Why are my mosfet fuses blowing?

I'll bet the Tannoys do fine- no fuse blowing. Your issue sounds remarkably similar to mine. If that's truly the case, you could, if the Thiels allow it, bi-amp or bi-wire the system with a second Classe amplifier. Be forewarned, though, you still may very well run into problems. You might be better off selling the one Classe you have and looking for something with a ton of current reserve to push the Thiels- Krell, Levinson or Rowland come to mind. Krells can drive a locomotive. But make sure you've diagnosed the problem before randomly starting to swap out equipment. Finally, is the Classe a Class A amp? Forgive me for asking a stupid question, but I'm not overly familiar with Classe products. If so, it's gonna get hot no matter what kind of load it's driving. Whether is blows a fuse or not should be the key with the Tannoys. Don't just look for power- look for an amp with high current reserve. You can get a high power amp- my Aragon was 400 wpc into 4 ohms and it couldn't drive the Kappas- with not a lot of current reserve that will run into the exact same problem.

Thank you Afc. I found a Stereophile article which seems to indicate that it is Class A/B (http://www.stereophile.com/solidpoweramps/297classe/) but I don't know how to verify. I will try the Tannoys and see what happens. Thanks!

The Classe's are largely A/B. Most are Class A for the first 30 watts and then A/B thereafter, I believe. Rest assured, to push those Thiels, you're going well into the A/B range of operation. An A/B amp will still generate quite a bit of heat as well. Not as much as an A, which loses about 70% of its' energy output as heat, but it will still generate a lot of warmth. A/B's lose, I believe, about 50-60% of their energy as heat. That's why those big amps have such huge heat sinks and weigh so much. Class D amps are about the only ones that don't generate much heat at all. There's practically no energy loss. You don't even have to turn them off usually, since they draw such little current at idle. The Tannoys will provide your answer for you. Good luck! Remember, heat by and large is not the issue- it's whether or not the fuses blow.

I just went back to the Original Post.

I'd redo / re terminate or otherwise look at the speaker connections / wire.

This problem came on 'over the last year' which tells me something is changing.

Speakers also change from cool/normal to hot. The impedance and other characteristics change. These are consumer speakers, after all, not Pro Audio stuff which will take it for hours on end.

And yes, an overheating transistor can work fine while cool than go non-linear. If you can get at your output devices while the unit is warm / hot but before you zap it again, try a can of freeze spray on the heatsink.......cool off the heatsink which will cool the output devices. You may hear something from a bad transistor. I wouldn't spray the devices directly.

Bearr38, this is the first time I'm seeing this thread. if both mosfet fuses are blowing, that leads me to believe your problem is with the speakers. I don't understand though why the mosfet fuses would blow under extreme load conditions but not the rail fuses for the output transistors.

BTW, Classé has a specific procedure for replacing the mosfet fuses that includes discharging the main power supply caps with a 100W incandescent bulb before doing anything in the amp, including replacing the mosfet fuses. Failure to do this risks damage to the mosfets. You also need to ensure that replacement fuses fit tightly in the fuseholder, crimping it a little if necessary. Loose mosfet fuses can translate into distortion on top of the audio signal.

I have service manuals for the CA-200 and CA-201. The older CA-200 manual is very basic, and does not have troubleshooting steps. The service manual for the almost identical CA-201 does have troubleshooting steps which you should find helpful. The steps are basically the same for all Classe legacy models. PM me if youw copies of the manuals.