Need help - Crossover mod for JBL 8330A

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Hello!
I have some JBL 8330A surround speakers with the "SMPTE/ISO2969 Curve X high frequency de-emphasis." The previous version, 8330,
uses the same drivers but has a switch that would disable that curve in favor of a flat response. The 8330A does not have the switch.

I think the 8330A sounds a little soft and would rather have the flat response of the 8330.
Looking at the 8330 crossover schematic it is easy to see the R2 2ohm resistor that is bypassed by the switch on the HF. I was
hoping the 8330A would have the same topology and I could simply bypass the same resistor.
Comparing the two crossovers, some of the differences make sense to me, and others do not. My goal is to make the minimum amount
of changes to the 8330A crossover in order to obtain the "Foreground Music / Flat" response of the 8330.


My observations:

1. The LF section looks close. L1 changed from 3.0mH to 3.2mH. C1 changed from a .01uF and 36uF, to a pair of 18uF, but the net
is still close. Why would they use a .01uF and a larger cap in parallel to begin with? It doesn't seem like it would really be to
get the extra .01uF. Just curious.

2. MF section also similar. The first pair of caps in parallel totalled 14.01uF and now it's 12. L2 went from .75mH to .8mH.
Pretty close. This last part I don't understand, a 39ohm resistor in parallel with the driver? This went away on the 8330A.

3. HF section (probably the important part) the first cap was a pair totaling 6.01uF and now a single 6uF. Instead of a
switchable 2ohm resistor in the 8330, there is the 5ohm resistor (the schematic seems to read 50 but pictures of the actual board
show 5ohm) in series, and a .5mH coil in parallel with that. I do not know enough about electronics to know what that is doing.
The last coil is the same in both versions at .2mH. Again like the MF section the 8330 has a 39ohm resistor in parallel, which is
gone in the 8330A.

It seems like I could jumper/bypass the R1 and L4 on the 8330A and get close to the 8330 crossover? But since I am not sure what
the 39ohm resistor was doing there (and how it is suddenly gone in the new version) I don't want to break anything.

Any help is appreciated, thank you!

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I love JBL documentation, and this forum.

I stumbled across this reference in documentation of another JBL speaker:

"The dividing network has bypass
capacitors wired in parallel with the larger active
capacitor values in the circuit to reduce hysteresis
effects on the signal. This provides improved resolu-
tion of complex transient waveforms. "

Doing research here about "bypass capacitors" was very useful.

It explains why the .01uF caps were there. Probably they found that parallel caps of same value provided similar enough benefit. I can't be sure because I have not studied the composition of those .01uF caps (my understanding is that they would typically be higher grade) getting rid of them maybe saved a few dollars.

As for the 39ohm resistors, it would have lowered the impedance, and added current on the branch, but I am not sure for what audio effect, or why it was ok to drop them for the "A" version.

Reading about parallel inductors and resistors, it could be used to change the phase angle, which is not something I would want to bypass if it was an improvement.

Any insight into the logic of these changes is appreciated. Also, anything to get me closer to the goal of removing the ISO2969 curve from this crossover.

Thanks!

[Earl K]

Just rebuild the existing networks into the 8330 standard ( you do, after-all, have the requisite information to do so ).

:)

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Thanks Earl! The boards are really different, finding space/ layout would be a challenge. Looks like the 39 Ohm 10W resistors are $2 each. Not too expensive but the idea of spending money to SUBTRACT a feature offends me, lol.

What computer program would I use to build these in simulation and see exactly what they're doing? Is it freeware?

[Earl K]

Thanks Earl! The boards are really different, finding space/ layout would be a challenge. Looks like the 39 Ohm 10W resistors are $2 each. Not too expensive but the idea of spending money to SUBTRACT a feature offends me, lol.

What computer program would I use to build these in simulation and see exactly what they're doing? Is it freeware?

XSim download ( .exe ) (http://libinst.com/Xsim/XSimSetup.exe) will do the simulation ( and yes, its freeware ).

:)

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Appreciate that you answered a question that's been asked by thousands before! I will get going on this!