I just received my new Magni, and I’ve run into what I think may be some sort of grounding issue. My super old house doesn’t have grounded outlets, and I’m experiencing odd conductance-related issues with the Magni. I’ve never used a dedicated headphone amp before, so I wanted to know whether the problems I describe are A) a function of the Magni’s design (i.e., perhaps I could get an Asgard and solve the issue), B) a generic problem with all mains-powered headphone amplifiers, or C) problems with the specific Magni I received (i.e., I should send it back for a replacement).
I performed a variety of tests, putting the Magni on a circuit with or without other electronics, on both Sennheiser HD 280 Pro (64 Ohm) and Sennheiser PX 100 II (32 Ohm) headphones. Since both headphones use a 3.5mm jack, I used a 6.3mm adapter (several different adapters, actually, to make sure none of them was the issue):
Solo circuit test, no source (Magni is the only thing on the circuit other than lights, no audio cables plugged in, just power and headphones):
Both the HD 280s and the PX 100 IIs performed essentially the same in this test, so I’m not listing the results separately:
• When I turn the volume pot between 9:00 and 10:00, there is a scratchy sound in the left channel. The sound isn’t persistent, just something that occurs when you’re moving the pot.
• 10-11: Scratchy sound in right channel, again only when moving pot.
• When I turn the volume above 2:00 or so on the pot, I hear mains hum. Touching the volume pot with exposed skin (or anything else conductive) amplifies the hum. Touching the chassis of the Magni amplifies the hum, but much less than touching the volume pot.
• Touching the exposed section of the 3.5mm to 6.3mm adapter at the same time you’re touching the volume pot (completing the circuit, presumably) kills the hum. Oddly, touching the adapter at the same time you’re touching the chassis only dampens the hum, doesn’t get rid of it completely.
Office circuit test, no source: (Plugging in the Magni on a circuit with computer, phone, printer, etc., still no audio cables/source.)
HD 280 Results, based on level of volume pot:
• 7-9: Silence.
• 9-10: There is a very quiet hum (not mains, different sounding) in the left channel that gets amplified whenever I touch the Magni (significantly when I touch the volume pot, less so when I touch the chassis.) This hum can’t be dampened or killed by completing a circuit between the headphone adapter and the pot or chassis). Still have scratchy sound in left channel when moving pot.
• 10-11: Even quieter hum in right channel, amplified in similar manner by touching pot or chassis. Scratchy sound in right channel still present.
• 11-5: Mains hum becomes noticeable at 11 now, instead of 2. Also, killing behavior is inverted from solo circuit test: Touching pot + adapter dampens the hum, while touching chassis + adapter kills it outright.
Performing the same test with the PX 100 IIs:
• 7-8: Hum in right channel, not amplified by touch.
• 8-9: Dead silence.
• 9-10: Quiet hum (amplified by touch) in right channel, opposite of HD 280s.
• 10-11: Silence.
• 11-5: Same mains hum behavior as with HD 280s.
Office circuit test, Xonar DX soundcard source (via 3.5mm stereo to RCA cable), but all sound muted on computer (essentially just looping the computer into the circuit):
• 7-11: Same results as no source test (hums, scratch, etc…).
o One exception: at right around 11, if I touch the volume pot, I can sometimes get a high pitched whistling sound.
• 11-3:30: Silence (no mains hum).
• 3:30-4: Scratchy sound in right channel.
• 4-5: Scratchy sound in left channel, and a hum similar to 9-10 hum in left channel, though seemingly not amplified by touch.
PX 100 II:
• 7-10: Same results as no source test (hums, scratch, etc…).
o At right about 10, I can get the same whistle I pick up at 11 o’clock on the HD 280s.
• 10-3:30: Silence.
• 3:30-5: Scratchy sound and hum in right channel, not touch amplified.
An office circuit test with music playing on the computer had the exact same results as the muted test, although the music somewhat masked the hum/scratch/whistle.