What is an acceptable level of transformer hum on a Krell S300i?


Greetings from a newbie. Just a quick intro:
  
Not rich, but I've been into hi-fi since college (Dual CS506 era). 20 years ago, I had progressed to Michell Gyrodec/Rega, Roksan Caspian CD/bi-amp, Sonus Faber Concertos.

The speakers never sounded as good as when the dealer had demo'd them hooked up to a Krell KAV 400i. But this was not a pressing issue as everything went into storage while I went travelling/working overseas.

Fast forward to present day. My brother has just got into vinyl (having discovered a killer high end McIntosh set up while staying at an Air B'n'B), so I unboxed my kit and set it up to show him what he should be listening to.

We both felt the sound was disappointing and my thoughts returned to that Krell demonstation 20 years earlier. My focus then wandered to eBay and I found myself winning the bidding on a S300i amp.

It is in excellent condition but it hums, even when in standby and without speakers or sources connected. I have tried different locations around the house, different houses, cheap mains cleaners and 3 into 2 mains plugs to eliminate the earth. Nothing makes any difference. When I press the mains switch on the back, the amp starts with a 'cough' which settles down to a constant hum after 2 seconds. I think it is higher than 50/60hz. Possibly closer to 200 hz. It does not vary at all after that. I convinced that this is the transformer because it also occurs in standby without any connections.

If the amp is on the floor and I stand next to it, I can hear the hum.   At 15 feet away, I can't really hear it, but with it in the rack and in my seating position, my ear is only 5-6 ft away and it is clearly audible when the room is quiet. It is annoying when reading in silence to the point that I need to disconnect the power.

My question to you... is this an acceptable level of hum? I would have expected that unless I put my ear next to the casing, I would not be able to hear anything. I have hunted the forums and web but can't find an answer for this specific issue. I use Transparent cables but these aren't relevant because it hums without any cables connected.

Your comments/thoughts will be greatly appreciated. I have already raised this issue with the seller who claims that it did not hum when he sold it. If the hum was really loud, there would obviously be a problem with amp. It is at a level that I can't be sure about without anything to compare it to.

Kind regards. 

rickx
So this is definitely mechanical hum. It is unlikely to be a ground loop, but it could be DC on the line, but testing this in a different home eliminates that.

Have you pressed your hand on the case? Is it something you can damp, or is it all sounding from the transformer?

Having the rectifier and caps checked out is not a bad idea, but I am afraid you may have a bad transformer, with only one, very expensive fix. :(

Erik
I still say don't even mess with it, demand a return from the eBay seller. Not sure how much you paid or what the description was but it is obviously far from functioning correctly.

Does not matter if his ad even states no returns, eBay will make them take it back if you state it is not as described.
Good luck
Thanks for all your constructive input, guys :-)

I did contact Krell and quickly received a helpful, if not conclusive, reply. I was hoping for a categorical, ’under normal conditions, our amps are virtually silent’ but it did fall a little short of that.

’It appears to be residual transformer hum and happens more frequently than one might think and shows up more in 50Hz environments. Usually residual hum is heard only when in close proximity to the units. 5 ft away is unusual... Other than replacing the transformer, which does not always fix this problem, I am sorry to say that there is not a fix for this phenomenon.’

He also said that you don’t need to short the balanced inputs on Krell’s integrated amps when not used.

I did press firmly with my hand on the unit which also has no effect.
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I use a PS Audio Humbuster III and it really works...no new ones (discontinued) but used or NOS show up here and there.