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View Full Version : Help w/ resistor values for active X-over ?



Tom Loizeaux
01-19-2004, 05:57 PM
JBL specs five .018 caps and five 24K resistors to make a 250Hz X-over card for their 5235 crossover.
They modify those resistor values to be two 27K and three 22K resistors to tailor these cards for use with the 4343 studio monitor.
I want to modify a Crown FFX-2 18dB/ octave active crossover to produce the best crossover for these JBL 4343 monitors. The Crown literature says to use six 19.3K resistors per channel for a 250Hz crossover point, 18K resistors for a 275 Hz point, and 16.1K resistors for a 300Hz point. The caps don't need to be changed in the Crown design.

Can anyone suggest what value resistors I should use to achieve the "corrected" crossover slopes in this Crown unit to optimize it for the JBL 4343 monitors?

Thanks,

Tom

4313B
01-19-2004, 10:01 PM
The 4343 used a 12 dB/octave active filter.

The easiest thing you can do with the FFX-2 would be to use all 16.6k resistors except R109 (R213) and R111 (R215), make those 18.0k. This will yield the 290 Hz crossover as used in the 4344, 4345, and 4355.

Tom Loizeaux
01-21-2004, 05:32 PM
Giskard, thanks for a very useful reply. You not only responded with a reasonable answer, but obviously went to the Crown schematic and gave me resistor numbers! I'm stunned!
But to complete the larger question, if I may...

Since the JBL 5235 crossover card for the 4343 is a 12dB design (180 degree shift), the cabinet wiring inverts the high freq, bringing it all back in phase. Maybe that's what makes the "adjusted" crossover cards needed. I understand the "adjusted" cards for the 4343 slightly reduce the level of the high freq. (2121 essentially) in order to achieve a smoother balance between the 2231 and the 2121.
So, if I were to use the Crown crossover, with an 18dB slope in place of the JBL 12dB crossover, wouldn't I be getting 270 degrees of phase shift, leaving me with a new 90 degree shift? If so, wouldn't this possibly work to slighty reduce the total output around the crossover point - and maybe even out the balance between the 2231 and 2121? If that's the case, then maybe I don't need to try to "adjust" the crossover point, but simply use the stock Crown values to get the crosser frequency I need. Is this making sense?
Well, what do you think?
I apologize for this being so specific, but I feel some on the Forum may be interested in more discussion on crossover phase realities.

Tom

4313B
01-22-2004, 02:10 AM
Here are some graphs:

JBL 5234A with 4343/4350 card - hp and lp are both 4 dB down at the crossover frequency ~ 255 Hz

Crown FFX-2 - hp and lp are both 3 dB down at the crossover frequency ~ 250 Hz

4313B
01-22-2004, 02:25 AM
17.5k in the hp and 20k in the low pass will get you ~ the same -4 dB point at ~ 255 Hz. You will end up with slightly more output from the 2231 below crossover and from the 2121 above crossover when using the Crown.