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Protection circuitry on mufi kw500--help?

We are not original owners of a mufi kw500. I believe it was 6 years old when we bought it in pristine condition.
Yesterday we had the unit on for 8 hours straight--3 hours sitting waiting for our company to be ready to listen, then 4-5 hours of rock. We played james gang, mumford & sons, wishbone ash, billy cobham, three dog night--our company loves drums and guitars. So it got quite a workout. We had just started deep purple smoke on the water, turned the sound up from 11:00 to 12:30 position-- when we lost the left bank, and the protection circuitry???? Kicked in to lower the volume. The legs went purple.
We turned the system off for a few minutes, then back on. It powered on, then immediately turned purple legs again.
It smelled a little electrical burnish. But not strongly so.
We tried a power up two hours later--same thing--powers on and goes to purple.
Is there a home reset option for the protective circuitry?
Has anyone else run into this? How was it fixed and what did it cost?
Any thoughts would be appreciated.
Obviously the unit isn't under warranty.
We will call the musical fidelity folks for ideas, also.
My husband is a mechanical engineer, and is willing to tackle the issue if we could find some schematics.
Thanks, folks.
Associated gear:
Clearaudio Revolution TT
ARC ph3SE
Tara lab interconnects
MIT z-series power conditioner
Vienna Acoustic Beethoven 4 ohm speakers
MIT 750 series 2 biwired with cvt speaker wires
REL Strata iii subwoofer
Mufi kw500 with RSA duke power conditioner
3 dedicated lines--one for mufi, one for subwoofer,
One for TT-associated equipment.
Rel is run by mufi in 2.1 mode.
5.1 additional equipment was turned off--Yamaha rxv1, Vienna acoustics mozarts speakers w MIT speaker wire and mufi 550ks.
Thanks for helping us!!
Deb
deebarnes
almarg's avatar
Ag insider logo xs@2x
almarg
9,670 posts
 
Thanks for letting us know, Deb.

I just found a couple of threads, including this one and another one at a different forum, indicating that the outputs of the KW500 are bridged. Which means that its negative output terminals, as well as its positive output terminals, provide signals to the speakers (as opposed to the more usual situation where the negative output terminal is grounded).

In general it is not good practice to connect a powered sub to the speaker outputs of a bridged amp, because depending on how the grounds are handled in the specific designs the sub might introduce a path between the negative output terminal and ground, resulting in that output being either loaded excessively or directly shorted to ground. If that were occurring, though, I would have expected it to have resulted in hum or other audible consequences all along, which leaves me a bit puzzled.

What I would fault in this situation is the manual, which makes no mention of the outputs being bridged, and does not provide the cautions that are usually stated in such cases about not connecting the negative output terminals to anything that could ground them.

In any event, I'm glad the problem wasn't anything more serious.

Regards,
-- Al
b_limo's avatar
b_limo
2,274 posts
 
Bummer about your amp. Curious though as to if you live here in the states and where you sent it for repair. Do let us know when you get it back as there seems to be some poor customer service for repair work regarding mf and it's not fair to sum up a whole companies repair assitance based on just a few bad examples.
Anyhow, I hope you get your amp back and up and running soon :-)
deebarnes OP
13 posts
 
I called the national sales office who referred me to a repair facility on the east coast.
Talking with Brett, he sounded knowledgeable and experienced.
Will let you know how he does. And will pass along his info if we are happy.
As for hums--no hums at all from anything when all was working "properly".
I guess we will use the line out for the subwoofer next time.
Bummer.
Deb
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