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Pass 250.5 shuts down driving Magnepan 3.7

This is my first post so please bear with me.

My Pass labs 250.5 protection circuit shuts the amp down when I drive my new Magnepan 3.7s at high volume. I'm talking about a volme higher than I might consistently listen, but not an unbearable level.

I'm using a Cary Slp 05 preamp with long balanced ICs (25 ft,) The amp is close to the speakers with less than 6 feet of speaker wire. The system works fine at normal volumes. I've tried the same thing with a pair of Cary MB500 (750 W @ 4 ohm) and they too shut down at high volume.

I believe the maggies drop below 4 ohms and I wonder if either amp can handle the impedance drop below 4 ohms.

Has anyone experienced this issue ?

Any suggestions ?
gcoxsom
onemug
784 posts
 
Suggestions:

Call or email Pass Labs about the problem.

Thoughts:

There could be something wrong inside your Maggies.

The Pass is stable into 4 ohms and lower. It has thermo protection. You may be overdriving it causing it to run hotter than designed and therefore shutting down "as designed".

How much does needle on the Pass deflect from the idle point?
magfan
1,583 posts
 
Are the amps in a well ventilated place? They very big and heavy with LOTS of heatsink.
If in restricted space, they could simply be not getting enough air.

I don't know that Magnepan has ever made a speaker that was a 'bad load'.
Low sensitivity? sure. Low impedance? yes again. But I've never seen any data showing bigtime dips or huge phase angles. Is the new 3.7 an exception? It does use a new design series crossover, so MAYBE, but don't bank on it.

If you haven't popped a speaker fuse, I'd look back at the amp......Onemug's bias meter suggestion is good.
charles1dad's avatar
charles1dad
10,250 posts
 
I suspect if could be a defective speaker. I`ve heard good tube amps drive maggies to loud levels with no sweat.
It`s hard to believe these large and powerful SS amps run out of gas with these speakers, they can`t be 'that' power hungry(given the sucess of the tube amps).
bvdiman
491 posts
 
Given two amps same problem, I'd first check your power source--wall AC. Make sure it is giving out ample supply, and preferably dedicated--for higher powered units.
magfan
1,583 posts
 
I LIKE that idea. If the wall voltage sags...badly, that'd do it.

Try a SINGLE channel at the offending level.

Borrow / buy a DVM and check line voltage during use. A 'Kill-a-Watt' meter would work, also. As long as you're on the same leg of the circuit as the amps, you'll be OK.

The Pass site also formerly listed output voltage of the amps, which would be another possible check.
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