The series crossover is an integral part of the Quasar design, crossing over the drivers around 150Hz. This removes the bass frequencies from the wide band driver (which can only be a good thing) and limits the bass helper to the bass notes. The series crossover does some nice things compared to a parallel crossover too.
So this is James' suggested design for my Fostex 208E sigmas
And as I intend to use the 208s eventually, this is the design I used to buy the bits. Fortunately the values are near enough for the Alpair 12Ps I'm currently using too. So without too much thought I soldered the inductor and capacitor together, along with a resistor to drop the efficiency of the Supravox bass helper a touch and clipleaded to the drivers.
I wasn't expecting much of an improvement, but was rather surprised by how much things cleaned up. Before, I was running the Supravox full frequency, so rolling off about 5kHz, and using a 6uF PIO capacitor to roll the 12P off at about 4kHz. And it sounded great. There was a bit of a muddle which I put down to the band between 4kHz and 5kHz where both drivers were operating, but the clarity of the 12Ps shone through, and the underpinning of the 285s was firm and taught.
With the proper crossover in place the muddle disappeared which helped to clean the sound up quite significantly. Not only did the muddle go, but the overall detail improved as the 12P was running lower and seems a touch more articulate than the 285 above the bass helper frequencies.
But then James reminded me he'd designed a parallel crossover for the 12P as his experience with the original Alpair 12 wasn't good with a series crossover. Mark Fenlon does suggest that the 12s need as clean a supply as possible. So I changed the components around for the parallel crossover below and reconnected the clipleads.
I wasn't sure what to expect, and didn't think it would make any difference in all honesty. Well, it's early days but I think there is a change. At first I thought there was a little less punch and dynamics, and possibly a little more sibilance. The more I listen though the more detail I can hear coming from the 12Ps. I need to listen over a longer period but I think I can hear greater subtlety and tone with the parallel crossover. Just shows how small differences can have an effect.
