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So I noticed my 3.6's haven't been sounding as awesome as I remember but realized I had moved them around and haven't done a ARC recalibration/correction recently.
I configured my laptop with latest ARC Genesis and did the measurement, and I saw a low dip from about 200 Hz to 1000 Hz in the midrange on the measurements, I didn't remember that before. There is enough air space behind, 3-4 ft, and the FR gets better for low bass so it isn't rear wave cancellation.
I checked and found both of the midrange fuses, left and right, were blown out. The worst part is when I replaced them with fresh fuses, nothing happened!
Is there some sort of crossover failure that does this? Anybody else have some ideas or experience with this sort of failure? I will call Magnepan tomorrow but I dread the news.
I could do a ARC correction curve which surprisingly didn't sound too bad as it corrected up most of the lower midrange. I'm attaching the ARC measurement.
Follow Ups:
I would agree with the others indicating deteriorating wires, but to have that happen simultaneously seems strangely odd.
I would get the amp checked out or at the very least run them through a set of 'throw-away' speakers for a while to rule them out.
I've got a pair of old Pioneer (Andrew Jones design) I use just for that purpose, you're welcome to borrow them if you want.
the amp is working fine driving the remaining two drivers, I'll check on other speakers but I suspect it's not a problem. It's a good amp (NAD M22).
I can only guess it was some transient that for some reason never hit the tweeters and somehow burned out both midranges.
Did you check continuity of both midrange drivers???
You can do that by disconnecting the external crossover.
Dave.
Can you give me any more specifics? If I were to disconnect the XO (which hasn't been removed since I bought them and requires that infernal tiny hex tool) what would I measure? Isn't the external XO split bass from high/mid so the external connections are for high & mid what XO being internal? My tweeters work.I have a cheap voltmeter/ohmmeter I use for testing batteries.
I'm concerned the drivers served as fuses---but it's hard for me to imagine some sort of spike that would blow out both midranges, including their fuses, and not hit the ribbon drivers and their lower current fuses. And the 3.6 has regular wire midranges, should be pretty robust, right?
I'm stumped to the problem and how it would happen on both channels.
I have never bypassed fuses and use 3AG fast blow fuses in recommended spec only.
Edits: 11/29/21 11/29/21
It is pretty well known around here that the drivers are very good at protecting the fuses - especially the ribbons. Also upteen years ago I had problems with the mid-driver fuses blowing (MG-IIIa), leaving the ribbon fuses intact. Problem was an underpowered mid-treble amp. At the time I was using an XO-1 to biamp".
As everyone has said check the continuity of the mid-drivers and a cheap meter is more than sufficient. Also did you check the new fuses? The condition that caused them to blow may still be present and they may have blown as soon as you turned the system back on.
BTW, as you have found out, when the mids go out, the effect is much more subtle than when you lose a tweeter or woofer. I guess the "missing" range is filled in partially by the other drivers and partially by our brains.PS: I am not sure of this but perhaps if the 200 uF "cap" which constitutes the HP filter for the mid has a short. This would allow the full signal to go to the mid-driver and cause the fuse to blow?? To check for the short disconnect the exterior box and place the meter across the positive input of the box and the positive output for the mid-treble. You should have infinite resistance, anything else is a problem. Shorted caps in a XO are unusual but you never know. The bad news is that more than likely the problem is upstream of the speakers since both mids went out.
"Our head is round in order to allow our thougths to change direction." Francis Picabia
Edits: 11/30/21 11/30/21
Yeah, well put.
In this case it wouldn't be surprising if the midrange transducer failed that reinserting another fuse would cause it to blow as well.
With the midrange driver open, the midrange crossover filter is a resonant circuit in the 800-1000Hz range. Nearly zero ohms load. :)
Dave.
I like that idea. It is possible that one driver went out some time ago and only when the 2nd failed did the OP notice. As I mentioned, a non-functioning mid-driver can be subtle. If that is the case, the good Doctor is lucky that the oscillation did not fry his amp.
"Our head is round in order to allow our thougths to change direction." Francis Picabia
Simply disconnect the external crossover and make a continuity measurement on the mid/tweet input with a DVM. You don't need to remove socks or do any sort of speaker surgery to perform this test.
You should see a measurement of approximately 4 ohms.
Dave.
How old are your 3.6? After many years of use the wiring on the diaphragm will deteriorate and may break.
I am afraid it is time to pull the socks off so that you can get to the internal crossovers and inspect the panels. First thing I would do is check resistance on the mid range. Should be 3.5 ohms.
Edits: 11/30/21
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