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Need help from Pass X-600 + ARC Ref 2 MK1 user

I am matching Pass X-600 with ARC Ref 2 (MK1) and encountering a strange hum issue. I have tried couple combinations and it seems like the hum is coming from only X-600 + Ref 2. X-600 and Ref 2 do not seem to have any problem individually. Following are the combinations that I have tried and they show no hum at all.

1. X-600 + Pass Aleph P
2. X-600 + Wadia 861 (using build-in preamp)
3. Mark Levison 331 + Ref 2

I have also tried couple power conditioners and even power regenerator, but the hum still. I was told the hum might be from power & preamp impedance mismatched. However, it shows pretty healthy from the specifications. Any comments on this will be appreciated. Thanks in advance!
jshaw1004
newbee
4,767 posts
 
Dumb question time. Have you tried using a cheater plug on the amp?
jshaw1004 OP
53 posts
 
Newbee: Thanks for your comment. Yes, cheater plug is the very first thing that I tried. I think I can pretty much rule out the ground loop issue.
cytocycle
1,043 posts
 
Does it humm out the speakers or is it a mechanical hum from the transformer of the amps?

Are you using balanced cables?
newbee
4,767 posts
 
Well, here's another dumb observation....If you are listening to noise with your ear near the tweeter/midrange and if your amp has a highish imput sensitivity you might just be listening to the noise floor of your amp, or its tubes. When I crank up my SP10II I get both a light hum and tube rush using two different amps, one with a .5 and the other a 1.v input sensitivity. With a third amp and a 1.5v sensitivity I get very little noise or hum. A different tube pre-amp is dead quiet into the same amps. Both with my ear at the speaker.
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