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The Project
This is a co-operative effort with my mate Bill from Subwoofer
Builder 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.
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