Dirty AC Power: Getting It Out of Your (Audio/Video) System
Article By Roger Sheker, Chief Engineer @ Audience

  For most readers of Enjoy the Music.com, it's no secret that the AC power coming out of your wall outlet is shockingly (pun intended) "dirty." In a perfect world, the AC power in your house should look like a perfectly smooth undistorted sine wave (when viewed on an oscilloscope). In reality, the AC can be corrupted with noise and distortion (dirty AC power looks pretty jagged and ugly on a scope). This noise can be heard as grain, "haze" and a general smearing or foreshortening of the sound field/stage, or can be seen as a reduction of the sharpness, color accuracy and detail of your video image.  (Note: an audible 60-cycle hum or 120-cycle buzz in the system is the result of a grounding problem, which is not related to power line conditioning, and is a whole other subject that could be addressed in a separate article.)

The power supplies in your audio/video components are supposed to block all that AC power noise, right? Well, not always. Some components do a better job at filtering this grunge than others. In addition, the voltage coming out of your wall socket can vary widely from the 120/240 Volts you're supposed to be getting (supply voltages can vary in other countries).

How can this AC power corruption be dealt with for us to achieve optimum audio/video system performance?

First and foremost: never take anything for granted. Assume nothing in your system is optimum and go on from there. For instance, when was the last time you cleaned all of your power connections? Connections should be cleaned at regular intervals, not ignored and forgotten. There are a host of other sources of AC power gremlins you might not be aware of. NOTE, never clean a live connector, unplug everything before starting.

For example: have you changed over to the new low-power florescent bulbs? These lights use a switching circuit with little filtering, and they can be a serious noise source. The older high-power fluorescents are generally more benign as they don't have noisy switching circuits.

Standard lamp dimmers can be very noisy (to the point of causing a very audible buzz through an audio system), and at the least should be either turned off or turned all the way up when listening or viewing.

You should turn off or unplug all of your unused "wall warts" (those ubiquitous black plastic AC power adapters) when listening to your system, or check to see that they're quiet and not affecting your system. These little trouble makers can be checked for noise radiation with an AM portable radio tuned off station. You might be surprised at how bad some of them are!

Have you upgraded/replaced your system's AC power outlets? This can make a surprising difference. In the old days of audiophillia, many enthusiasts swapped their standard outlets for hospital-grade outlets; this is still a viable option. These days, a number of aftermarket AC outlets are available and one can't assume that because a product is expensive it is better.  We have found that some of the expensive AC outlets color the sound and do more harm than good.  Our recommendation is the Hubble hospital grade outlets with solid unplated brass pins. Cryogenically treating also gives a further improvement and is recommended.

Are all of your high frequency wireless transmitters, such as those found in all wireless products like portable phones, cell phones, wireless routers, etc. turned off or at least out of the room? These products can be a serious noise source.

The above factors are just some of the most common and simple things to consider when cleaning up your AC power.

A more costly and complex way of optimizing your audio/video system's power would be to install a dedicated AC power circuit just to power it. This dedicated circuit should be rated at 20 Amperes or more and overbuilt as if it were going to be used for 30 Amperes. This expense is to be considered as an investment in sonic truth as this weak link will always be a problem otherwise.  For this, three leg, stranded 10 AWG, shielded and UL rated for in-wall applications is recommended as well as the best dedicated and house mains breakers you can find.  Again, cryogenic treatment is recommended for the wire.

After all of these considerations are addressed, the final step in optimizing your audio/video system AC power is to employ quality power conditioning for all of your audio components. This is such a complex and misunderstood subject that I have included some salient points here.

Currently in common use, there are five basic types of AC power conditioning products. They may be employed singularly or in various combinations:

A pure passive power conditioning design is an implementation of various types of passive noise filtering such as inductors and capacitors of varying quality and size. The best of this type of conditioning will have independent filtering for each outlet with the filtering covering a broad band of frequencies. The very best of this type will also pay particular attention to having very low DC resistance from input to output insuring full peak power delivery. This is particularly important for preserving system dynamics. If not properly designed, a loss of sonic dynamics can occur due to the fact that most power amps draw far higher currents at dynamic peaks than the average value. DC resistance in the power circuit will cause a voltage drop, compressing this peak power draw leading directly to compression of musical dynamics.

As a class, AC power conditioners providing active regulation will be either partial correction/replacement, or full regeneration of the AC power. Active regulation of necessity will also include some of the filtering found in passive units, thus providing some degree of input/output filtering and isolation.

There are various types of active power regulation products in the marketplace, and some work very well for low-power type front end equipment (such as a CD or Blu-ray players). However, active regulation will often limit dynamics and current delivery when used with power amps. Active regulation can also introduce other problems into the regenerated/regulated power, including reduced peak power delivery and broadband noise. The best of this type of conditioner will offer very low power distortion, and often includes active power factor correction, reducing distortion of the input power. The regeneration type power conditioner can even include the option of changing the output frequency.

Isolation of AC power input from the output is always implemented with a transformer. By nature, transformers inherently "separate" the incoming AC going into the primary winding from the outgoing AC from the secondary winding with no direct DC connection. (Transformer design and the possible sonic consequences as well as other effects are a vast topic in and of itself.)

Isolation transformers will be one of many differing types, from a simple standard EI core design (referring to the core shape, implemented with two pieces shaped as the E I letters) all the way to a more complex balanced power toroidal type. Input/output (I/O) isolation can also be implemented with a magnetic amplifier type transformer implemented as a ferro-resonant regulator/isolator. All ferro-resonant-type transformer units of necessity must be big and heavy to allow full system dynamics and power delivery. Ferro-resonant units can be, and most are mechanically very noisy giving off a power line frequency noise/buzz requiring installation in a remote location. Larger units may also require a dedicated AC line.

As a class, isolation transformers do not provide broadband noise filtering, however they can be effective in lowering residual powerline frequency hum problems. Balanced power isolation also can be very effective in reducing powerline frequency noise if it is compatible with all the equipment being powered. This balanced power is implemented by splitting the AC voltage into equal but opposite phase voltages that are balanced from ground. This can prevent corruption of the ground as the noise will also be equal but opposite thus canceled out. The weak link here is the quality of the power transformer of the equipment being powered. If the high and low side input into the transformer have much of a difference in parasitic leakage the inputs will not be balanced so the balanced advantage will be lost.

More about power factor correction: PFC is not produced as a standalone power conditioning unit. It is implemented with either active or passive circuits. Active PFC can be found in many of the active regulation type power conditioners greatly reducing induced power source distortion. Often a power factor correction circuit can be found in the power supplies of power amps or other high power equipment. This type of correction/restoration insures full power delivery by reducing distortion of the input power. This restoration is done by regenerating the clipped peaks using stored power.

Power factor correction can rightly be thought of as power line distortion reduction, and it is required in some countries for certain classes of electronic equipment. Typically it is actively implemented in the power supplies of these components.

Passive PFC is usually done by the power utility companies before it reaches your home and is implemented using large banks of capacitors. These capacitor banks can often be found on power poles or in power sub-stations. Typical full passive PFC requires very large high voltage capacitors and is impractical for home use. Passive power factor correction can only be an approximation to an optimum solution as one never knows what will be connected to the line.

Another often found problem with AC power is residual DC on the line indicated by a mechanical buzzing heard coming from power amplifiers occasionally, not constantly.

Since all power is delivered from the utility company power lines coupled through transformers, this DC will average out to zero volts; however it can reach several volts in the short term. Power transformers are AC only devices and require a changing magnetic field to work. They have no direct electrical connection from input to output and therefore cannot pass DC. The big transformers used by the utility companies can hold an apparent DC voltage for up to several seconds. This apparent DC polarity will be reversed while being present again when the equipment causing the problem is turned off.

This residual short term DC can cause significant problems with equipment using large toroidal transformers in their power supplies. Toroidal power transformers have very low DC resistance on their inputs and any input DC present will cause very high currents to occur. This high current can cause saturation in the core during peaks in the AC power cycle.  This saturation causes the input AC power to see a greatly reduced inductance and as it is going into a virtual dead short on these peaks, there is very little to impede this peak current flow and it can reach hundreds of amps.

This high current will generate intense magnetic fields that the transformer core no longer can absorb and will cause the transformers to mechanically buzz. At the same time, this core saturation will cause massive distortion on the transformer's output. The only practical cure for this in the home is to eliminate the cause, or shunt this DC and reduce it as much as possible. The only available way to do this is to use a large isolation transformer or place a large inductor across the AC line to act as a shunt. This shunting is done by the inductor having a very low DC resistance or short to the DC voltage. An inductor stores energy when a current is flowing through it and returns this energy by trying to maintain the current when the source is diminishing or cut off. An abrupt cut off of this current can cause the inductor to release this energy as a very high voltage. As an example this is how the ignition coil in your car generates enough voltage to fire the spark plugs with only 12V applied.

This sounds like a simple enough solution – just put the proper value inductor in a power conditioner and be done with it – but it is not easy to implement benignly, as inductors stored energy must be dealt with or huge voltage spikes can be created. These spikes are potentially damaging to audio/video components and can generate large amounts of noise. Also useful inductors are quite heavy and can be large. The best long-term solution is having the equipment causing the problem removed from the power source, note this is sometimes easier said than done. Cooperation can sometimes be gained from the utility companies in tracking down and correcting the cause of the unwanted DC.

In the design of a power conditioner with the goal being quality audio and video reproduction, the best rules to be followed are to first do no harm and to be safe. Too often people have experienced a degrading of dynamic peaks, colorations and so on.

Because most of the newer front-end components and some of the new power amps are equipped with regulation circuits they are relatively insensitive to low order harmonic AC power distortion and long term/slow AC voltage changes. These components can usually operate quite well to specification over a very broad range of voltages other than the desired 120/240 Volts AC (in the U.S. and some other countries). This is usually implemented using highly regulated switch mode power supplies, including in some equipment, very effective power factor correction. However, what this new technology cannot do is effectively nullify/eliminate noise coming in on the power line. At the same time, this new technology can contaminate the power line with its own generated noise. Proper and effective noise filtration can only be assured by using quality dedicated power conditioning.

Many people do not understand and/or discredit the use of power conditioning in high end audio.  This is understandable due to some of the bad effects that come along with many power conditioning products.  One must understand, however, the fact that all of our electronics are simply modulators of AC power. Transforming this AC power into useful forms doesn't change this basic fact and the end result can only be as good as the power that we start with.  Thus, the need for well designed power conditioning products that do no harm.

--- Roger Sheker
Chief Engineer @ Audience

Audience, LLC
Voice: (760) 720-9611
E-mail: info@audience-av.com
Website: www.Audience-AV.com