When Goodvibes at Head-Fi.org, sent me a repaired Grado GR10, he also set me up with his modified CMOY amplifier from eBay. Couple of things pointed out by NwAvGuy have been carried out by Goodvibes, and the result is quite promising.
Measured @ 0.5V rms
First, the gain has been increased from 1 to a factor of 3 by changing R1 to 47k, and R2 to 22k, which is about 10 dB boost. 20 x log( 1 + [ 47k / 22k ] ) = 9.9 dB
Third, R3 has been reduced, in order to tame the DC offset even further to 0.1mV @ the unity gain. And the resulting -3dB corner frequency at the input is ~ 3 Hz. 1 / (2 x π x 22k x 2.2uF) = 3.3 Hz
Output impedance characteristic
It is a nice & simple portable amplifier, suitable for the purpose, with no colouration and good details. I don't think you can ask for more at this price. The revise plan set by NwAvGuy, executed by Goodvibes, has been briefly analyzed by udauda.