Thanks for your reply and sorry for my late answer.
I spent my night for bulding the system.
First, I have some earphones, those who you use as a speaker capsule in your ear.
I found them in a local shop, and the product label did not contain anything.
So, I googled and found that these earphones has an (real) impedence of about 150 Ohm.
So, putting 8 earphone in parallel gives about 18 Ohm, still enough for the power aplifier.
The power amplifier is based on the Philips TDA7052, and it is 1W aplifier for coil-based speaker (8 Ohm). So, ther could not be any problem for a tenth of earphones.
In fact, I tried and the sound is good at every earphone, and the IC does not heat too much.
The preamp was broken and it was based on 2 transistor (emitter follower or/and base-follower scheme). So, I built anew one based on OP AMP with band-pass filtering.
I also used shielded cable and now the sound is very good and strong.
The next problem is to add another "box". In fact, this box is for 8 people only.
I would extend the system for 16 and so on people.
My idea is to put an OPAMP -based voltage-foller at every input of every box, such that
the original output of the pre-amp is carried to every box.
I think it is a good idea, because the OP-AMP will drive the cable for interconnect the boxes and also will provide an high input impadence such that subsequent boxes are not
jeopardized. But I still did not implement the idea.
Any suggestion about it?
P.S. When I was at high school I knew all these stuff. Now I forgot almost everything.
For example, the use of shielded cable is a must in audio applications, short pins, good connection on the PCB, and yes the final insertion in the plastic box are humble but not less important things!