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In Reply to: RE: Help needed: Tympani IVa midrange driver problem posted by mikebarney on April 07, 2021 at 12:50:52
I'm afraid I will not be much help but looking at the schematic that you posted one score years ago, the mid-driver does not appear to be doubled. The mid impedance is marked at 5 ohms + the DCR of the coil. A better measurement is to remove the tweeter fuse so that the tweeter does not interfere. However, how this helps diagnose the problem is beyond me <(. I assume you already did the various cable and channel switches so you are convinced the problem is in the left channel speaker and not upstream. This is supported by the normal behavior of the tweeter and woofer.
Addendum: Here's a thought. Pull the mid-fuse on the right panel and replace it with a 4 ohm resistor (the fuse is useless anyway). That will drop the output by ~ 5db : 20*log(5/9)=-5.1 db. Not a cure but a work-around. The remaining issue would be the balance of all three drivers.
I married the perfect woman - the downside is that everything that goes wrong is my fault.
Thanks for your note. You are correct, that all upstream components have been ruled out. I checked my original TIVa schematic from years ago, and although the online copy is difficult to read, I believe the original shows the midrange resistance as 3 ohms (exactly what mine measures), not 5 ohms.
I like your work-around with the additional resistor to reduce the other channel, however the bad channel exhibits a suck-out in the midrange relative to the woofer and tweeter, so I'm afraid I'd then make both channels sound bad!
Correct! :-(( And the tweeter.
3 ohms is correct - that's what mine measure (well, actually 3.2 ohms).
If you convert your T-IVas to active, then you will be able to boost the faulty mid panel back to 'normal' volume.
The midrange driver without the crossover should measure about 3.6 Ohms. There have been a guy who let Magnepan re-wire the mids of the 3.6 with foil conductors. I did it myself, it was the smallest cross section foil, the one used in the mids of the 20-series. Will give the same impedance (resistance) but slightly lower output (as the foil is not sitting in strongest spot of the magnetic field). My 3.6 shows a difference between right and left speaker, tweeters are 2.5 dB hotter on one side. I am not impressed by the quality of work of Magnepan (I have four sets). One of my foiled T-IVa mids has also a lower output, the Mylar is further away from the magnets. It seems to be common that the Mylar leaves the double side tape, that will change the performance quite a bit.
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