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|11th June 2019, 04:11 AM||#521|
Join Date: Aug 2016
Location: Tucson, AZ
Modifying an LXMini crossover for ADS L980 2-way biamplification
Here is how I went about designing and modifying an LX Mini crossover board for my biamplified ADS L980 studio monitor loudspeakers.
The ADS L980 is a 12” 3-way loudspeaker that was manufactured in the mid-1980s. Since it was ADS entry into the studio monitor market, it features high quality, matched drivers in mirror image cabinets. Components consist of a12” sealed box woofer, a 2” high excursion dome midrange and a 0.75” dome tweeter. Crossover points are nominally 450Hz (very low for a 2”) and 5000Hz. Efficiency is 90dB 1W/1M. Passive crossover slopes are 12dB/oct Butterworth. In biamp mode, direct connection is provided to the woofer voice coil but the midrange HPF and LPF and tweeter HPF stay in the circuit.
Listening tests using a miniDSP digital crossover/processor determined that my preferences were for a 500Hz 24dB/oct Linkwitz-Reilly alignment between the woofer and the midrange, leaving the mid-high passive crossover set at the -1.5db position for the tweeter. Since a 24dB/oct L-R filter is two 12dB/oct Butterworth filters cascaded, I decided to design the active crossover with a 12dB/oct Butterworth alignment HPF for the midrange, which would add to the passive crossover Butterworth filter to yield a 24dB/oct L-R alignment, and an active 24dB/oct L-R alignment LPF for the woofer.
I started with TCJ Filter Designer from Glass-Ware. This program is a bargain at only $10. John Broskie has done the DIY audio community a real service here. Moving away from the optimal component values, which are non-standard, I went with the closest standard cap values first and then the closest 1% resistor values after that. I plugged these into a modified LTSpice version of the LxMini crossover model (link available on one of Papa's sites). This confirmed I was on the right track.
I have posted the schematic mods, the LTSpice plot and file and some photos. Should be pretty self-explanatory. The caps in the resistor spots are a bit twisty as are the resistors in the cap locations. There are a few Jfet current source resistors I stuffed and soldered in by mistake in the L and R section two of the HPF. Ignore them.
How does it sound? The improvement over the miniDSP in detail, imaging and overall clarity is almost shocking, it is so obvious. Well worth the time and investment. Thank you, Papa, for making this wonderful Jfet analog crossover board available through the DIY store and also the matched Jfets and current source resistors.
Last edited by avdesignguru; 11th June 2019 at 04:29 AM.
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