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:

Intercom terminal diagram

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:

Simple 12V pre-amp circuit

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.


  1. How are the values for the capacitors and resistors calculated ?
  2. Is it possible to make it even simpler ?
  3. If I want to add a potentiometer, where would it be located ?

Thank you for taking your time to read this :)


A preamp like you show has an output current that is much too low to drive a speaker. A speaker is usually driven by a Power Amplifier that is usually an IC.

But if you use a power amplifier IC then its output will always be connected to the speaker so then the speaker will no longer work as a microphone.

  • \$\begingroup\$ Yeah, you would need to put the amplifier on the other side of the talk/listen switch, which sounds like it's more invasive than the OP is willing to consider. \$\endgroup\$ – John D Jul 14 at 23:00
  • \$\begingroup\$ Yes, I see, I forgot about the microphone part. And yes, putting the amp between the talk/listen switch and the volume switch seems to be the only viable option, but I'm not willing to modify the circuit board to try this out. I'm thinking of attaching the power amplifier right before the speaker (which is a 8Ω, 0.5W, forgot to mention) and adding a switch on the talk/listen button itself, so the amp will be connected only when listening... something like that. If it works out, I'll add the final diagram here, for future reference. Thank you for your time :) \$\endgroup\$ – Adi Jul 15 at 8:04

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