http://www.6moons.com/audioreviews/burson5/3.html
House keeping items - noise: First attempts at running the HA160D as DAC or preamp in my big rig caused a fat hum as soon as the Burson threaded into the signal path. This couldn't be cured with a floating ground*. The hum occurred already
without a power cord plugged into the Burson. Whilst my ModWright KWA 100SE power amp has a ground-lift feature, this merely affected the hum's amplitude. It did not eliminate it. The original HA160 had suffered a ground loop issue but with certain sources only. Burson successfully addressed this with a subsequent circuit alteration when other users reported the same hum. Then it had only affected headphones of course. Now headphones were in the clear over all sources, be those iMac, iPod via Sieben Technology dock, Onkyo digital-direct dock or the digital and analog outputs of my Ancient Audio Lektor Prime. This noise was a 'downstream' rather than 'upstream' affair and occurred
only when the Burson's pre-outs were used.
Wyred4Sound's ST500 amplifier was as noisy as the ModWright KWA-100SE. The low-power low-gain FirstWatt J2 reduced the noise to what most would consider normal for a large number of expensive valve amps. But preamps ought to be inaudible when no signal passes. My three in-house preampsEsoteric C-03, ModWright DM 36.5, Bent Audio Tap Xall are. I finally lucked out. Even with the Burson's volume control fully open, my 25-watt Trafomatic Audio Kaivalya tube monos betrayed
zero noise right at the tweeters. What caused the noise on three different amps but not the fourth I didn't know at this juncture. Burson reported back that they only ever experienced hum (with sources) when their interconnects were unshielded. I indeed run unshielded 5-meter links between preamp and amps. In two years I'd simply never had an issue with
any preamp/amp combination. Hence I don't yet own any trouble-shooting shielded 5-meter equivalents.
"The preamp output of the 160D has higher impedance than some other designs. If the power amp also happens to have a higher input impedance, the combo will pick up interference in the air through an unshielded connection much like a TV antenna - especially a 5-meter long unshielded cable. Most branded interconnect cables should be shielded but if the customer is unwilling to part with his favorite pair of unshielded cables we can provide a free modification to lower the output impedance of the 160D here in Australia or at any of our authorized retailers like 32ohm Audio in Portland." To remove my suspicion that I suffered a ground loop instead, team Burson requested that I let them submit a 5-meter run of shielded interconnect. I accepted. Their cable proved that indeed I had
not suffered a ground loop. My hum had arisen from long unshielded cables. Once shielded cables replaced them, the hum was gone.