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.
You may have a short in one of your speakers.
To clarify IMO: the "short in one of your speakers" may be a problem with a crossover rather than a direct short in a speaker voicecoil.
The crossover in any speaker is a strange device and parts of it breaking down could allow too much current to flow without showing in other ways.
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".

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?


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.
Thank you all for the input. I did contact Classe, and they thought it may be a short in the speakers. I am not highly tecnical but if any one knows of a way to confirm if there is a short, I would greatly appreciate it. Thank you for all the help!
A short in the speaker should be audible at moderate levels; one or more of the drivers would not be working. It is also an unlikely scenario. Failures in speakers are typically something burning up because they were over driven. This results in an open, not a short. It is possible that something is physically touching inside that causes a short but you should be able to put your ear up to each driver and see if it is working.

The other simple test, assuming you have access to another set of speakers, is seeing if it blow fuses on another set.

I like Herman's call. Try other speakers.

I'll bet it still gets warm.
Re bias makes sense as do output devices breaking down under heat / stress.

The temp of the output devices is much hotter than the heatsink and all it would take would be some bad / devitrified heatsink compound to really cook some transistors.

Also, please check out this link to the CS2.3 Thiel. This is the measurement panel from the Stereophile test.
IF this is your speaker, I wouldn't use a bridged amp for any reason. It is a demanding load with a pretty beefy low impedance dip and a pretty nasty hi phase angle, to boot.

A short somewhere shouldn't be something that comes and goes. You only blow when you listen at high volume, correct? If it were a short somewhere, it should blow at any volume level. I would check with Thiel. The impedance of many larger speakers systems varies with the amount of power being put into them. I used to own a pair of Infinity Kappa 9 speakers- large, complex load. Although they were "rated" at 8 ohms, when I ran into the identical problem you are faced with- I kept blowing fuses on an Aragon and Audio Research amp- I made a few inquiries, and found that the actual resistance on the Kappa 9's could drop below 1 ohm at full load. That means you need a ton of current to drive it, and if you don't have that, you'll blow fuses left and right. If that's not the problem, check the amp bias. Checking and paying to have the amp rebiased won't fix anything if the problem is the speaker load. I got around this for a while by bi-amping the Kappas, but it was still a problem. Eventually, I got rid of the speakers. Alternatively, if you love them, you could try a current monster amp like a big Krell and see if that solves your issues...but that's a pricey fix. Good luck.
Thank you all for the great input. The speakers sound excellent (until the fuses blow) so it may be a power issue. I do have another set of speakers (Tannoy Monitors)which I can try to see if the amp still gets hot. P.S. Thanks for the link to the Stereophile article!
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.