My setup has evolved to such a level that the M553 I use as a crossover has by far become the weakest link in the setup. Therefore I decided to examine the circuit, do measurements and see what mods could be made to improve the soundquality. One could say that there are a great number of other active crossovers available, but none of those had what I was looking for out of the box like the M553 has. As the M552 circuit is exactly the same save for the 2-way vs 3-way, the below also goes for the M552.
In the circuit are about 32 capacitors in the signal route. As in high end systems the sound of even one capacitor is deemed critical to the overall system sound, you can image what 32 can do to the sound. But if all these had a function, upgrading them would be quite a lot of work and cost quite some euro's, although the time involved would be much more of a burden for me.
So I analysed the circuit and made measurements with multimeter and scope to see what happened after each opamp. It turned out the DC was quite low at all points in the circuit. Capacitors are normally used to prevent potentiometers from crackling by blocking the DC. But here the DC is only in the area of 40mV max. This could still be a problem if the amplification was large, but in all the steps in the M553 process, there is about unity gain with all pots set at 0dB. So the ratio between DC and signal was always about the same.
Therefore I thought it would have to be possible to bridge 26 out of the 32 capacitors. Incredible, 26 out of 32 could go. And I just did it by bridging all the 26 pairs of legs by small soldered jumper leads of about 5mm. And the M553 now sounds awesome. Depth, air, body, mellow, lucid, name it. It can finally keep up with the rest of my system.
So, why did JBL include all those capacitors in the first place? I think for predictability in how the M553 would function. With the capacitors it is more foolproof, less of a worry I guess, as the potmeters can't crackle, there is no plop when muting etc. But in reality, in my living room, the 26 could go.
6 remain as they are the output capacitors and are the final block for DC leaving the M553 (both hot and cold of the balanced outputs have one each).
So, if you use the M553 in a high end setup (or any home setup I'd say), bridge the 26 caps and free your sound!
ps., and see to it that the subsonic filter is bypassed by setting the jumpers the right way...
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Originally Posted by frank23
… Traditionally in Opamps, coupling caps are used to reduce the effects of DC Bias offsets on the front end of the Opamps from the preceding Opamp stage, the voltages are just a few mV or lower and will vary greatly with temperature. Now imagine a series of cascaded Opamps one after the other all with a DC Offset, the first Opamp multiply it’s DC Offset by its gain and then follow through on all the Opamps multiplying by their gain … by the end you could quite easily end up with offsets of some many volts in either polarity … all subject to temperature drift, now superimpose an AC waveform on that DC Bias of say 10 volts …. Then this is going to reduce the overall AC level you can use (by the DC Bias level) as you approach the supply rail voltages and all unstable and temperature dependent and it goes without saying this will also severely reduce your headroom as well …. Capacitors are NEVER used for “to prevent potentiometers from crackling by blocking the DC” 
… if you want to stick with the M552 and improve the sound then replace C9, 91, 97, 98, 40 and 41 with Elna Cerafine caps or some other audio grade cap …. These will be the main culprits and if you want to continue then replace all the signal caps with polypropylene film caps or so …. And contemplate doing a Opamp upgrade ….. personally I would be removing all those wire links you have soldered in and your speakers might appreciate it as well! .. good luck !!
