power cables + unshielded phones cables = hiss? nope. not really. you should see the tangle of cables next to my comp. I am afraid to even start organizing it, but nevertheless the hiss is not present.
now cell phones + unshielded audiocards/amps = OMG pain.
now without terrible abstract math, to add to audiomagnate.
Number of dedicated headphone amplifiers and sources produce hiss nevertheless due to their imperfect AC(alternating current) to DC(Direct Current) conversion. This problem usually does not occur with battery-run sources (which nevertheless can have their own problems in that field), but it the AC 60hz artifacts can show themselves up in unexpected places. The headphone amplifier in the Stax mkii combo is designed to operate with batteries but comes with a power supply too, and on batteries it is gorgeous. On outlet power number of its users have found problems with ac60hz artifacts, which usually take form of mild hum or hiss, which may sometimes increase volume depending on the specific frequencies played back. (If you look at this from physical approach, when the capacitors used in the amplifier hit a particular stance (inverse of capacitance multiplied by frequency is essentially the current multiplier) in sync with the source's frequency, the current amperage goes up quite a bit, depends on the situation (capacitance mostly, hum is at 60 hz), but something like 3-4x is easily expected for that soft spot, which is about 4-5 dB.
This cute effect combined with audiomagnate's explanation is more or less the result of most of the hiss and hum we have to deal with. Keep in mind, most of the components mentioned by folks on this site WILL NOT this problem. We don't like hiss
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