Originally Posted by Cecala
Jsplice, did you end up inserting either felt or rubber between case and transformer and if yes, any improvements?
I did put rubber foam weather stripping between the transformer and the side of the case, and it did nothing. I've been researching this issue all over the Internet, and people are saying that DC in the AC power lines can be a cause of transformer hum. I'm not positive as to whether or not the humming transformer is the cause of a loud buzz through my speakers when using the dac in preamp mode, but I do suspect that to be the cause. There are several commercial DC filtering products, such as the PS Audio Humbuster, UPC 200HB (I actually own the normal PS Audio UPC 200, which has no humbusting capabilities), and Channel Islands Audio also makes a DC filtering product. There are a few DIY DC filtering projects I've seen online as well.
I emailed the seller on ebay about this issue, and doesn't seem to think it's a transformer issue, but rather it's an issue with impedance matching between the DAC and my power amps. I don't believe that to be the case, and if it were the case, then this is a serious design flaw. The NAD C272 power amps are a widely used commercial product, and I'm sure their input impedance is well within the acceptable level.