<https://audiophilestyle.com/forums/topic/4735-lengthy-primer-on-ac-with-some-thoughts-on-reducing-the-computer39s-impact-on-yours/>
<http://schema.org/text>
"This was originally posted in this thread: http://www.computeraudiophile.com/content/Mac-Mini-Linear-Power-Supply \n  \nI've re-posted it here in the event that a broader audience might be interested. \n  \nAs I said in the earlier thread, I reached out to the person I go to for all things AC - a former poster here who shall remain anonymous per his request. \n  \nHere is what he had to say: \n  \n\"After thinking for a while about a response, I realized that doing even a moderately complete job would require a substantial chapter in a substantial book. I know this because some experts have done just this and it took that much space. And that was in books targeted toward electrical engineers with some years of experience. For an audience primarily of people who are not electrical engineers, or better yet physicists, it would take much, much more than what anybody would want to read in some hobbyist forum. More than I’d want to type, too. \n  \nSo, this will be as simple as I can make it, hoping that people will spend time researching ideas that they aren’t familiar with and knowing that the subject will barely be scratched. \n  \nThis will also be as apolitical as I can make it, with one exception. You can easily find billions and billions of posts on various internet forums about these audio topics. Many of these are from highly educated and smart engineers who wave their arms and proclaim that none of this is really important. Here’s the problem: From the beginning, engineers (and some other professionals – you know who you are) are trained to make various simplifying assumptions about any problem. This is done not only to streamline the educational process, but also to make really complicated problems understandable for us puny humans. But, that doesn’t mean that the details that were ignored actually go away. In the end, they may not really be significant after all. In many cases they are not. Maybe just as often the details are significant. If you wanted to calculate the planetary motion of the Earth around the Sun, you might not include the effects of some piece of rock the size of a Buick that is floating around somewhere beyond the Moon. It probably wouldn’t make any meaningful difference in your calculation. But if that rock decided to land in your neighborhood one evening, you might find the effects really significant to you. \n  \nThe power system in your house is really, really complicated. It may not look it, but it is. It is not only the delivery mechanism for 60 Hz power (50 Hz in many parts of the world), but also is an unwitting connection system between everything plugged into it. \n  \nFor now, we’ll entirely ignore the radiative aspects of the home power system. By those I mean radiation of noise as well as capturing various undesirable signals that are already floating about, like radio signals, wireless signals, TV, and so on. These are real bona fide conditions that can be major PITA problems, but that goes way beyond Mac Mini power supplies. \n  \nThink about what happens in your house, after the breaker or fuse box. (Lots of badness takes place outside the house, but let’s skip this one, too.) You have cables of various lengths that often have non-linear loads attached along their way at various points. What’s a non-linear load? Anything that distorts the waveform of the 50/60 Hz voltage waveform. Unfortunately, almost every load in a modern home aside from incandescent lamps and electric heat is non-linear. If DC is used in the load, the AC must be converted to DC. This is usually a non-linear process. More on this in a bit. \n  \nNon-linear loads draw current in some way different from a pure sine wave. This causes harmonics of the line voltage to be placed on the line as well as various artifacts that are nominally secondary to the actual power process. \n  \nThe reason these harmonics and artifacts are important is because audio equipment is imperfect with regard to “ignoring” impure AC supply voltages. The simple harmonics of the line frequency usually are not so much of a problem, since similar harmonics are generated within the audio equipment and are normally considered in the design and execution of the product. The ones that really are a problem are normally between 50 KHz and 5 MHz. Since these are outside of what is normally considered the frequency range of what people can hear and loudspeakers can reproduce, why do we care? \n  \nAudio amplifiers and processing equipment, even those “competently designed and manufactured so that they all sound the same,” often respond to higher frequencies. Yup – it’s true. Not only do they respond linearly at these frequencies, they also often respond non-linearly to higher frequencies. Even look at the data sheets for the most popular opamps. As a simple example, notice how even some simple amplifiers can detect and play back radio signals, sometimes even quite loudly. This is one of those situations where you’ll need to do your investigations as to why this may be – it’s a long explanation. For the moment, please just go along and accept that this is true. \n  \nThe non-linearity of the amplifiers at these frequencies can cause problems in the audio band, even if the amplifier is essentially perfectly linear within the audio band itself. Not that any amplifier really is perfectly linear… For example, imagine that somehow two high frequency signals manage to find their way into your amplifier. Let’s assume that these two signals are at 211 and 212 KHz. Way outside the audio band, right? Well, if applied to an amplifier that isn’t especially linear at ~200 KHz, you’ll get harmonics of the 211 and 212 KHz signals at the output, which you can’t hear. You’ll also get an intermodulation product at the sum of the two (423 KHz) as well as at the difference of the two – 1 KHz. Even I can hear that – you probably can, too. That’s not even thinking about the higher order IMD products that fall into audio range as well as out. Nor is it considering the effect of these signals on the operating conditions of the devices within the audio band. Conceptually, it’s easy to imagine that if for some horrible reason there was a signal at 1 MHz that was driving the amplifier to clipping at that frequency, you couldn’t hear the 1 MHz, but that clipping certainly would affect the performance within the normal audio band. \n  \nOf course, that was really a highly simplistic example. It’s more likely that the high frequency signals will not be simple sine waves, but rather complex signals that are likely additionally modulated by other complex waveforms and noise. The products of these that fall into the audio band are equally ugly and can sound like modulation of the noise floor or other nasty garbage that your auditory system will have a hard time processing. Simple harmonic structures occur in nature and even in your ears, so harmonics of the music are the least of your worries. \n  \nSo, back to the power supply. \n  \nIn a “linear” supply, the rectifier diodes are commutated (look it up) and the output smoothed and filtered to make it a close approximation of DC. Sadly, most diodes are not perfect and can produce outputs at several hundred KHz (tube rectifiers, imperfect in other ways, don’t do this). These high frequency signals not only can travel forward into your amplifier stages but also backward out of the power supply through the power cord and back into the power system in your house. How nice – they can now get into every other electrical appliance plugged into your power system. The type of smoothing filter affects the level of these as well as resonances that are found within the system. That’s partially why choke input filters have a good reputation in some circles. Toroid power transformers are inferior in this regard compared to split bobbin transformers, since the windings are right atop each other in a toroid and there is a lot of capacitance between the primary and secondary windings. Those are just two of many details that affect transmitting high frequency garbage both out of and into a piece of audio equipment plugged into the power mains. Or any other applicance for that matter. \n  \nAn additional concern with a linear supply is any regulation that is used. In many applications it’s desirable to maintain constant voltage at many points in a circuit. A linear regulator does this fundamentally by dissipating any unused power that is due to the wrong voltage as heat. Basically, closed loop linear regulators are servo controlled resistive voltage dividers. The servo watches the output voltage wherever the designer chooses and adjusts the voltage drop across one of the regulator elements to maintain the desired voltage at the sense point. If you were really diligent and had really fast fingers, you could do the same yourself manually by turning the knob on a potentiometer. Obviously, that doesn’t work because of the speeds and resolutions involved. But the same principles apply as if you were doing it by hand. Making these adjustments at signal speeds makes the regulator act as a filter. The faster the regulator, the better the filtering at high frequencies. Most, but not all, regulators don’t offer much rejection for the range of 50 KHz to 5 MHz. In fact, since most regulators use the same types of servo amplifiers as found in audio amplifiers and are susceptible to high frequency interference. Whoops. (Some regulators make use of some circuitry or component properties to maintain voltage in an open loop application. These are usually less susceptible to outside interference.) \n  \nSwitching regulated supplies are a little different from linear supplies. Switchers are used for one simple reason: Efficiency. The way a switching regulator works is that it creates a high frequency AC signal from a DC source, which is then rectified and filtered. Yup – you might be using two rectifier systems in series if the switching regulator is powered from the AC mains. The switcher adjusts some aspect of the high frequency AC waveform to vary the DC output voltage from its filter. It uses the same kind of servo action as the linear regulator does. Since the frequency of the AC output from the switcher is high – usually somewhere between 50 KHz and 5 MHz (sound familiar?) the components needed to nominally filter the DC are smaller than what is needed for filtering rectified AC mains frequencies. Lower value caps are needed as well as smaller inductors. That is space efficient as well as money efficient. The waveform used is made such that the diode rectification process is made to be as efficient as possible electrically. That makes for less heat, better power usage, as well as a smaller package. Since a transformer is often not needed, that also reduces cost and size, as well as offering a side benefit of allowing the supply to be used in many countries where the AC mains can be 50 or 60 Hz and anywhere from 100 to 250 VAC. The bad thing is that the high frequency stuff can blow right through your power system into your audio components, where they may have an effect that you don’t appreciate. \n  \nIn the case of the Mac Mini, there is an additional consideration. The external DC supply (note that new Mac Minis have built-in supplies) only produces a single output voltage of somewhere around 18 VDC. Inside the Mini there’s a bunch of other switching regulators that provide the actual operating voltage for the various internals - none of any consequence actually run from 18 VDC. So the artifacts from this process can easily migrate out of the Mini and through the external DC supply to your AC mains. That’s not to mention all the switching noise from the actual processing components doing their job… \n  \nWhat do you do? \n  \nOne approach is to build a suitable linear DC supply that, if properly done, eliminates the first level of problems. If done really well, the switching noise from within the Mini back into the AC mains will be minimized and filtered, too. \n  \nA second approach is to filter the AC power going into and coming out of the raw DC switching supply for the computer. There’s numerous commercial units available for this. Unfortunately, most of the ones I’ve measured have limited attenuation for between 100 KHz and 2 MHz where most of the power supply garbage is located. Some of the ones that are OK for common mode signals (look it up) aren’t great for differential mode signals. In addition, many, many use filters that try to bypass common mode noise to the safety ground in your electrical wiring. Unfortunately, at least in the United States, that safety ground is connected directly to the “neutral” wire of the system back at the breaker/fuse box. For high frequency signals where the wiring in the wall is a substantial fraction of a wavelength, that may be OK. But for the 100 KHz to 2 MHz region, that effectively can short out and negate part of the filtering. (Investigate: Balanced Power) That’s why some manufacturers use what’s called doubly isolated AC wiring so that no safety ground is used. In many countries, the safety ground plain does not exist. Read on your own the reasons given for using a safety ground – it’s not for noise filtering. Most of the general purpose AC mains filters are designed to minimize conduction of frequencies well above 1 MHz, so that they don’t get onto the AC mains system, radiate, and interfere with radio and other wireless devices. In addition, making filters that really are good for between 50 KHz and 5 MHz often requires pretty large components or circuit tricks that require large components. Even most of the AC filter guys who target the audio market don’t want to go there. For one thing, there’s a whole dogma that includes the idea that anything in series with the AC mains will wreck the performance (see below). Plus, it just plain gets large, heavy, and expensive and that isn’t what most customers want. \n  \nSo, again, what do you do? \n  \nSince the AC system in your house is pretty much guaranteed to be different electrically from everyone else’s, you need to experiment. The close to infinite number of variations of what is plugged in where and all that makes a pure analytical approach pretty hard. One thing that can be done is to try some of the products used to change the impedance characteristics of your AC mains system at various points. As just one of many examples already mentioned, Alan Maher makes products that attempt to minimize voltage peaks of noise where you don’t want them. Another approach is filtering the AC mains. This includes AC power cords, which really do act as lossy cables at high frequencies because of the dielectric used and the configuration. Or, you can use a cleaner DC supply for your Mini, even including batteries. Done right, batteries may be the best performance solution for everything audio. I mean everything. They just are so tough to manage in so many ways that most consumers just don’t want anything to do with them. Not many manufacturers want to build equipment they can’t sell. \n  \nDon’t forget, it isn’t just the Mini. If you have an external disc drive that has its own switching supply, that can be a problem. Same for your TV. Or dimmer controlled lamp. Or the microprocessor controlled dishwasher in the next room. \n  \nAs I said, this is complicated. \n  \nOne more specific point about how AC filtering can affect dynamics and all that… Even if you have a perfect AC filter in every way, the problem may be the amplifier itself. I have found many amplifiers that are “voiced” to take advantage of the power supply artifacts to give the impression of increased dynamics. It’s an illusion, but if it works for you and you find it more realistic, great! The point is that if you remove these artifacts by cleaning up the AC or the DC by using batteries, some or all of that thrill can go away. Keep in mind that in many audio products, the signal current goes right through the power supply with limited isolation. That means that any crap in the supply will somehow affect the desired audio signal. \n  \nI told you this is complicated.\" \n  \nHopefully this will help some of us (begin to) understand what we're dealing with. \n  \nenjoy, \nclay \n  \n  \nPS, Chris, if it's unwieldy to have this much info in two places, please feel free to delete one and direct traffic to the other. \n  \n  \n  \n\n"
<https://audiophilestyle.com/forums/topic/4735-lengthy-primer-on-ac-with-some-thoughts-on-reducing-the-computer39s-impact-on-yours/>