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Help needed: Tympani IVa midrange driver problem




I recently purchased a pair of Tympani IVa speakers, still in their factory sealed cartons after a complete rebuild at Magnepan last year. I've been having a problem getting centered images between the stereo pair, and after some diagnostics, discovered that one midrange driver is approximately 5dB lower in volume over its entire range than the other channel. I measured near-field to eliminate any room effects, and every other driver in the system (tweeter and both woofer panels) measures virtually identical in level left to right. My first thought was that one of the two runs of voice coil wire that make up the mid driver was not connected, causing the lower efficiency/volume. However, the left and right midranges both measure identical DC resistances (3.25 ohms including the 0.6mh coil), and if only one of the two runs of wire were connected, the DCR would be much higher (double) in the bad channel since they are connected in parallel.
I've checked the internal wiring and crossover (both external and external), and they all seem fine. I also jumpered the fuses, as I've had problems with intermittent fuseholders in the past, but that's not the case here.
Any suggestions for diagnosing/repairing the problem would be most welcome, short of shipping them back to Magnepan, which is my last resort.
Attached is a screenshot from OmniMic of the frequency response graphs showing the midrange channel to channel imbalance.
Mike
PS: I've communicated with Gary Edwards at Magnepan, who was helpful but admitted he doesn't have the technical expertise to help. He suggested I call Wendell, but I got no response there. Hoping someone here has some suggestions. Thanks.
PPS: This is actually my second pair of TIVa's - I foolishly sold my first pair many years ago. I posted the schematics of the TIV and TIVa crossovers in MUG Tweaks over 20 years ago!

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Topic - Help needed: Tympani IVa midrange driver problem - mikebarney 12:50:52 04/07/21 (33)
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