Their is a lot of discussion about changing the er4p to er4s by using an adapter cable or FixUps replacement cable.
Is it also possible to change the Er4p to Er4B (the binaural version)? I actually has a lot of binaural recordings so it would be interesting to be able to switch, but very expensive to have two pair of Etys!
Say you've got the 4P and you're using the 75 ohm resistor mod to emulate the 4S. Here's how you could emulate the 4B as well:-
Instead of a single 75 ohm resistor, use a 22 ohm resistor in series with a 56 ohm resistor (total 78 ohms). Then add a 680nF capacitor across (ie: in parallel with) the 56 ohm resistor. This will boost the top end by about 5dB with a turning point at about 4kHz, bringing you pretty close to the equalisation of the 4B:-
You probably want a switch in series with the capacitor so you can A/B test the modification. A dual-pole of course, one for each channel. In fact if you are really cooking you can use a special double throw switch with three positions, one of which is centre-off, i.e. no connection. By connecting the centre pin to the join between the 56 and 22 ohm resistors, one side to the capacitor and the other side to the other end of the 56 ohm resistor you get three headphones in one:-
Position A: capacitor in circuit, "4B" equalisation
Position B (centre-off): "4S"
Position C: 56 ohm resistor shorted out. This is the 50 ohm "4X" Ety, a compromise between the 4S and 4P.