construction and use of a FILVORA (Fixed Insertion Loss Variable Output Resistance Attenuator) and DFILVORA (Dual Fixed Insertion Loss Variable Output Resistance Attenuator) for measuring the impedance and sensitivity of headphones, including modern stereo headphones, magnetic headsets and piezo-electric ceramic headsets.
http://headwize.com/projects/showfile.php?file=tongue1_prj.htm
Koss' The Plugs are inexpensive canal-type headphones that suffer from a "peaky" frequency response, weak acoustic isolation and a lightweight sound due to inadequate acoustic damping and poor ear cushion design.
http://headwize.com/projects/showfile.php?file=cmoy4_prj.htm
The mods are grouped into 3 categories: Transducer, Earcups and Headband, Earpads, Cables. There are even instructions for how to fix the infamous Grado "grattle." The article specifically mentions the Grado SR-60, SR-325 or the Alessandro-Grado MS-1
http://headwize.com/projects/showfile.php?file=grado_prj.htm
The magnets in old-style high impedance headphones can gradually lose their potency resulting in diminished output volume. The authors describe two methods for restoring de-magnetized headphones.
http://headwize.com/projects/showfile.php?file=provost_prj.htm
This passive EQ corrects a rising characteristic in the frequency response of the beyerdynamic DT990 Pros and turns them into one of the most natural and accurate sounding headphones on the market.
http://headwize.com/projects/showfile.php?file=beyer_prj.htm
Make surround headphones from a pair of circumaural headphones and earbuds. The author uses these headphones with the SB Live PC soundcard, but they will also work with any 4-channel surround recording.
http://headwize.com/projects/showfile.php?file=connors_prj.htm
This state-of-the-art portable headphone amplifier is based on the AD823 and AD812 opamps and employs several techniques to minimize opamp input errors such as balancing input impedances and composite amp output stages.
http://headwize.com/projects/showfile.php?file=kemhagen_prj.htm
Tips for troubleshooting and repairing electrostatic headphones.
http://headwize.com/projects/showfile.php?file=rehorst1_prj.htm