The Tempest
By Scott C. Blaier
31 Mar 1996
CALSOD Crossover Design
The crossover was designed with the help of the excellent CALSOD computer model. At the outset, it was determined to keep the crossover as simple and uncomplicated as possible. This was possible because of the excellent characteristics of the drivers, which do not require substantial crossover manipulation to compensate for shortcomings intrinsic to the drivers. The crossover presented here is the one that "sounded" best to us after listening and tweaking. Second order filters are used to keep driver distortion low and preserve power handling. A schematic of the circuit is included with this article Fig. 2. Crossover schematic along with a CALSOD plot of theoretical response, impedance, and phase response Fig. 3. CALSOD model of the SPL response.
The tweeter crossover occurs at 3200 Hz using a 2nd order electrical filter to provide the required acoustic transfer function. A bypass capacitor bypasses the tweeter's series capacitor to reduce hystersis and improve transient response.
The midrange crossover consists of a bandpass circuit that rolls off above and below 3200 Hz and 300 Hz respectively. The bandpass circuit employs 2nd order electrical filters to yield the required acoustic transfer function at each end. The bandpass filter works in conjunction with the midrange's acoustic rolloffs (determined by the subenclosure's resonance characteristics and the driver's high frequency rolloff). The bandpass circuit uses a transposed component layout instead of cascaded sections because it yielded better phase and frequency response. The midrange polarity is reverse phase relative to the tweeter and woofer to place the drivers in phase around the crossover frequencies. The midrange series capacitor is bypassed by a 0.47 polypropylene capacitor to minimize hysteris loss and improve transient response.
The woofer is crossed over at 300 Hz using a second order electrical filter. The woofer is well attenuated at the point where its upper frequency response suffers from cone breakup and directionality beaming.
The crossover uses Bennic and Solen polyproylene capacitors throughout, except for the large value capacitor in the woofer filter, which is a 100 volt electrolytic. All inductors are air core except the large value in series with the woofer; it is iron laminate to keep dcr low and cost down. The inductor's dcr and its effect on woofer efficiency and Qes/Qtc are considered in the CALSOD model. The large 70 mfd. Solen polypropylene capacitor in series with the midrange was on sale for $14.00 (about ½ price). If you are really on a budget, you could gang together several mylarTM and/or polypropylene capacitors in parallel to achieve large capacitance values. Often you can buy "surplus" mylarTM and polypropylene capacitors at bargain prices from Madisound, Speaker City, and other distributors. I do not recommend electrolytics in series with the midrange and tweeter. There is a limit to reducing cost, and at some point sound quality will suffer disproportionately to monetary savings.
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