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What could cause this amp to fail?

Hi Everyone,
Apologies if this is the wrong forum for this question. I just got into serious speakers (been into headphones most of my life). I picked up a pair of Thiel CS1.5’s that have blown me away. Anyways, I also bought a used Perreaux EX-120 (circa 1994 MOSFET Class AB 100wpc) Integrated amp on ebay for pretty cheap (seller had no feedback and I decided to take a chance) which is the subject of this question.

For the first few days after plugging it in it sounded great: warm, smooth, powerful until on around the fifth day, I noticed the sound cut from the amp and what sounded like a click. I powered it down and let it rest. The next day I heard the same thing but it started to happen more and more frequently. It would start about a minute after starting to play music and then quickly increase to the point where it would be clicking and cutting out multiple times per second (it doesn’t happen if no signal is running through). There was also a noticeable clicking coming from within the amp itself. The problem now starts happening immediately after putting signal through it and doesn’t stop.

I took it to a local audio repair who thought it was a relay issue but was unable to fix it after several tries. He referred me to another repair shop who has now told me that 8 Mosfets in the amp are blown and the input selector has ’aged out’. To repair it would cost about $450 dollars as he would have to replace many components in the amp and that it would end up as a just a power amp since the selector is a custom component that Perreaux doesn’t sell anymore.

I’m trying to get my money back from the seller, but he’s pretty much refusing saying that this is impossible and suggesting that I must have overloaded it since it worked when he sent it out. One thing to note is that I have carpet (not deep, quite short) and the binding posts on the Thiels are at the very bottom where I imagine it’s possible that the carpet could come into contact with the connectors. I have 2 other amplifiers (an entry level Cambridge Integrated and a B&K receiver which is a MOSFET design) and neither have had this issue.

My question is: could static electricity or contact between the carpet and the binding posts/cable connectors cause the amp to fail? If so, how come it took 4-5 days? If not, what is the possibility that it is my fault?
joezuu
ozzy62's avatar
ozzy62
3,111 posts
 
I am sure there are reasons why people sell of their equipment.
Yeah, it's called upgrading. You've never sold anything?
mlieber507
12 posts
 
There probably is a junction transistor phase splitter circuit driving all of the final output MOSFETS . If this develops heat sensitivity it could put a DC potential at the speaker terminals. The relay circuit is there to detect this DC and shut down the amplifier before it blows your speakers out. You need a repair person who understand how high power amplifiers function. Youtube has a number of videos posted by people who really understands this stuff and could probably help you.
joezuu OP
10 posts
 
Thank you all for your replies, this have been very enlightening. As I mentioned earlier, I'm new to the speaker world and know little about amp pairings, etc.

I’ve decided to not go through with the repair since given what @8th-note, @mlieber507 and some others have said. It seems likely that the amp was working ’fine’ for its age but failed due to the stress place on it by the speakers. I may end up repairing it in the future but certainly not with the current repair guy since some of what he said doesn’t seem to make any sense. In the meantime, I will be eyeing an Adcom GFA 555 II.
Feel free to leave some more recommendations for amps that pair well with Thiels, integrated or otherwise.

hartf36's avatar
hartf36
206 posts
 
In the next three weeks, if you DO somehow make a determination that the amp you received was defective in some way and was not as it was represented, forget what the seller says as far as refunds.  He can refuse 'till he's blue in the face.  It was stated earlier that eBay "usually" sides with the buyer.  In my recent experience, eBay damned near ALWAYS sides with the buyer regardless of the documentation you send them if a case is opened against you.  I recently had a "switch and bait" scammer run the game on me, and of course eBay sided with him.  It was a low-dollar item, so no big shakes, but it left a bad enough taste in my mouth that I no longer sell anything worth more than $50 on eBay.  They refunded the buyer out of their own pocket, and I got a used item returned to me.  Point being, if you eventually learn that it was defective and they seller refuses a refund, open the case.  You'll get your money back.  They need people buying stuff on their platform - that's how they  make their money.
boomerbillone
355 posts
 
I had a problem similar to yours in a church auditorium. It turned out to be poorly soldered wires inside a phone pug with an all metal body. A strand of "hot" wire had come loose and touched the metal body of the phone plug used to connect the amp to the speakers. Double check the wire used to connect the amp to the speakers. Any wire strands loose at either end? Since the amp has played in a shop and folks have had it out of the case, it shouldn't be dirt and fuzz on the heat sinks. If you have a fan available, point it at the air inlet of the cabinet and turn it on high before you turn on the amp. Does it take longer to shut off with the air blowing on it? If so, take the cabinet off and see if that helps. Also, be sure the small screws holding the output transistors in place are tight. Look for stray bits of conductive material in the cabinet or chassis f the amp. The two techs should have cleaned it out very well. Any stray wires around the amp's output connectors? Use a strong flashlight and wiggle it around. Tiny strands of wire can be very hard to see. Good luck. 
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