There is a humming in the door intercom, which is connected to the TSA-a/b. What could be the cause?

If the door intercom should work after installation, but still have an unpleasant humming noise, the following points can be considered:

  1. The microphone of the door intercom ages, loses amplification and becomes sensitive to hum. Usually this can be recognized by the fact that the signal from the door in the called telephone is too quiet. And if you put your finger on the microphone from the front, it will hum louder. The only thing that helps here is to replace the microphone capsule with a new one. Due to the existing environmental influences (heat) this damage pattern is quite common.
  2. The TSA-a/b is powered by a bell transformer. An audible ripple current is produced when one of the bell transformer's terminals is directly or indirectly connected to the 0V, U+, MIC, LSP or NF terminals. Make sure that there is no connection here. If necessary, you will need a second bell transformer.
  3. A ripple current can be coupled into the connection cable between TSA-a/b and doorbell station by electrical fields. If the hum changes by touching the cable, this is probably the problem. Use a shielded cable. The shielded cable is connected to earth/protective earth or to 0V (one end is sufficient).
  4. The bell transformer can inject a capacitive ripple current due to its internal structure. Here the connection of the 0V connection to earth / protective earth helps.