Going Topless

Frank, while I do not disagree with your experience, there can be an additional factor at work here. Is the ARC preamp tube type? If so, tubes seem to prefer having all metal removed from their proximity. My Wolcott amps are run with the covers off for this very reason. My experience with the performance gain this provides, pretty much mirrors the benefit you received with your ARC.

Removing the lids on components can have a DRASTIC effect sometimes. Besides tubes receiving more direct stimuli via vibration transmitted through the air with the covers removed, circuit stability may also be effected. While most of my experience in this area is with RF based products, i have watched some amplification circuits go from completely unstable and on the edge of oscillation ( covers on ) to running beautifully ( covers off ). While my situation probably pushes things a little more due to the nature of RF wanting to float, removing the lids may reduce the overload that a low level circuit was seeing due to being overpowered by a high level signal. Instead of the fields "jamming" up the guts internally, it is now "freewheeling" around in the room, producing less concentration inside of the component. The reverse is true of our audio based gear. With the lids off, RF can now enter the gear more easily although you may have also minimized the magnetic interaction between incorrectly routed wires or the field generated by a transformer. Either way, run it how you think it sounds best. I, for one, will not doubt that the unit IS performing differently. Just keep in mind that you may have to clean the internals out due to collecting dust and that there may be safety issues to deal with if you have pets or kids. Sean
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I prefer topless:

In fact, I even took the metal cover off my breaker box because my system sounds more dynamic (less constricted). I replaced the cover once, but didn't like the resulting affect on my system, so I removed it again and have been enjoying it without the cover for the past few years. (Kids don't try this at home!)

I also had good results removing the top covers from some tube preamps and amps. I just cover the gear with plastic when not in use to keep the dust out. --Bruce

frap, which arc do you have? can removing the covers be cooling the tubes and causing this effect? do you have tube rings on?
just a thought.