Here's how the crossover board pulls out of the back of the speaker column.  

After replacing components, such as electrolytc capacitors, on the various cards of an old Studer 2-track,  I wondered if some of the components in my loudspeaker crossovers might have aged to the point that they were not holding their original value - such as the capacitors.  The speakers are only about 8 or 9 years old, however.  Furthermore, advances in component longevity by the time the crossovers were assembled, had already resulted in more robust duty cycles.

As it happens, the resistors are all the right value resistance, and the capacitors are all at the right capacitance, and have very low measured ESR.  However, I noticed that there were some differences with a single resistor on different places on the crossover networks of the left and right channel speakers.








 










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This shows a pair of 100 Ohm 10 Watt resistors which are on the right speaker's mid/tweeter crossover section.   Notice how the right resistor's leg gets wrapped around the left resistor's leg before it goes through the through-hole.   The other legs are also wrapped together, with only one going through the other through-hole.


 In parallel, two 100 Ohm  resistors equal 50 Ohms 

( Rp = ______1_____)
 (_____1/Ra + 1/Rb...)


On the left speaker's mid/tweeter return path, however, the right-most 100 Ohm resistor's bottom leg was  either broken, or deliberately cut, and was thereby no longer attached to the left-most 100 Ohm resistor.  Therefore, the part of the signal that went through this network saw twice the resistance as the other speaker's crossover (100 Ohms insead of 50).  


 






















Believing it to have been diconnected as the result of an accidental break, rather than a deliberate tweak, I initially soldered a piece of resistor leg as a jumper to make them work in parallel.  My gut feeling, however, now, after having heard it play for a while both ways  is that, D. A. L. had determined that the right-most resistor on this particular crossover was not needed (due to mechanical impedance issues of the tweeter and,  possibly, mid range drivers), but, rather than deleting it, altogether, the connection on one end was, simply, cut.  Since the resistor was already epoxied to the PCB, it would have been difficult to remove, anyway.  But the remedy they chose was just as effective.  After trying it both ways, I believe that the left loudspeaker sounds better with the right-most resistor of the ordinarily-parallel twins deleted.  Without the notes, or an anechoic chamber with precision meaurement microphone, signal generator,  MLSSA, and impulse spectrometry hardware, such as was used by D. A. L., however, I  would not know as an undisputable truth what the original history of this resisotor really was...



















 


This picture shows the installed set of resistors for R6, R3, R4, and R5 on the left speaker's mid/tweeter crossover send section.





































Here's how R5 looks on the right speaker's crossover.  R5, also part of the mid/tweeter path, but in the _send_ section, had apparently not been installed at the factory.  A cable that connects the underneath of R5 (on the left speaker) to a nearby capacitor is instead connected to the bottom of R4 (which is a 51 Ohm 10 Watt resistor).   

Having evaluated the performance of the loudspeakers using the crossover as-is, and also with a 39 Ohm resistor placed in R5 on the right channel crossover, matching the left one's configuration, I decided that it sounded better with the R5 empty, rather than packed (on the right speaker).   Also, an article in Stereophile magazine has an interviews of John Dunlavy, in which he explains that his crossovers are adjusted to match the drivers (which are cherry picked, also), after the cablinets are filled, by placing the cabinet on a forklift and driving it just inside the anechoic chamber,   This way the phase coherence, impulse and step response can be made as close to the design goals as possible, even though the components, themselves, are not paricularly exotic.  The crossovers for a version 1 SC-V are visible in a D. A. L. brochure, and there is no resistor in R5, there, either. 



Please forward suggestions to andrew@serifsound.com



-Andrew