The Project
This is a co-operative effort with a mate to create a centre speaker for his sound system. After looking at speaker specs he settled on a pair of 6.5" Polypropylene coned Woofers crossing over to a Jaycar 1" Dome Tweeter through a fourth order Linkwitz network at 3.0 kHz. He designed the crossover from a calculator found at www.ajdesigner.com/crossover/crossoverfourth.php. The following drawing was emailed to me with a request to source the components with exact values. I measured the capacitance values in store with a handheld meter and re-checked all component values later against a HP LCR meter. Here's how the values worked out:

Building The Crossover Network
Capacitors were individually measured to bring the installed values within 1% of the calculated values. Inductors were unwound one turn at a time and measured. They are accurate within half a turn when measured on the LCR meter. When the Tweeter was measured, the samples I had showed an inductance between 30 and 35 uH at 10kHz, so a Voice Coil Inductance correction circuit of a 6.8 ohm resistor and a 0.82 uF capacitor was added across the tweeter output. Woofer Equalisation consists of a 3.4 ohm resistor in series with a 20.5 uF capacitor. The complete circuit was assembled on a piece of veroboard with the components close mounted onto vero pins. This allows the components to be glued down to prevent microphonics. Input/output cables of 45cm length were attached to complete the job.

Initial Impressions
The project is still not complete as my mate has not yet built the enclosures. When the crossover is delivered, the enclosures should be complete and the results ready to post on the web site. Initial listening tests in the workshop with a set of similar Jaycar 5" polypropylene drivers demonstrated a seamless crossover to the tweeter, although the woofers sounded a little harsh. This was corrected when the Voice Coil Inductance equalising network was introduced across the woofer output.

Testing now evinced a sweeter sound with a suprisingly wide dispersion angle. The bass/midrange response will improve tremendously when the woofers are mounted in the enclosure. I expect the cabinet to be a tilted fascia floor mounted d'Appolito design (Woofer-Tweeter-Woofer) capable of supporting a TV. If this works out as planned, I might consider building a set for myself - this is my first experience with a fourth order Linkwitz crossover and I am impressed. The total cost for the crossover worked out at just over $110.00 for all the parts.

Details:

The Centre Speaker Crossover

Bench Testing The Crossover

The Crossover Network