The impedance of headphones is of concern because of the output limitations of amplifiers. A modern pair of headphones is driven by an amplifier, with lower impedance headphones presenting a larger load. Amplifiers are not ideal; they also have some output impedance that limits the amount of power they can provide. To ensure an even frequency response, adequate damping factor, and undistorted sound, an amplifier should have an output impedance less than 1/8 that of the headphones it is driving (and ideally, as low as possible). If output impedance is large compared to the impedance of the headphones, significantly higher distortion is present.[11] Therefore, lower impedance headphones tend to be louder and more efficient, but also demand a more capable amplifier. Higher impedance headphones are more tolerant of amplifier limitations, but produce less volume for a given output level.
Ok, it’s been a month and a lot of new experience. I’ve auditoned ATH M-50 (clamp’s too tight; they sweat my ears just from few minutes of exposure) and Senn HD 600 (the velour pad is amazingly comfortable & size is ok). So, long story short, now I can refine my search to: circumaural&non-pleather/foam pads, which brought me to Senn HD 439 (cloth) & 518 (velour).
Garbage in = Garbage out (GIGO) is a popular phrase used to emphasize the importance of a good source. This can be the soundcard in your laptop, the quality of your portable audio player, or the CD player you’re using for music listening. Those fall into the “Source” category. The better your source is, the better the sound will be at the headphone end. This is why we are seeing more and more audiophile digital audio players (audiophile DAPs). They are expensive but they sound good.
Once you’ve got that all order, to put a cherry on top it would be ideal for the amp to play nice with my laptops (PC at work, Mac at home) *and* my *iPhone* 4S.  I haven’t found a headunit that works with PCs and iDevices.  If one doesn’t exist it would be a big plus for the amp to include inputs so I can get digital sound out of my iPhone (with the Pure i20 or its ilk) and into my headphones.
In early powered radios, the headphone was part of the vacuum tube's plate circuit and carried dangerous voltages. It was normally connected directly to the positive high voltage battery terminal, and the other battery terminal was securely grounded. The use of bare electrical connections meant that users could be shocked if they touched the bare headphone connections while adjusting an uncomfortable headset.
We run every pair through a rigorous testing process over several days or weeks. That includes playing them in all sorts of scenarios — be it on a bus, in the listening room, or at the office — and playing back from a wide array of sources. We know most people use their headphones with a smartphone, often with lower-quality MP3 resolution tracks, so we do, too.
Wireless and truly wireless: These connect to your devices using Bluetooth, so you’re never physically tethered to your smartphone, tablet, or computer. Wireless headphones don’t use a wire to connect to an audio source, but they do use a wire to connect the two earpieces together. In contrast, “truly” wireless headphones come as two separate earpieces that don’t need wires to connect to anything. Wireless headphones are incredibly affordable; truly wireless headphones can cost anywhere from $100 to $400.
This is better than I thought it would be. Worst case is the T51p won’t have the excitement of the more “V”-shaped headphone sounds, but you’ll hear more of what’s actually in the recording. The more neutral headphones are most often a little bright (that’s what most users say), and can irritate on electronic and improvised music, but the T51p didn’t show any of that with these 3 tracks.
Headphones are made in a range of different audio reproduction quality capabilities. Headsets designed for telephone use typically cannot reproduce sound with the high fidelity of expensive units designed for music listening by audiophiles. Headphones that use cables typically have either a 1/4 inch (6.35mm) or 1/8 inch (3.5mm) phone jack for plugging the headphones into the audio source. Some stereo earbuds are wireless, using Bluetooth connectivity to transmit the audio signal by radio waves from source devices like cellphones and digital players.[4] Due to the spread of wireless devices in recent years headphones are increasingly used by people in public places such as sidewalks, grocery stores, and public transit. Headphones are also used by people in various professional contexts, such as audio engineers mixing sound for live concerts or sound recordings and DJs, who use headphones to cue up the next song without the audience hearing, aircraft pilots and call center employees. The latter two types of employees use headphones with an integrated microphone.

The impedance of headphones is of concern because of the output limitations of amplifiers. A modern pair of headphones is driven by an amplifier, with lower impedance headphones presenting a larger load. Amplifiers are not ideal; they also have some output impedance that limits the amount of power they can provide. To ensure an even frequency response, adequate damping factor, and undistorted sound, an amplifier should have an output impedance less than 1/8 that of the headphones it is driving (and ideally, as low as possible). If output impedance is large compared to the impedance of the headphones, significantly higher distortion is present.[11] Therefore, lower impedance headphones tend to be louder and more efficient, but also demand a more capable amplifier. Higher impedance headphones are more tolerant of amplifier limitations, but produce less volume for a given output level.
Of the tested codecs we met, aptX and aptX HD fared the best out of all our candidates. While that may seem strange to say, on the whole their results were right where they needed to be in order to stand in for a wire for commuters, and listeners over 40. You’d really only run into issues at high volumes (90+dB), so while aptX isn’t quite able to keep up with CD quality, aptX HD is able to get extremely close to the mark with a little processing creativeness. Both codecs fall short in the highest frequencies a human could potentially hear, but the vast majority of people can’t hear sounds over 18kHz anyway.
Telephone headsets connect to a fixed-line telephone system. A telephone headset functions by replacing the handset of a telephone. Headsets for standard corded telephones are fitted with a standard 4P4C commonly called an RJ-9 connector. Headsets are also available with 2.5 mm jack sockets for many DECT phones and other applications. Cordless bluetooth headsets are available, and often used with mobile telephones. Headsets are widely used for telephone-intensive jobs, in particular by call centre workers. They are also used by anyone wishing to hold telephone conversations with both hands free.
In early powered radios, the headphone was part of the vacuum tube's plate circuit and carried dangerous voltages. It was normally connected directly to the positive high voltage battery terminal, and the other battery terminal was securely grounded. The use of bare electrical connections meant that users could be shocked if they touched the bare headphone connections while adjusting an uncomfortable headset.
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