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RE: Chokes for MMG

When I got my first pair of Magneplanars (Tympani-I) in 1972, the speaker came with a simple symmetrical 1st-order 6dB/octave x/o built in, at 3200Hz iirc. Magnepan and ARC (who were distributing the speaker at the time) recommended bi-amping the T-I for increased performance, and ARC offered a passive outboard x/o box for the speaker, the x/o frequency being 1kHz. The 1st-order high-pass filter part (a single resistor, I believe, though I could be mistaken) value was determined assuming the input impedance of the tweeter panel power amp (a requirement in a passive x/o) to be 100k ohms, the input impedance of the then-current ARC amps.

To use the outboard x/o, a pair of interconnects were run from the pre-amp to the x/o, then a pair from the x/o to each of the two power amps. In this scenario, an alternative way to create the low-pass and high-pass filters was to install the passive parts found in the x/o directly onto the input jacks of the power amps. That would eliminate the extra interconnect and the x/o box itself, BUT would require the pre-amp to have two pair of output jacks (or the use of a cable-splitter). It would also not provide the level controls found in the ARC x/o box, which enabled balancing the high and low T-I panels. Some people installed the part in their high-pass amp, and used the x/o box for the bass panels, which allowed the panels to be level-balanced.

The above is simple to implement for 1st-order filtering, but higher-order filtering is a different story. A unique approach was taken by Dahlquist in their combination passive/active x/o---passive high-pass, active low-pass. There is no reason to assume an active x/o need necessarily sound "dirty"; there are plenty that sound as clean as any other piece of high-quality electronics. To think otherwise is the product of ignorance, if not attitude.

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  Kimber Kable  

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