A couple of years ago, I bought a set of the now-famous Grado SR-60 headphones. I use them when I travel or when I don’t want to bother anybody with music at home. The headphone amp circuit is basically a simple non-inverting op-amp gain stage with external buffering.
http://headwize.com/projects/showfile.php?file=stokes_prj.htm
The headphone amp is a 2-stage design with Analog Devices opamps. The are two completely separate ±9V battery supplies, one for the input buffers and voltage amplifiers and the other for the output stage only.
http://headwize.com/projects/showfile.php?file=kemhagen_prj.htm
The entire amplifier has a differential topology from input to output to get a balanced input and for lower noise, less ground loop problems. The first stage is a differential amplifier with feedback directly from the output stage.
http://headwize.com/projects/showfile.php?file=gilmore4_prj.htm
The op-amps should be chosen with care. They have to be able to deliver relatively high current values and to drive low impedance loads. I decided to use the National Semiconductor LM6171. This is a wide band (100 MHz) voltage-feedback opamp that is able to deliver 10V into a 100 ohm load.
http://www.headwize.com/projects/showfile.php?file=meier_prj.htm
In the process of broadcast matrix encoding and decoding, balancing amplitudes between the two channels of a stereo audio path is very necessary. It is necessary for the maintenance of proper headroom, dynamic range and, of course, separation.
http://www.benchmarkmedia.com/appnotes-a/lrtestst.html
It has a grounded cathode input stage with an idling of about 3mA. The output stage is a push-pull White follower, which provides low output impedance without the need for global feedback. It idles at about 10mA and can swing ±20mA in push-pull.
http://headwize.com/projects/showfile.php?file=cmoy5_prj.htm
It is an op-amp type of circuit which is built with discrete components (BC559 and BC560 transistors, with a BD139/BD140 output pair). It runs in class A and has no output coupling capacitor.
http://headwize.com/projects/showfile.php?file=waarde1_prj.htm
The circuit topology is a SRPP (Series Regulated Push-Pull) input stage which is AC-coupled to a Parallel-Triode Cathode Bias output stage.
http://headwize.com/projects/showfile.php?file=frazer_prj.htm
I recently purchased the Sennheiser HD600 headphones second-hand and realised that their full potential could only be met with the highest quality amplifier. Hence this project!
http://headwize.com/projects/showfile.php?file=busbridge_prj.htm
The design is a two-stage, but with an octal tube - the more linear indirect-heating 6SN7. These tubes and the cathode follower configuration of second stage give very low distortion, low output impedance and an incredible low frequency response.
http://headwize.com/projects/showfile.php?file=ciuff3_prj.htm