Discrete Symmetrical Power Conditioning for Home Theaters

How did I wind up with rolling bars on my video screen? Can I remove them? I added a new sub-woofer across the room from my equipment cabinet, now the system hums - why? I don't have any hum or rolling bars in my theater, what could Discrete Symmetrical Power do for me?

Today's electronic circuits are more sophisticated than ever, and their susceptibility to AC line problems is far greater. Add to this the fact that today's AC mains supply is extremely noisy and we have a recipe for poorly functioning equipment. Further, the complexities of these systems have forced many installers to connect source components to multiple AC taps throughout a home or office. This is a recipe for ground loops which may lead to rolling hum bars in a video presentation or annoying hum and buzz in audio. Additionally, there is also the consideration of eliminating all RF and EMI noise from the AC line as we possibly can.

Most AC line noise is asymmetrical (transverse mode), meaning that the noise level or waveform on the AC Line wire is different than that present on the AC Neutral. This noise comes primarily from appliances and switching power supplies. There are a number of ways to combat this noise, the best being a low-pass device with linearized filtering. However, a premium system will most certainly suffer from any form of AC line noise, as its superior resolving power will be jeopardized in part by the noise's masking effect.

Simply put, any AC noise that is coupled into your components' circuitry is often higher in signal level than much of the low-level signals your home theater is attempting to reproduce. If this occurs, a masking effect will certainly take place. This is key, as today's high resolution audio-video is defined by its ability to accurately reproduce complex low-level signals.

The rarer but still critical second type of noise found on an AC line is symmetrical noise (common mode). Many conditioners are not capable of reducing this noise, because to do so with passive circuitry one is severely limited in the overall frequency range of noise reduction. Balanced amplification-regeneration schemes are troublesome in their limited current capabilities and heat production and cannot break a ground loop. The only effective way to reduce common mode noise (particularly in the frequency range associated with buzz and rolling hum bars) is with an isolation transformer, and the most effective isolation transformers for a home theater are Discrete Symmetrically Balanced isolation transformers.

Discrete Symmetrically Balanced power is achieved by running the incoming AC line into a 1:1 ratio isolation transformer, with a precisely placed center tap on the transformer's output or secondary. The incoming voltage, (120 VAC on the Line terminal and 0 VAC on the Neutral and Ground) are split in perfect halves on the secondary of the transformer. The output Line terminal now has 60 VAC, and the Neutral terminal has 60 VAC, when referenced to its center tap Ground, which remains at 0 VAC. What's significant about this is that the two 60 VAC terminals are now in opposite polarity. So the symmetrical AC noise (or common mode) fields cancel. Further, this noise reduction is extraordinarily efficient and linear across a huge frequency range (bandwidth). Recording and broadcast microphones have utilized this same noise reduction principle for over 80 years.

Though there are many types of isolation transformers, all are not equal! An isolation transformer that is not symmetrically balanced will reduce a great deal of common mode noise as long as it's been created with proper ground strapping and an effective faraday shield to minimize capacitive coupling. However, without precise balancing, the noise reduction is typically not as great or as linear, which is critical for maximizing system performance.

There are also many form factors (shapes) for isolation transformers. The traditional E-I type is the squarish package most people associate with a transformer. This type of transformer has two significant problems. The first is its tendency to propagate a huge magnetic field when any significant AC load is present. This can easily couple hum into adjacent equipment for a distance of up to 5 ft in many cases! The second is the relative inefficiency, or poor regulation of this type of transformer. As the current demand rises, your output voltage will drop significantly. Toroid (or donut shaped) transformers have a big advantage here, as the field of radiation is considerably reduced (allowing rack mounting near sensitive equipment), and regulation can be maintained at nearly 98 percent. This means that as the current demand rises in your system, the voltage coming out of your transformer will be very close to that going in. This is simply not possible with E-I isolation transformers.

Another important attribute with Discrete Symmetrical Power is total isolation from ground. This has the advantage of breaking the noise that is present on the AC mains, but the greatest advantage is breaking ground loops.

In a perfect world, all homes would be wired as it is done in broadcast studios -- star grounded, (sometimes called a home run). When this is done, all of your theater's three-prong AC cords are brought to one point or conditioner. This puts all the AC ground plugs at nearly the same potential from Neutral to Ground, because the resistance from one plug to the next is nearly zero ohms.

However, we live in a world where many folks will not pay for this level of AC wiring. For that matter, the addition of a new remote component can create an unwanted ground loop as well. This is because one or more of the theater's components are plugged into an AC tap that is far away from the main AC conditioner or strip. Ground is carried by the signal wiring as well as all three-prong AC cords, so if there is a significant voltage drop across part of the system's AC source, a ground loop will appear. This is the cause of the hum and rolling video bars we wish to avoid.

With an isolation transformer that has an isolated or floating output, we may effectively isolate one or more offending components, thus breaking the noise loop. Of course, to do this it is critical that the transformer be safeguarded with a GFCI (ground fault circuit). This circuit assures that if any current leakage from Line to Ground or Neutral to Ground exceeds 5mA, the circuit disconnects. This in turn assures safe operation well beyond a typical household AC tap.

It is important that the transformer have sufficient capacity as well. If we wish to remove the rolling bars from a 50-inch plasma television and improve its performance through common-mode noise reduction, we will require at least a six amp transformer. Further, even a 50 amp isolation transformer will have some inductance as well as voltage loss, which can cause current compression in large power amplifiers. For this reason, it is generally best to avoid any isolation transformer where power amplifiers are concerned. Additionally, most power amplifiers are designed to eliminate common-mode noise by well over 90 dB, so there is no substantial benefit. However, there is considerable benefit in asymmetrical AC noise filtering and power factor correction for these amplifiers.

There are many other considerations for advanced power management but what must be understood is that the sensitivity and sophistication of today's electronic circuits require serious AC power conditioning. Anything less is too costly to consider, and will limit performance.