2. amps-preamps
  3. 1406857519

Audio Research M100 Thumping

I just picked up a pair of ARC M100 mono blocks. When I turn them on, all is good. They operate fine for about 15-20 minutes, then the output cuts on one, and it begins a three-beat thump which sounds like a loud heartbeat. This occurs even if the volume is all the way down on the preamp. It also bleeds over to the other channel, but if I pull the speaker wires from the offending channel, the other channel is clear.

Ideas? Should I simply try replacing some of the output 6550s?

Thanks,

Brian.
bkrpdx
bkrpdx OP
24 posts
 
Thanks for the good advice. Anyone know of an authorized ARC tech in Portland, OR?

This morning I went to test if the tube switch had worked, and it blew a fuse. I switched the fuse from the other amp, and the amp seemed fine. Not sure why it blew the fuse. Now I need to go buy more fuses.

But it makes me even more inclined to have them checked.

B.
jperry
2,460 posts
 
You should have them checked pronto. They have some issues and you don't want them to damage themselves or your speakers.
bifwynne
2,206 posts
 
Bkrpdx ... call Kalvin Dahl at ARC. He can properly advise you.
hifigeek1
770 posts
 
This is called motor-boating and it means the amp is oscillating. M100's run very hot. Make sure the driver tubes are okay. I believe (from memory) they are either 7119's or 7044's. Those could be problematic. I would suggest you find a place to get both your amps checked out. I would not run the damaged amp.
bkrpdx OP
24 posts
 
I exchanged emails with Kalvin. He suggested switching out the output tubes and all of the fuses and running them for a while. I did that, and about five hours in, they really sound fantastic and seem to have no issues. I think these amps sound better than my old ones. Much more bass. Much cleaner high end. Sharper sound. And the others were "better" amps - conrad johnson Premier 8As with factory C1 upgrades.

Thanks, all.

Brian.
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