Power Conditioners

APC S20 Power Conditioner with Battery Backup


Sequenced and Remote Power Control

Sequenced control of AC power to various components is desirable in that you want source components and preamplifiers to be stable before turning on power amplifiers. Otherwise if the amplifier is live as source components turn on, one may get annoying, and potentially damaging “pops” going to the speakers.
The APC S Type Power Conditioners have four output banks of AC outlets that are turned on and off sequentially, two of which are delayed on system turn on by an adjustable time of up to 10 seconds.

Sequenced on/off is fine for turning the WHOLE system on or off, say from the front panel on/off button. Unfortunately, that is not what is needed for normal daily use. In practice there will typically be one or more components that you will wish to always have powered on, e.g. a video recorder like my DISH DVR, so that programs can be recorded while the rest of the system is “off”. Other components are likely best left powered on, but in standby mode. You will want all these components protected by surge suppression, battery backup, etc.

So, in practice the best strategy is likely to leave the power connected to most components and to turn the system on and off using the typical on/off sequencing available with a well-programmed universal remote control. There may be some components, however where there is not a standby mode, or as in my case, where that standby mode uses excessive power. In such cases you will want to control AC power to those components by remotely controlling the individual banks on the power conditioner.

Remote control of the various outlet banks is one area where the S20 and the S15 differ. With the S20, each of the four banks can be switched on and off via RS-232 control. The S15 allows control of the banks via two 12V triggers. Thus the S15 will be easier to integrate into system that have 12V triggers available, but not RS-232 control.

Battery Backup and Voltage Regulation

Power supplies in AV components are designed to work with a certain range of input voltages. Lets say a power supply for use in the United States is designed to work at 117 Volts, and may work efficiently with a line voltage of, for example, 105 to 130 Volts. If the input voltage to the component drops below 105 V then the power supply will not work as designed and potentially have a much harder time keeping up with the demand for power.


The APC S20 senses the incoming line voltage and acts accordingly to assure that the output voltage remains close to the nominal 120V output voltage.  The S20 accomplishes this in two stages using a Line Interactive topography that combines battery backup with a tap-changing transformer.   The first stage of voltage regulation therefore is, accomplished by adjusting transformer taps as the input voltage varies. If the input voltage drops, a change in transformer taps boosts the output voltage.  The transformer taps have a spacing or granularity in their output voltage of about 8 volts.  Thus by changing transformer taps the output voltage can be maintained at 120V ± 4 volts, thus assuring the power supplies in the connected AV components always see a voltage well within their design specification.

In Line Interactive UPS systems, such as the S20, the battery-to-AC power converter (inverter) is always connected to the output and the inverter operates in reverse to provide battery charging during times when the input AC power is available. As the inverter is always on and connected to the output terminals, battery power is available to cover power failures, brown outs, and especially high power demands. The line interactive design also provides additional filtering and reduced switching transients compared to the standby UPS topology where the inverter is typically not powered or connected to the output during normal on-line operation.


The second stage used to maintain the output voltage involves using power from the backup battery. If the input line voltage falls below an adjustable point, 100V is the default setting, (or above 134V, also adjustable) the APC S20 switches to the “Battery” mode where the battery alone supplies the AC power via the inverter and a transfer switch isolate the system from the utility power until a proper input voltage is restored.

Protection against power failures is particularly valuable with front (and rear) projectors, such as the one I use in my home theater, as in use, the projection bulbs are very hot and their lifetime, and perhaps the lifetime of the projector itself depend on a cool-down cycle when the projector is turned off where the bulb is off, but the projectors cooling fans are running for a few minutes to remove the heat from the bulb. With projectors, power failures are not cool, and a battery backup system can allow the user time to shut off the projector after the end of the cool down process.

It is also common today to have hard disc-based components incorporated into an AV system, such as a media server or a satellite receiver and Digital Video Recorder. Such systems also can benefit from an orderly shutdown procedure.

Surge, Spike and Over-voltage Protection

There is no question that it is prudent to protect your AV equipment from voltage spikes and surges due to lightning storms, large appliances switching on and off, etc., and the APC S20 is specified to adsorb spikes of up to 4080 Joules. Surge protection is certainly a case where it is better to be safe than sorry. Having all your AV components plugged into a single well-protected source of reliable AC power is a good thing. Lightning happens.

Line Noise Filtering

As discussed above, chances are pretty good that if you are considering a power conditioner in the price range of the S20 you probably have pretty good components in you system already, and the power supplies in those components are probably doing a pretty good job of filtering RF and other noise sources present on the AC lines. However in the case of line filtering, more of a good thing can’t hurt, and could well help, unless, of course, the filtering is inappropriate for the current demands of specific components.

The APC S Type Power Conditioners have four AC outlet banks with specific filters designed for different types of components. These are labeled and described in the manual as:

Switched 1 – Digital Filter, for “digital” components such as CD, DVD, DVR and Satellite receivers etc: 6 outlets

Switched 2 – Video Filter, for devices such as TVs or VCRs: 2 outlets

Switched Delay 1, for analog based devices such as tuners, preamps and receivers: 2 outlets

Switched Delay 2, for high current devices such as amplifiers and subwoofers: 2 outlets

My contact at APC suggested that other than the outlets designed for high current amplifiers and subwoofers, that the other outlets banks and their respective filtering were similar enough that I should not worry too much whether I plugged my digital preamp into the “digital” or “analog” outlets. It should be noted however that the various outlets banks do vary in their timing as the system is powered on, and individual banks can be are turned on and off separately. Use the various outlets accordingly. The high current outlets will be discussed in more detail below.

Environmental Monitoring

One of the nice things the APC S20 provides is a way of monitoring the AV system environment. The front panel status display can show a number of useful things about the system including the line input and output voltages, the total power in watts being used at any given time, the remaining “fuel” in the battery and the remaining time battery backup operation with the current system load.


Also very handy with the S20 is a temperature probe that can be used to monitor the temperature inside your component rack or cabinet. A temperature/humidity probe is available as an option for the S20.

Communications and Control

This is the area where the S15 and S20 differ the most. The S20 is network enabled allowing control and status monitoring via either RS-232 and/or a network management card to connect to any Ethernet network. A USB port is also included that interfaces with APC’s “Powerchute” software to gracefully shut down computers with PC or Mac operation systems.

While I have not made any use of the Ethernet network management aspects provided with the S20, I have no doubt those with AV systems containing laptop or desktop computers may well find the S20’s network management capabilities very enabling.

The S15 on the other hand does not have Ethernet or RS-232 control capabilities, but rather allows control of AC power to the various outlet banks using the more common DC triggers (12V) provided by many preamplifiers and receivers. Many users will therefore find the S15 easier to incorporate into their systems than the S20 that these lacks DC trigger control.