Connecting

Wired

Jack

Normally a headphone is connected with a jack.
As a jack is tip, ring, sleeve you have three wires, the signal for the left and right channel and a common ground for both drivers.

 

The ¼ inch (6.35mm) TRS jack is the standard.

As it is about as thick as a mobile, for portable gear a 3.5 mm connector is used.

 

Symmetrical

An alternative is to connect the headphone with 4 wires, a separate + and – for each driver.
This is exactly how we connect our speakers  to an amp but for some reason it is often called a “balanced” connection in the headphone world.

A true balanced connection is using 2 signal wires, one carrying the signal and the other the inverse and a third for the earth.

 

To do so, you need to rewire the headphone and need a headphone amp with 4 pin output.

 

There is no standard for connecting headphones “balanced”
A common solution  is a 4 pin XLR.

 

A mini TRRS jack is another option

 

 

 

Headroom BUDA balanced headphone amp

 

Sennheiser’s HDVD 800 DAC/Headphone amp

The HDVD 800 has symmetrical outputs for the headphone.

USB-C

More and more manufacturers of mobiles starts dropping het 3.5 headphone jack.
Indeed, without this jack it is easier to make the mobile waterproof.
Another trend is replacing the micro USB connector with a USB-C connector.

Although USB audio is not mature on Android, it is possible to connect a DAC to the USB C.

 

Moshi 24 bit / 192 kHz USB DAC - € 25.00

 

 

This is what one expects, a digital bus hence a DAC to convert the digital to analog.

 

Google USB-C Digital to 3.5 mm headphone adapter - $9

 

A fully analog adapter.
The 24 pins USB-C allows for analog audio!

The device must set the bus in Alternate mode to obtain this.

Obvious, USB-C is the most analog sounding digital bus!

Wireless

A wireless headphone allows you to move freely.
A wireless headphone is an all in one solution.
Sound quality depends not on the drivers only but on the inbuilt DAC and amp as well.

IR

Infrared is light hence it almost free of interference (WIFi etc)
The downside is that sender and receiver must literally see each other.
Infrared is in general limited to 10 m.

Sennheiser Set 900 infrared headphone

RF

Wireless headphones often use a RF (Radio Frequency) transmitter.
The advantage is its long range (up to 100 m) and sender and receiver don’t have to be in the same room like in case of IR.
RF transmitters often use the 2.4 MHz  band.
This might cause interference with WiFi.

Sennheiser Flex 5000 $199

Base/docking station with optical and analog input.

Receiver with headphone out turns any wired headphone in a "wireless" one.

Added 2017

 

Bluetooth

Bluetooth is a very popular protocol to connect a headphone wireless because its power consumption is low.
This is crucial as beside the Bluetooth receiver the battery also powers the built-in DAC and amp.

Standard Bluetooth audio is not lossless.
If both sender and receiver support it, it is possible to use non-mandatory codecs like APTX.

More detail information about Bluetooth and its audio codecs can be found here.

 

Novero Rockaway bluetooth headphone

 

Although wireless, almost all wireless headphones use wires to connect the left and right part.

Very few have a solution where left and right are connected wireless.

 

Onkyo W800 BT
$299.99

WiFi

As WiFi is a very popular protocol and is not handicapped by limited bandwidth and range like Bluetooth.

It looks like the ideal protocol to connect a headphone wireless.
Unfortunately, power consumption is high compared with Bluetooth resulting in a limited battery live.
The Koss Striva is the only WiFi enabled headphone known to me.

References
  1. Balanced Drive vs. Unbalanced Drive - Headroom
  2. Balanced audio - Wikipedia
  3. Headphone Connectors & Pins - DIY audio blog