RTS PS-4001 4-Channel Power Supply for RTS ES-4000AThe PS 4001 power supply supplies four (4) isolated channels of intercom system phantom power to down line components. Use the PS 4001 as a stand alone power supply to supply four (4) independent partyline channels. Or, use it in conjunction with an ES 4000A expansion station to create additional intercom channels when using a US 2002 PS 2001L or US 2002 SPS 2001 Master Station configuration. The PS 4001 can operate in either balanced or unbalanced