The FTX0617 coaxial isn't an easy driver to work with but after taming it's high frequency response I find it provides excellent transparent sound quality in the treble without any objectionable traits. The high frequency dispersion characteristics from this driver are also excellent, it's rated at 100 degrees nominal and it certainly provides wider, more consistent coverage in the top octaves then any of the other 6.5" coaxials I have used.

The high frequencies blend near seamlessly with the midrange on the FTX01617 with all the benefits a coaxial provide, namely the point source like dispersion pattern without the drawback of off axis nulls or uneven power response in the vocal range inherent in more conventional multi driver designs. With the the load of the lower frequencies lifted by the woofer the 6.5" midrange on the FTX0617 really opens up resulting in an effortless and clear midrange presentation even when the volume is pushed.

The woofer may have surprised me the most, mainly from the bass extension it gave in the small cabinet being quite unexpected, as I was anticipating an f3 around 65hz when I was designing the cabinet, not the sub 50hz extension that resulted after the crossover implementation. In such a compact enclosure the woofer has no trouble staying in control with lots of power either. Excursion remains at fairly safe ranges up to 400w and the speaker really starts to come alive when you feed it decent power.

This design certainly met all the expectations I had for it when I first set out designing/building them and I would have no trouble recommending them for compact high quality LCR or surrounds if extreme output requirements are not a consideration. They are capable of getting loud enough for reference levels in small/medium rooms given enough power but won't win any SPL shootouts against proper Pro Audio designs due to the medium-low sensitivity and efficiency of the design. That said these clearly outclass more conventional Hi-Fi designs when it comes to their dynamics, clarity and composure at high volumes/power levels.

I didn't really hold back on the crossover design as I was trying to extract the most I could from the drivers, the only real limitation being what I could physically fit into the enclosure.

Most of the work ended up going into the HF driver as it's response did not lend itself to a simple filter. Essentially it's a 2nd order highpass (though electrically acts more like a 3rd order near the crossover) but with a pair of notch filters and another pair of contour filters mixed with padding. I would have adding something to damp the peak at the top end but that rise starts so high (>18k) you don't really notice it and the peak also flattens out some as you move off axis.

The midrange was much easier to work with and therefore is a much more straightforward circuit, a 1st order highpass and 3rd order lowpass with some padding and paralleled components to get the required values and keep the size down. The highpass does interact with the impedance of the midrange resulting in an electrical slope that's somewhere between 1st and 2nd order (~9dB octave).

The woofer uses a 2nd order lowpass with a coil in series with the parallel leg of the filter. The coil results in a notch and increases the slope of the filter without disturbing the phase much. You loose the lowpass slope as it flattens out above 2 kHz but it stays at least 30 dB down for the remainder of the range and this woofer isn't doing much at those frequencies anyway. A large capacitor is placed in series with the woofer this helps to give a more linear electrical response near the crossover and the interaction with the impedance peak at the system resonance causes a high Q highpass which provides over 4dB of passive gain just below 50 Hz allowing the design to achieve it's surprising bass extension.

The crossover between the midrange and the tweeter is ~1650 Hz and between woofer and midrange is ~250 Hz.

The Slipstream was designed and the crossover tuned specifically for in/on wall use, because of that there is minimal baffles step compensation needed with how shallow the cabinet is. One of the other benefits of the shallow/wide cabinet is the minimization of SBIR. The reflection off the front wall does not cause as large or sharp of a cancellation as a narrower/deeper cabinet would and if mounting in wall that eliminates the issue entirely.

Slipstream Bill of Materials

I now have the crossover circuit board kit for this design available