Duntech Thor IB-20 CC crossover - diyAudio
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Old 4th January 2015, 11:48 PM   #1
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Default Duntech Thor IB-20 CC crossover

This is an active crossover for the Duntech Thor self-powered Subwoofers circa 1986.
I'd like to move the crossover point (3dB down point) from 110Hz down to 90Hz or so.
I sent John Dunlavy a fax inquiring how to change it to 160Hz (1987), and he sent me a fax of the schematic and some suggestions on what to change, both on the active circuit passthru to the midrange power amp, as well as in the subwoofer's equalization circuitry. He suggested .086uF instead of .1uF which is the value of the caps in both places he told me to change. I forget why I wanted to raise the crossover point, but in hindsight, that's the wrong direction!
I never actually took his advice. Instead, I simply bypassed the active pass-through circuit which drives the power amps [to the mid-range speakers] with a .047uF MIT Multicap (which turns out to be the 3dB down point of 60Hz). (As calculated for a 60,000 ohm input impedance of the Amp).
I lived with that for 25 years. But now I want to also shift the 3dB down crossover point in the circuit which drives the subwoofer down to 90Hz or less.
I began by changing the .1uF cap that John suggested I work with - to a .147uF.
It is indeed starting to shift the 3dB down crossover point downwards but it's also reducing the output at say 40Hz and 30Hz as well by a small amount - 1dB or so. Not exactly the direction I want to go with my bottom Octave! So, I am looking for ideas as to where in the circuit to start modifying the crossover point without reducing the bottom Octave's output at all! I have the schematic and can email it to you, if you need to see it. thanks in advance. -TMack
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