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Why are my mosfet fuses blowing?

I have a Classe CA-200 Power Amplifier/200 watts per channel into 8 Ohms (side heat-sink version)which is driving a pair of Thiel CS2.3s with upgraded coaxial tweeter/midrange. Sound is very good. I listen at relatively high volumes and recently (over the last year) the amplifier is getting hot within 60 to 90 minutes of listening and the mosfet fuses (2AG 1/2 PT, 1/2 amp fast blow) have been blowing. Do I need a higher powered amplifier to listen at high volume? Should I look for a used CA-200 and use one to drive each speaker (700watts into 8 Ohms)? Thanks.
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rwwear
2,867 posts
 
You may have a short in one of your speakers.
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bob_bundus
1,834 posts
 
The way you've written this I'm assuming that initially you had no problems at higher output levels but now the B+ rail fuses are failing? Both channels doing this? You might have the amp bench tested by a tech. Have it run at rated power into dummy loads of similar impedance to the Theils for a couple of hours to find out if it holds up.

Something in the load is possibly degrading if the amp is OK; possibly capacitors breaking down in the speaker crossovers is what she's alluding to. Or there could be a voltage protection device in the crossovers (such as zeners or varistors) which are now stressed from repeated overloading.

If it's always been this way then yes, you'd probably want "more power Igor".
herman
2,253 posts
 

You are blowing the fuses for the driver stage
The Classé CAM200 equipped with a rail currents sensors to protect the output drivers under extreme condition such as short output, and Mosfet fuses (2AG 1/2 PT, 1/2 amp fast blow) to protect the Mosfets, which are used as pre-drivers for the output stage of the amplifier.

That leads me to believe the problem is with the amp, not the speakers or the volume. There is no reason for the driver stage fuses to blow unless the amp has a problem like one of the output transistors breaking down when it gets very hot.

Have you contacted Classe for their opinion as they suggest in the manual?
IF IT APPEARS THAT PROTECTION FUSES DID BLOW, CONTACT YOUR LOCAL DEALER. DO NOT TRY TO CHANGE A BLOWN FUSE. REPLACING A FUSE WITHOUT CHECKING FOR COMPONENT FAILURE COULD RESULT IN FURTHER, SERIOUS DAMAGE TO VITAL COMPONENTS IN YOUR AMPLIFIER. CONTACT YOUR LOCAL DEALER or Customer Service at the Classé Audio, Inc ---(514) 636 63 84.


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mrderrick
292 posts
 
I drove my CS5i's with a CA400. I blew the Mosfet fuses once while listening at only moderate levels. After replacing the fuses I found that one side of the amp (channel) was running hotter than the other. It tuned out that the output devices needed to be re-biased. (yes, the bias can drift) It is rather simple to do. First you adjust the DC offset, then the output device bias average. (all of the output devices will not read the same, I think because Classe uses several different types together) All you need is a multi meter and a small "tweaker" screw driver.
I'm not certian of the internal layout of the CA200. The adjustment pots are located behind the Mosfet fuses on my CA400. You will also need to find out the bias level for the CA200. Ask this question over at the Audio Asylum in the "Amplifier" forum. That's were I was able to get alot of very good help.
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