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My tube amp causes isolation transformer to buzz

So I have an ARC Classic 60 tube amp that was causing a hum/buzz on my speakers regardless of volume.  In an effort to eliminate variables I plugged the amp into a Variac Transformer.  The sound from the speakers was almost completely eliminated, however once the power was turned on at the amp the transformer, which was otherwise silent, started to buzz.  Not super loud but audibly.  My amp draws 350 watts at idle. My Variac can handle up to 800 watts. 
So my question is, does applying a load to a transformer cause it to buzz, or can the amp cause the transformer to buzz?
last_lemming

Showing 2 responses by atmasphere

atmasphere's avatar
atmasphere
11,640 posts
 
“The transformer can make significant distortion if loaded above 50%. That distortion can cause diodes and transformers downstream to become noisy”
Sure. Harmonics are the result of distortion. The big nasty is the 5th harmonic (300Hz) which is the one that causes the problems. Any transformer will make more distortion as you put more energy through it, since they are inductive devices. Fluke Instruments published a paper on this topic back in the 1990s; I'll have to poke around to see if I can find my copy, but I think its online too. Here's one I found that deals with this topic in greater depth:
http://support.fluke.com/find-sales/Download/Asset/1260362_6001_ENG_K_W.PDF
atmasphere's avatar
atmasphere
11,640 posts
 
Variacs are not built with the intention to be particularly quiet. The only one that was to my understanding was made by MacIntosh back in the 1960s.


Also something to be aware of with any isolation transformer- 50% of rated capacity is a safe limit. The transformer can make significant distortion if loaded above 50%. That distortion can cause diodes and transformers downstream to become noisy.


I think Erik might be on to something. If you have a ground loop, its not uncommon for an isolation transformer to fix it. But its usually better if you find what is causing the ground loop and fix that. Even if they don't hum or buzz, ground loops degrade the sound overall by increasing intermodulations.
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