Building a power budget for a partyline intercom system.

As with any audio devices that operate over a wide variety of levels and usages the power requirements of an intercom system can vary greatly.

Clear-Com specifies partyline stations for idle and maximum power consumption to allow the user to appropriately specify adequate power supplies. Partyline stations draw power to activate their electronics which is the idle power consumption. The maximum specification is a worst case situation where the partyline station is both talking and listening at loud levels, is receiving call signals and generating a call signal. The largest jump in power consumption will be for generating a call signal as this supplies power for the call signal for all other stations, yet cannot be expected to exceed the specified maximum power consumption of the station.

The power budget of the system should be determined by adding together the power consumption for all stations on the system. As with any audio system power headroom should be specified for the system. A conservative power capacity provision would be using the maximum power specification of the intercom station and provisioning a minimum of equal, or greater, peak capacity from the systems power supply.


* Powering multi-channel partyline systems;
Multi-channel systems can be powered by Clear-Com's power supplies such as the PS-702 two channel and PS-704 four channel units. The MS-702, MS-704 and SB-704 mainstations are additionally capable of powering a multi-channel partyline system.

All Clear-Com multi-channel units capable of powering a partyline system share an identical internal power supply which is capable of a 1.2A average and a maximum of 2.0A peak current capacity.
The power capacity is shared between all channels of the unit and dynamically allocated as needed to the units channel.

It should be noted that a maximum of 1.8A peak can be drawn from any single channel, leaving 0.2A in reserve for other channels.

Clear-Com's Partyline Calculator
https://clearcom.com/DownloadCenter/technicaldocs/EncorePowerSupplyCalculator/EncorePowerSupplyCalculator.xlsx


* Adding multiple power supplies to a system;
While it is preferred to use a single power supply in a system, there may be instances a system may require multiple power supplies to power the system. This can be done in different ways depending on needs and budgets. Different options are covered below:

GOOD - Parallel connecting multiple power supplies
Clear-Com power supplies, such as the MS-702, MS-704, SB-704, PS-702 and PS-704 are designed with a special power softening circuit that allows users to parallel power supplies for more current capacity to a system. A perfect summation will not be achieved, yet a user can expect to get approximately 75% of the second power supplies capacity when bridged in this manner.

When paralleling power supplies the following should be noted;

* The power supplies must be powered from the same outlet for minimal ground differential.
* Adding a second power supply will not perfectly sum the capacity of the two units.
* The power supplies protection will not react as effectively as if the systems were power isolated.
* Care must be taken that the system has one point of termination per channel.

BETTER - Connecting audio only between multiple power supplies
Alternatively it is possible to only connect the channel audio between partyline power supplies. This can be done by connecting only pin 1 (ground) and pin 3 (audio) between the power supplies. The system now consists of two power-isolated sub-systems that only share audio during normal operation. Should the system fail due to reasons of cable shorts or faulty power supplies most likely only one sub-system will fail.

When connecting audio only between power supplies the following should be noted;

* The power supplies must be powered from the same outlet for minimal ground differential.
* Power does connect between the two sub-systems when call signals are used.
* Care must be taken that the system has one point of termination per channel.

BEST - Electrically isolated multiple power supplies
The multiple power sub-systems can additionally be fully isolated by using a specialized partyline isolation devices such as the Clear-Com MT-701. The MT-701 separately isolates call signals, audio and power between two systems making them fully independent and able to be powered from different grounds.

Electrically isolating partyline sub-systems is highly recommended.

When isolated isolating power supplies the following should be noted;

* Care must be taken that the system has one point of termination (MT-701) per channel.

It should be noted that the operation of Mainstation specific functions such as the Channel Link (can double up on termination when done from a main station not providing terminations) and Remote Mic Kill (shorts the line to unlatch talk keys) may vary depending on how power supplies are added to a system.