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Audio Research Reference 150 - buzzing input transformer

Hello everyone, 

I have bought a second hand ARC REF 150 power amplifier (non SE, KT120 tubes) and there was a buzzing noise coming out of the input transformer after few hours. I have detected the noise appears as the amp fully warms up - after about an hour. It goes pretty loud within another 30-45 minutes.

I have taken the amplifier to a friend, a reputable tube amp engineer. He told me the input transformer must be replaced and also that in his opinion it was poorly designed.

I live in Europe so I didn't have the chance of sending the amp to the factory, nevertheless, a new transformer is on its way and I'll get it fixed by an authorized dealer and service. 

I was also told that the problem may happen again and that I should get rid of the amp. 

My question is whether any of you had similiar problems and what the cause can be. I really love the sound of this amp and of course I'd like to keep it. I'd also like to do as much as possible to keep the amplifier safe. 

I have found a very similiar discussion here on Audiogon, I have messaged the OP and he replied he had his transformer replaced afterall - 

 

Is there anything that can cause the transformer to overheat and melt inside? The noise was caused by a vibration of the plates inside the transformer. 

 

My speakers are Tannoys D700, they go down to 3 ohms and I can imagine they are current hungry even tho their sensitivity is 92dB. They worked well with a Classe CA-200 but the ARC sounds a lot better of course. I'm planning of upgrading the speakers in about a year but for now, I'm still very happy with these, it's always a pleasant surprise when I upgrade any piece of gear. 

I have asked the ARC company whether the speakers won't be a problem for this kind of amp. They ensured me I'd be fine and that they test their gear with much more difficult loads. 

The amp never got too hot when I placed my hand on the top of the chassis, the tubes were measured and they're fine, about 1100 hours on them. The amp wasn't in use for a few years before I bought it. 

Thank you for any suggestions.

128x128filipm
jetter
3,033 posts
 

@filipm 

I'd like some advice what to check so I can listen to it with ease. 

Perhaps the individual repairing your amp can give you an educated guess of what caused your problem and recommend how to avoid a similar occurrence. 

From your description of your cabinet, it sounds like your amp had plenty of space to breathe and was not overheating, especially as your mention the top was barely warm (surprising to hear as I would have GUESSED it to be toasty even in "regular" use).

Hopefully someone here will address the problems that can occur with your house/apartment's power, i.e., too much or too little voltage, DC in the lines, etc. just in case that is a problem.  All over my head and happy listening.

 

filipm's avatar
filipm OP
55 posts
 

The ARC REF150 manual says that the operation temperature is about 40 degrees of Celsius. Of course the tubes are much warmer but the chassis was always about that.

filipm's avatar
filipm OP
55 posts
 

So the traf is finally replaced. I just had a first listen. 

Does anyone know how hot the input transformer can get? I have left the cover off so I can touch the transformer. I guess it gets to about 80 degrees of Celsius after 2 hours. The output transformers are quite cool. The volume wasn't loud.

Tannoys D700's are connected at 8 ohms, bi-wired. 

It works without any problems of course, I'm still a little nervous. Thanks!

atmasphere's avatar
atmasphere
11,794 posts
 

Is there anything that can cause the transformer to overheat and melt inside? The noise was caused by a vibration of the plates inside the transformer. 

Does anyone know how hot the input transformer can get?

@filipm To the first comment (sorry I didn't catch this earlier), to my understanding ARC uses 'EI' core transformers for their power transformers (its not an 'input' transformer).

Over time, the bolts holding the transformer together (not the ones that secure it to the chassis) can loosen up. This is because they are insulated from the transformer by nylon washers, which shrink over time. If they get loose enough, the transformer can rattle. I suspect, since the amp was running this whole time, that simply tightening these bolts (there are four of them) would have solved the problem.

EI core transformers are quite resistant to DC on the power line. It takes quite a lot to make them rattle. Toroid transformers OTOH are quite sensitive!

Live and learn...

Regarding the second question its normal for the power transformer to run warm. 80C is fine. If the transformer gets so hot you can't touch it that's bad, but 80C is acceptable.

petg60
1,486 posts
 

@filipm

Better get a cook’s (meat) thermometer, like the ones sold at Ikea, to be sure.

 

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