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  PWM power stage explained PS Audio's super linear analog power stage represents a revolution in audio power amplification. Our analog power stages are an artful blending of analog and digital topologies that, together, bring forward the beauty and power of recorded music. The analog half of of our power amplifiers was developed over the last 25 years of listening and designing by Paul McGowan and is now found within every audio product we make at PS. Called the Gain Cell™, this analog module is the analog 'front end' for the power output stage. The analog power output stage is a PWM or Class D amplifier that is extremely efficient, extremely linear, low distortion and exceptionally good sounding. But, what is a Class D PWM (Pulse Width Modulated) amplifier? How is it different than a traditional class AB, class A or other power amplifier? What the heck is a PWM power amplifier anyway? A PWM power amplifier has an efficient output section that is PWM based code (more about PWM shortly). Instead of the output devices moving current up and down to match the musical signal at the input to the amplifier, a PWM power amp has only an on or an off state at its output. Like a CD or DVD player, a digital power amplifier uses only 1’s (on) and 0’s (off) to create the musical power used to drive your loudspeakers. Sometimes referred to as a 'digital amplifier' a PWM Class D amp varies the length of time the 1 or the 0 stays on to create the signal. This differes from a traditional PCM (Pulse Code Modulated) digital scheme which converts the analog music signal into a numeric representation. Let’s look at a traditional power amplifier A traditional tube or solid state power amplifier uses the output devices like a constantly adjusted valve. Picture water going through a pipe with a faucet valve regulating the flow of the water. Your hand (in this example) is the musical signal, and the water is power to your speakers.  If you could move your hand on the valve quickly enough, you could smoothly turn the water faucet up and down creating a flow of water that mimicked the action of your hand.With very little force from your hand, a lot of high pressure water can appear at the output of the faucet. This is the nature of an amplifier, a very small force controlling a very large force. The small force is the output from your preamplifier, and the very large force is the output of your power amplifier. The problem with this approach is two fold: it isn’t very accurate and it isn’t very efficient. In a standard analog power amplifier, the musical signal at the output of the power amp is not an exact copy of the musical signal at the amp’s input. When an amplifier's output does not match its input we would say it has a linearity problem. There are a number of ways this can be compensated for and that fact, in a nutshell, is what power amp designers have been struggling to correct for - in one way or another – for years. Good for cold weather Ever noticed the use of big heat sinks on power amplifiers? These are necessary because traditional power amplifiers generate lots of heat and that heat needs to be dissipated away from the output transistors. The heat that is generated in a power amplifier is wasted energy. A typical class AB amplifier is only 50% efficient. This means that 50% of the energy consumed goes into powering your speakers and the remaining 50% is converted into heat. Why the heat? Traditional power amplifiers produce heat when the output of the amplifier is somewhere between one of two possible states: all the way on or all the way off. In a class A or class AB amplifier this is virtually always. There is rarely ever a time that a power amp is all the way off or all the way on, so they are always generating heat. Remember our valve explanation above? The entire time the faucet is anywhere other than all the way on or all the way off, it is producing heat. So too with a traditional power amplifier. Now, let’s look at a Class D power amplifier One way to create an amplifier that produces little to no heat is to have the output devices either all the way on or all the way off. Sound familiar? Yup, the use of all-the-way-on (1) or all-the-way-off (0) is what we are all becoming familiar with: PWM. But while this is a great idea, how the heck do you make all of the musical signals in between all-the-way-  off and all-the-way-on? The answer to this perplexing question has been around for many years, but when it was first invented it wasn’t used for producing music, rather, it was used to regulate the speed of electric motors in trains and industry. A bit of history on PWM amps The first usage of a Class D approach to power amplification we could find was produced by a man who gave Bill Gates a shot in the arm, Sir Clive Sinclair. Now, here’s a bright guy who invented the world’s first pocket calculator and is credited with being amongst those that started the PC revolution. The Timex/Sinclair computer and the Executive calculator were genuinely innovative products from a genuinely innovative kind of guy. In 1964, Clive Sinclair started a company called Sinclair Radionics Ltd. that produced a Class D PWM power amplifier known as the X-10, designed  by engineer Gordon Edge. This tiny power amplifier was purported to produce a whopping 10 watts of power but, in reality, produced only a couple of watts. This was later to be replaced by the Z-12 that actually produced a real 12 watts.PWM is born What engineer Gordon Edge did, to solve the problem of heat, was to use a system of 1’s and 0’s in varying lengths. By controlling how long each of the two states remained on (or off), a signal could be produced that, after the appropriate manipulation, would actually produce music. That process is known as Pulse Width Modulation or PWM. The first Audiophile version of the PWM process that we are aware of was an Infinity systems  product known as the SWAMP amplifier. The SWAMP was a revolutionary product as envisoned by Arnie Nudell and his then partner in Infinity, John Ulrich (now running Spectron). As the story goes, research efforts for this product nearly killed the company in its infancy so expensive and back breaking was its design challenges. The SWAMP was finally produced in the mid 1970's and despite some horrific reliability problems, the amp received critical acclaim.Following the cancellation of the Infinity switching amplifier, Infinity's very next attempt at a power amplifier was the Infinity Hybrid Class A, an artful blending of tubes and a traditional Class AB solid state design.  Modern day uses of PWM include the Laser disc, DLP projectors (Digital Light Processing displays), DSD (Direct Stream Digital) or SACD, Jeff Rowland Designs 501 mono blocks and of course, the PS Hybrid Class A power amplifier and its more modern counterparts the GCA and GCC series.How PWM works Think of PWM as a digital data stream that does not require a D  to A processor to convert its signal to analog. In fact, the PWM process is 100% analog.A digital, or PCM signal (Pulse Code Modulation), which is what is used in a CD or DVD, converts the musical signal into a series of numbers – each number representing a voltage level. The higher or lower the number, the higher or lower the voltage. The CD disc itself merely stores these numbers in a computer like memory. To play back a CD or DVD using PCM you need another computer to read these numbers and process the conversion. This “computer” is called a D to A processor. PWM does not need another conversion process, so it does not require another device to interpret its code into audio. PWM is a far more direct approach than PCM, and as mentioned is not digital in nature. The long and short of it PWM is simple to understand. At the output of a PWM amplifier there is an electronic switch, rather than an analog “valve”. The switch has one of two states: on or off. In order to duplicate a musical signal, the output switch turns on or off in longer and shorter time periods. If the volume of the music is low, the on/off times are very short. The louder the music gets the longer the switch stays on. The switch is connecting your loudspeakers to the amplifier’s power supply (like a battery). So, when the switch is on, the full power of the amplifier is applied, and when the switch is off the power supply is  disconnected from the loudspeaker.The longer the power supply is connected to the loudspeaker, the more your woofer or tweeter move, thus creating sound. If you do this fast enough, then the speaker doesn’t know it is being turned on and off and it sounds smooth, like music. Going to the movies A good example of how this works is a moving picture or video. What you view on the screen looks like it is fluid, smooth and not full of one after another transitions. We all understand that a movie is really just a series of individual still photographs, transitioning quickly enough to fool our eye into believing they are “moving”. Our eye can be fooled if we present it images 24 times a second. So too with a PWM amplifier. Moving from on and off quickly, we can fool the loudspeaker and our ear/brain mechanism into believing it is smooth and lifelike (because actually it is). In this instance, we need to go a bit faster than the movies and turn these on/off transitions at a much quicker rate: 350 thousand times a second. Let’s look at an example In this example, time is represented in the horizontal plane from left to right. Amplitude (volume) is represented in the vertical plane, or up and down. Note how the individual bars, which represent the on time of the output switch, stay on longer as the amplitude goes up and down. This is the code that PWM uses.  To convert this code into a smooth output, we need to remove the transition marks and fill in the gaps.To do this we require a filter. A filter uses elements like capacitors and inductors (coils of wire) to store and release energy. At the beginning of a switch transition, some of the very quick energy is used to charge up the filter elements, thus making it smoother. When the output switch turns off, the energy that we previously stored in our filter is released, thereby filling in the gap between the on and off. This is the smoothing action that we need to complete the conversion. And, that is basically it! The advantages Remember when we started this discussion we mentioned that analog amplifiers have two problems: they are not linear and they produce heat. Well, guess what? PWM amplifiers are absolutely linear and produce almost no heat. With respect to linearity, which is the ability of the power amplifier’s output to accurately follow the amplifier’s input, traditional power amplifiers aren’t very good because they rely on the valve’s characteristics to function perfectly. In a PWM amplifier we don’t care how accurate the valve or switch is, because it is only turning either on or off. Perhaps a good analogy would be the difference between an artist and a non-artist. Anyone can draw a  simple line on two planes, while only an artist can draw everything in between and make it look like a photograph. How close the drawing looks like the real thing is completely dependant on the skill of the artist.So too is the problem with using a transistor or a tube as a perfect valve. At either end of their amplifying limits they become non linear - they are NOT perfect valves. So, the linearity of the PWM or switching amplifier is nearly perfect. Voltage in equals voltage out. And when it comes to heat, a PWM amplifier loses only between 5% and 10% to heat, the rest of the energy being sent directly to the loudspeaker. The disadvantages PWM amplifiers are rather tricky to design, they can radiate airborne noise, and they are very sensitive to different loudspeaker impedances (the later problem of output impedance finally solved in the GC series of power amplifiers from PS Audio).  A design challenge Designing a traditional power amplifier is almost trivial to designing a proper PWM based power amp. The disciplines are entirely different and require a unique knowledge of RF and digital techniques not required in a traditional power amplifier. PWM circuits are also pushing the envelope of technology. The output devices used in these designs are typically MOSFET’s and in a full range digital power amplifier these MOSFET’s are required to operate under conditions that semiconductor manufacturers couldn’t even dream of meeting even just a few years ago. A question of noise Perhaps one of the most daunting of tasks with a PWM power amplifier is noise; radiated noise. The on/off switching of the power supply as described above can create a radio signal. In fact, if you wanted to build a radio station transmitter you would build something very close to a digital power amplifier. Instead of an output filter you would use an antenna.  While radio station engineers do their best to get as much of the energy developed by their digital power amplifiers into the air, Class D audio engineers do their best to keep as much as they can out of the air. This task is quite a challenge for every engineer that tackles the dream of a high efficiency power amplifier and can be absolutely daunting in its successful completion.PS Audio GCA series amplifiers solve this classic noise problem quite well and are quiet enough to place even the most sensitive of equipment near the amplifier with no ill effects. In a PS power design, noise is not a problem. Load problems From a practical standpoint load variations of loudspeakers, as powered by a PWM amplifier, are one of the biggest hurdles that digital amplifier designers face in making an audio amplifier that sounds good on every system. In fact, the PS Audio GCA series of power amplifiers, integrateds and multi channel amps are perhaps one of the very few amplifiers in the world to really solve this problem (in an Audiophile friendly way). Remember the output filter we spoke of used to smooth out the switching transitions of the PWM circuit? This is where the problem resides. The problems with filters All filters are designed to work into a specific load. That means that a filter’s action will be consistent only if the input and the output to the filter remain constant. This is true for most every filter. The problem is the loudspeaker itself. 4 Ohms? 8 Ohms? The loads are different. To make matters worse, a 4 Ohm or 8 Ohm speaker is not really what it claims. Ever notice in loudspeaker manufacturer’s specifications the word “nominal” impedance? This refers to the fact that the speaker has an overall impedance of whatever they specify (4 or 8 ohms). Unfortunately, the actual impedance of a loudspeaker varies with frequency. If you are playing a bass note, for example, the loudspeaker impedance is anywhere from 2 Ohms to 30  Ohms! This is quite a swing. What’s a poor output filter on a digital power amp to do? Remember, its performance is dependant on the load being constant.The results can range from a dull muddy sound to a hard and bright sound all from the same amplifier. It is very dependant on the type of speaker you have. Beware of Class D amplifiers and their interaction with speakers! PS analog power stages solve this classic problem The analog PWM power circuitry is revolutionary for several reasons, among them the elimination of the load variant problem. PS GC amps will sound identical regardless of the type of speaker you have. With the GCA Series of power amplifiers, integrated amplifiers and multi channel amplifiersprovides high damping factor load invariant power to the loudspeaker in such a way that your speaker will perform the way it was intended to sound. High damping factor As discussed above, one of the classic problems with PWM power amplifiers is a low damping factor due to the requisite output filter. Damping factor is a means of measuring how well the power amp can control the loudspeaker. A damping factor of 100 to 200 is considered excellent. The first PWM amplifier we built, the HCA-2, had a damping factor (at low frequencies) of just under 100, a mighty feat in itself when you consider most PWM amplifiers are lucky to have damping factors above 25 or 30. The new GCA series power amplifiers, utilizing the analog power stage, has a damping factor in the thousands, a 10 times increase in control over the loudspeaker than any other amplifier on the market. When you hear the authority and control any of the three GCA amplifiers presents in your listening room you’ll be setback on your heels. In conclusion There are advantages and disadvantages to any type of power amplifier scheme. There is no perfect solution, and that’s the plain truth of it. PS Audio's analog power PWM stages solves a number of the traditional problems associated with power amplification and the accurate  portrayal of recorded music.But once you accept the fact that there are no perfect solutions then you have to ask yourself 'what is the best solution' and, in our view, there's no contest. The GCA Series of power amplifiers with Gain Cell™ technology solves so many problems that it is as near perfect as a designer can get. The high damping factor, incredible control over the loudspeaker, breathtaking dynamics and musically pleasing presentation puts the GCA series of power amplification head and shoulders above any other technology. This is about as close to perfection as we're likely to get. Pure Power™, Pure Resolution™ from the leaders in purity, PS Audio. |
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