Your preamp opamp will not work because its + input has no DC reference voltage. With a dual-polarity supply the DC reference voltage should be 0V. For a single positive supply the DC reference voltage is half the supply voltage.
A resistor from de + input to ground then, but I do not know how to calculate its value.
The gain control needs to be adjusted so that loud sounds do not cause the preamp opamp to produce clipping.
Yes, loud sounds cause hearing damage. Maybe two diodes back-to-back can clamp the level.
Could you show exactly what to try and where, AG?
ANY opamp circuit can have simple changes to its biasing so it can use only a positive supply.
It would be good to use just +9V but the filter, as is, it requires +/- 9V.
I am not sure how to polarize properly its four op amps. Maybe two 100K resistors in series, between +9 and ground and their junction to the + input?
If so, may I use just
one pair of 100K resistors as above and polarize
all the op amps in my circuit?
The calculation for a capacitor value to pass a certain low frequency into a certain resistance is simple.
What is the lowest frequency?
I am not sure, maybe 30 Hz? I have no experience in audio circuits, AG.
It looks like you have the earphones connected in series and are probably out-of-phase instead of connected in parallel and in phase. Out-of-phase sounds weird.
Yes, they will go in parallel. In my case, no matter what, I have just one ear in sound condition (pun intended). The left one was damaged by an infection in a voyage from Canada to Italy. In less than 48 hours I started to perceive just noise. When it is strong, it means PAIN to me. Anyway, brother goes OK.
Gracias for your help AG!