Important information for MOH and telephone system installers

Important information for MOH and telephone system installers

The audio output port of the CUBE is electrically similar to that of a CD player, iPod, PC, MP3 player, radio, satellite receiver, etc. Such audio ports cannot be connected directly to so-called “wet loop” telecommunications ports, as damage may result to either piece of equipment.

1.  If you’re connecting to a 3.5mm jack on the phone system, then you’re probably ok.

2.  If you’re connecting to a phone port on the phone system, then you may not be ok.  - If you know that the wire pair connects to a “handset port” (ie the same type of port to which you would connect a phone handset), ie “FXS” port, then you’ll need the isolator.- if you don’t know what the wire pair connects to, use a multimeter and measure the wire pair to see if there is a DC voltage before connecting.  if you measure a voltage,  you need the isolator.

3. In our experience, we see more chips blown on Ayava systems than anything else — these almost always seem to be FXS ports
When the installer does not know what they are plugging into, the installer should be using a voltmeter/multimeter to check.  If they can’t for whatever reason, and they really cannot find out what kind of phone port they’re connecting to, then the simplest solution is to always include the ground loop isolator in the install.

A “wet loop” is a connection directly to a telecommunications port that carries DC voltage (e.g. the FXS connection from a PBX or switch to an analogue extension), or which may carry ring-signal voltage.

The CUBE should preferably be connected to a designated MOH input on the PBX / switch. To connect the CUBE to a wet loop, a proper barrier device (isolating transformer) must be used.

  • On certain premises, installers may be unable to determine whether the provided pair is connected to the switch’s MOH input, or to a wet loop. We recommend in such cases that installers utilize a DC voltmeter to measure across the provided pair. If a DC voltage is measured, the provided pair is a wet loop, and an appropriate barrier device must be used. An example of one such barrier device would be a ground loop isolation transformer such as the Boss B25N, available multiple places, including

  • Some switches (e.g. Avaya) do not offer any MOH inputs, but require that MOH sources be wired up to a telecommunication line card instead. In such cases an appropriate barrier device must be used. See the switch’s guide (e.g. Avaya) for more information.

  • Avaya documentation regarding the MOH connection can easily be misinterpreted. The Avaya guide contains two sections referring to connecting “FCC approved” vs “non-FCC approved” MOH source equipment. Since the CUBE carries FCC approvals, installers may incorrectly pick the first option. The Avaya reference here, however, pertains to FCC approval as a telecommunications (telephone) port (which is highly uncommon on MOH equipment), therefore the “non-FCC approved” section applies.

• If in doubt: 

  1. Follow the PBX / switch manufacturer’s instructions for connecting standard CD players, radios etc.
  2. Use a voltmeter to measure whether or not there is voltage between the pair provided before connecting your audio player!


Only approved telecommunications (telephone) ports may be connected to wet loops. To our knowledge, no MOH device on the market (barring those with actual phone jack outputs), have outputs approved as telecommunications ports. Connecting such MOH outputs directly to a wet loop risks damage to either piece of equipment, as well as deterioration of the audio quality. 

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