My apartment has a simple door intercom system which uses a hybrid two-wires line to carry both the receiving and transmitting audio signals for each floor unit. The wires are connected to each other in parallel. Wires other than Audio are the Power (Vcc), Ground, and a Video line which isn't relevant to the question.


simulate this circuit – Schematic created using CircuitLab


My goal is to build a circuit which could feed a normal audio line (not a microphone) to the system, and extract another normal audio line out from it. The problem is that the idle voltage of the intercom audio wire seems to be an odd 14.6V which I can't interface. (I tested and it isn't the intercom power.)

I don't know how to get a normal audio signal out (which could be connected to a recorder or a sound card) and how to feed the line with audio from an mp3 player, or any regular AUX jack.

I need a simple, modern circuit to incorporate into my modern project, with these requirements in mind. Also, feedback/echo elimimation is preferable, but not a must.

From my understanding, a normal audio output has a typical maximum peak-to-peak voltage of 3 volts.

I googled quite a lot of keywords, but none seemed appropriate. Please provide an understandable explanation on how would I go with designing such circuit.

  • \$\begingroup\$ This isn't a design service. Asking how to do something is cool, and somebody will probably post a a simplified circuit to explain how it could be done. Asking for a complete design is NOT cool, and may well get your question put on hold. \$\endgroup\$
    – JRE
    Commented Dec 7, 2017 at 12:25
  • \$\begingroup\$ I'm actually looking for something simple & generic which I hope will also be beneficial for anyone looking it up on Google. \$\endgroup\$ Commented Dec 7, 2017 at 12:33
  • 1
    \$\begingroup\$ A diagram would say a thousand words. Both of the connections and the signal levels. \$\endgroup\$ Commented Dec 7, 2017 at 12:46
  • \$\begingroup\$ The two wires also carry power, which is why you see the 14.6V - it should just be a matter of using capacitors to block the DC. I would say the word "normal" wouldn't really apply to audio levels in this system. \$\endgroup\$
    – Finbarr
    Commented Dec 7, 2017 at 12:52
  • 1
    \$\begingroup\$ Ok, while not technically carrying power, it still has power on it. However, it does mean that when the opener button is pressed your audio circuit could see a 14V negative pulse, so you need to make sure that doesn't cause it a problem. \$\endgroup\$
    – Finbarr
    Commented Dec 7, 2017 at 13:54

2 Answers 2


Simply driving the line takes an amplifier and a coupling capacitor, and simply monitoring the line takes a coupling capacitor and an amplifier. However, the received signal will include the transmit signal. Keeping the tx signal out of the rx signal is called sidetone cancellation, one of the main functions of a hybrid. The transformer-based telephone hybrid is over 100 years old, but solid-state versions are much easier to deal with.

Here are two concept schematics from National Semiconductor. The first is the bare basics: a driver, a receiver, and a subtractor. The second has a few more parts to compensate the signals for the fact that the impedance of a phone line is not a pure resistance. It has inductance and capacitance that the line drivers does not. Compensating for them makes the sidetone cancellation more complete, especially at higher frequencies.

In your case rather than a direct connection or an interface transformer, one coupling capacitor should be enough - IF - it is ok for your interface circuit to share the intercom line ground. Note that knowing the impedance of the intercom line is an important starting point, but you can start without it and adjust later.




Because your intercom system is a bus-type, there is a pretty good chance that it functions similarly to most professional wired intercom systems where the line impedance is a fixed value and each station uses a current source to get the audio onto the bus.

RTS Intercom systems use something called a "bilateral current source" which, I believe, originated from a National Semiconductor app note several decades ago. RTS obtained a patent for using a bilateral current source for closed-circuit intercom systems. Google has the patent available for viewing and figure 2 is pretty close to what you need.

Bilateral Current Source patent

Notice that this circuit both drives the bus as well as listens to the audio on the bus. Pot 42 in figure 2 is the null control that minimized your own talk audio on your own receive line. The 20k & 60k resistors have to be closely matched - I normally match to 0.25% or so. The gain of the 2nd op-amp stage and the 100R resistor at the output of the 2nd op-amp sets the transfer gain of input voltage to output current.

RTS sets the bus impedance at 200R. The Telex balanced-audio version of this type of intercom system sets the bus impedance at 600R. Clearcom bus impedance has changed many times over the years but 220R was a common value in their early days.

If the 4 resistors around the 2nd op-amp are closely matched, the effective impedance of the bilateral current source is several tens of kilohms. This presents a tiny impedance change when the station is attached to the 200R bus.

This is a simple circuit and will be easy for you to try with your intercom system.

  • \$\begingroup\$ IVC used a similar system in their studio cameras. The current source circuit is a Howland Current Pump. A small resistor network gets you four tightly-matched resistors that temperature and ratio track for much better performance. 50 stations on one bus was no problem. \$\endgroup\$
    – AnalogKid
    Commented Dec 7, 2017 at 20:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.