In my apartment building, we have an intercom system with one central unit and multiple terminals, one per apartment. Unfortunately, after some unknown tweaks done a couple of years ago to the central unit, the volume of the ringer (when someone is calling from the central unit on the ground floor, to request access) is not even a quarter of what it used to be. We were told that that's how it is, nothing can be done; we can't touch the central unit (administrative reasons). All that's left is tweaking the terminals themselves.
The terminal's diagram is as follows:
Signal comes from COMP, at 12V, be it the ringer or the voice transmission. The Volum (volume) is set on HI, so there are no parts between COMP and SP (speaker), the sound is heard as it comes.
Most of it is a nice circuit board, that I don't want to touch. The only part that I'm willing to try my hand at is between Dif1 & Dif2 and the SP. That's where I want to add a nice simple 12V pre-amp circuit. Problem is I never actually done one. In my mind, it's something as simple as:
INPUT would be Dif1 and Dif2 from the first diagram (intercom terminal), OUTPUT would be the wires leading to the speaker (SP).
One config. I found for this diagram, at 12V, is:
- R1 = 2.2 MΩ
- R2 = 4.7 KΩ
- C1, C2 = 10 µF, 10V
- T1 = BC148B
- 12V, in my case, will come from a 12V battery.
- How are the values for the capacitors and resistors calculated ?
- Is it possible to make it even simpler ?
- If I want to add a potentiometer, where would it be located ?
Thank you for taking your time to read this :)