I've used an adaptation of the James-Baxandall tone control network, which has a bass/treble control. Interestingly, I've managed to somehow cut-down the number of components used by experimenting in pSpice. The left and right set of two resistors are actually variable resistors of 10ohm. The output is after the 22ohm resistor, or between the right two resistors.
The green line is something close to what I'm using to filter the input signal, but note that the output still goes through filters before reaching the drivers. The red line is just another example of possible tuning, but the treble is out of control there. Top circuit is green, bottom is red.
I was testing the earphones against shure 846s and JH customs (JH5 and JH11) at Jaben. With this control, I can adjust the response and get something fairly close to a Shure 846 signature, but at the expense of volume. On my sansa clip, I can drive the shures at -40dB, while my earphones need around -10dB to get the same volume.
The trick now is to shrink this plus the filters into a circuit small enough to fit into my earphones. At the current count, I have 14 different resistors/capacitors per ear.
Incidentally, the Shure doesn't have very nice highs, but really impactful bass. The JHs doesn't have the same kick to the subbass, but more forward mids.
Edited by vector1 - 7/5/14 at 5:22am