste (that is one interest in the Crown DSP thing
3 one will anyway to tune the speakers to the room they will live in

Hope you project goes forward though...

[davidpou]

Are you sure it is glued?
The gasket tends to stick after a while, and a lot of force is required to pull them off the baffle board.
A spatula can help, but you may destroy the gasket in the process (not that easy to make though, using some thick paper and a pair of scissors...).
All in all it is better to leave the horn in place :D

Well the last M. F. in France who touch my L101 glued the medium of the crossover to the back of the speakers instead of screwing it (plus he screweed them....:banghead:)
when I realised his work sounded hawfull I tried to teke the crossovers out and I damaged the paint in the back of the speakers badly... they were absolutly new looking !!!! this traumatised me a little from forcing things out...
I bought the 4425 mainly to play and learn so it won't be as painful but still.... as you said better to leave them like that for now... I really tried softly everything I could....

[pos]

do you know anyone near Paris who would take some € teaching me... ?
There are several companies that do that sort of things for recording studios and concert halls, and could probably tune your system, but I don't know if they would teach you how to do it yourself.
As I live in Paris I would gladly/gratuitously help you to the limited extent of my knowledge/experience, but alas finding spare time is very difficult for me these days :(


by the way one related (naive) question: what is the use of sticking to JBL DSP for the M2 if:
1 you can find back some results by mesuring yourself
2 one can taillor the sond of the M2 to it s taste (that is one interest in the Crown DSP thing
3 one will anyway to tune the speakers to the room they will live in

You have to distinguish the crossover part and the global EQ part, both addressed in said preset.
The independent tuning (EQ, filters, level, delay) of each driver is paramount to get a complementary acoustical summation around the crossover, there is no way around it.

Now regarding global EQ, and your questions :

1- you could get something close, but the measurement technique used by JBL is very difficult to mimic in a home environment (listening window average in an anechoic situation, and other aspects of the spinorama).
2- Toole/Olive experiments tend to prove that there is no such thing as personal preference: people always seem to prefer a well behaved speaker (flattish on axis response, smooth power response, no resonance) regardless of their age/gender/culture/musical taste.
3- a well behaved system like the M2 (controlled off axis response, mostly constant on a wide angle, and smooth power response) should adapt to any decent room/situation without EQ over the Schroeder frequency. LF is another thing of course.

See the end of the video here: https://www.youtube.com/watch?v=zrpUDuUtxPM
(and then watch it all :D)


Hope you project goes forward though...
It does :)

[davidpou]

As I live in Paris I would gladly/gratuitously help you to the limited extent of my knowledge/experience, but alas finding spare time is very difficult for me these days :(


Very kind of you. I live in Suresnes. I will arrange my time table according to your availability.... no hurry I still have a lot to learn by myself .
A box of wine it will be then....