Originally Posted by Heliac
With the caveats mentioned to protect your horn driver this is what you may wish to try.
I suggest you use a non polarised capacitor per the JBL 5235 manual for protection from pops or low frequencies. Go larger x 2 or 3 times as otherwise this will interfere with the crossover function on the horn. You can use a non polarised electrolytic crossover (60uF or thereabouts) capacitor 50-100 volt rating bypassed by a 0.1uf 250 volt rating polypropylene capacitor.
Going into the menu do not engage any of the PA functions such as limiter or compressor.
Go to the menu for the crossover filter type (refer to the manual or you will go crazy) and select Butter-worth 18 db slope for the woofer amplifier Frequency 800 hertz. Select Butter-worth 800 hertz 6 db slope for the high frequency amplifier.
Using the mic and analyser (refer to manual) measure the loudspeaker at 1 metre on axis with the horn.
Then refer to the tech sheet link posted by Robert H and look at the acoustic crossover response of the woofer.
Look in particular at the scale and the slope the woofer curve.By adjusting the crossover frequency either up or down in increments of 25 hertz you should get reasonably close to the tech sheet acoustic crossover curve of the woofer.
Do the same with the high pass crossover filter looking at the acoustic crossover curve for the horn.
Then combining both filters again measure the overall loudspeaker response. It should be a smooth response in the crossover region. The vertical mic position will impact on this measurement an you might try several measurements at different height levels in front of the horn.
As a test of the correct crossover setting you can reverse the polarity of the high frequency amplifier speaker leads to the loudspeaker. Then measure the system response again at 1 metre. What you should see is a uniform null or notch in the crossover region of 15-20 db.
If you see this notch you are close to the original passive crossover characteristics.
You can fine tune the crossover frequency of both the woofer and the horn slightly to obtain the most uniform and deep notch. The put the speaker leads back in the correct phase.
As a further step you can also make fine adjustments to the frequency response below 300 hertz. This is to compensate or the woofer operating directly from the power amplifier. You may find you need add a low frequency shelf filter below 100 hertz of 2-4 db boost. You may also find that a low Q PEQ filter at 290 hertz of -1 tp-2 db improves the mid range clarity as this is often a problem frequency in domestic listening rooms.
DO NOT attempt to use further equalisation offered by the active crossover. Use the adjustments on the loudspeaker only.
This will take you a whole Saturday afternoon and you ae best to do it without interruptions. Send the wife or girl friend out shopping.
I take no responsibility for any accidents that may occur. I suggest you keep the volume settings at a low level and follow all the guidelines in the DBX PA2 manual.
You might find you prefer some subjective aspects of the bi amp mode of operation but not necessarily other aspects. This is a multi faceted problem. The human mind/ear takes time to adjust and you need to be patient and re learn the subjective changes. The S9800 was optimised for the passive crossover. When you bi amp it does have a real effect on the bass. This is because the woofer is influenced by the passive crossover losses and this has a subtle but audible effect on the woofer low frequency response in terms of damping and the apparent low frequency extension. You might find while the bass is preferred overall that you somehow find the transparency or fine high frequency details are different. This is because you are processing the signal and some changes are to be expected. Like all things with high quality sound reproduction its subjective and personal taste comes into it.
This is only your first attempt and it may take some time to reach at pleasing level of enjoyment
Good luck
Ian
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