Class DSD

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Who knew so many of you were happy reading about common mode rejection? Thanks for the kind notes about yesterday's post. Today let's focus on what a Class D amp does, how PWM works and why this process is so closely related to DSD. Remembering that the simplest understanding of a traditional class A/B amplifier can be broken into two parts: input voltage amplifier and output current amplifier - it's somewhat easy to visualize how an analog amp works. The preamp feeds the amp a small signal, perhaps a volt or two and the power amp increases that voltage by 25 to 30 times. For sake of understanding and easy math, let's just call it 30 times. Put one volt into our imaginary amp and the voltage amplifier part gives you 30 back. Simple. If it's a well designed amplifier, the 30 volts looks identical to the input 1 volt - no difference means no distortion. The output current stage of our imaginary amplifier takes the 30 volts and adds wattage ability to it - so it can drive your loudspeaker's low impedance load. To summarize, in a traditional amplifier the voltage gain stage makes the input signal bigger, the output power stage adds current to that bigger voltage so you can have enough power to drive your loudspeaker and you get music. The simplest understanding of a Class D stage can also be broken down into two parts: the input modulator and the output power stage. Today we'll focus on this first input stage, the modulator. Here's the idea. To create an efficient low heat producing power amp we need to only allow our output power stage to be either all on or all off - anything in between will create heat and lose power delivery to the speaker. To accomplish this "all in" or "all out" idea the Class D input modulator has, instead of a voltage amplifier as in our traditional amp, an on or off comparator. A comparator is a really simple electronic switch that compares two things to each other. If one of its two inputs is higher than the other, the output of the comparator will be high. Should the input of the first one be lower than the input to the second one, the output of the comparator goes low. Simple. So what are these two inputs we are comparing? The first one is our music - the second one is a triangle wave - which is nothing more than a steady up and down voltage at a very precise and fixed time. This triangle wave, which we mentioned the day before yesterday, is really dependent on time for its perfection - and the sound of our amplifier can only be acceptable if this up and down ramp is exact and equal in its timing. The up part has to be identical in time as the down part, over and over again. Perfection here is the key. So, now put the two ideas together - the one input is our music moving up and down in voltage - the other input is a precise up and down voltage ramp. As long as the music input is higher than our voltage ramp side, the output of our comparator is on - and as soon as the music voltage is less than the ramp - the comparator is off. This activity makes a kind code that is directly related to the music in a very linear fashion. Here's a picture of what this looks like: Note on the top half of this picture the black sine wave, then the red constant triangle wave and in the lower picture, the on and off length of the modulator output. It's kind of easy to see that there's a direct relationship between the time the triangle wave is on - and where the sine wave is - resulting in our longer and shorter output. But also note that as soon as the sine wave drops below the red triangle wave the output goes away. The sine wave is our music - and, if you put this sine wave into your loudspeaker, you will hear sound. Funny thing is, if you put the modulator output into the loudspeaker you will also hear sound. For those of you following along in my posts you may recognize a similar pattern to this statement - yes, it's the same as DSD. Remember that DSD can be placed directly into a speaker (ignoring the watts it takes for the speaker to move) as can a sine wave as can a PWM or Class D signal. In fact, Sony's SACD process is very close to PWD or class D. Digital audio, on the other hand (known as PCM) cannot be placed directly into a loudspeaker - for if you do you'll only get noise. So, we'll cover more tomorrow, but to sum up - Class D is an analog process and not one that requires conversion to a discrete numeric representation like digital audio - and Class D is as close to DSD and analog as you will ever get. There are a number of shortcoming for Class D and we'll start to understand these soon, but at least now you get an idea how Class D should maybe be called Class DSD.
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Paul McGowan

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