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Yesterday I asked how it could be that a switching power supply can regulate its output without yet another power amplifier being inserted into the supply?

We know that to regulate a traditional power supply of a power amplifier (something we'd love to do for best sound) we need to add a second power amp just for that task - making our power amplifier far more expensive and large.

Well, the answer is that it can't. It too needs its own power amplifier but in a switching power supply, that feature can be added with very little trouble and without many new components. I know that probably is confusing so let me explain.

If you recall how a switching power supply works, there an electronic switch between the SMPS (switching power supply) and the AC wall socket. This switch is turning on and off very fast so that the incoming AC is chopped up into little pieces easily "digested" by a very small transformer. That's how come SMPS are very small and efficient in their designs. Slower AC needs bigger transformers, faster AC smaller.

But now imagine that this switch on the input of the SMPS is variable, rather than just a a fixed on/off switch working very fast. And let's further imagine that the variable part isn't the frequency (how fast) of turning the AC on and off - but rather how long each switch stays on.

To picture this, think of a train moving down the tracks at 60 MPH. The speed of the train is quite steady and fixed - this is the same as our switching frequency. Now imagine that the train is made up of different length cars: some extremely short and others extremely long with everything else in between. If each car is filled with coal and our imaginary train is dumping that coal in one spot as it is moving, then the flow of coal delivered by the train would be variable. Short cars don't contain much coal while long cars contain a bunch of it.

Lastly, imagine that we have a train yard master at the beginning of the train's journey who arranges the order of the cars to meet the demands of the coal hopper collecting that coal. When the coal hopper needs a lot more coal the yard master has made certain a long car is dumping coal and when the hopper's pretty full, only a short car delivers its load.

This is what's happening in the input of our SMPS. As our power amp demands more power to make our loudspeakers play, the smarts inside of our SMPS says "we need long cars of power" and when the amp is just idling "we need only small cars".

So the input switch that speeds up the line frequency from 60Hz (60 times a second) to 100kHz and permits tiny transformers to be used serves yet another function if you add some smarts to the system. Cheap switchers don't have these smarts because they're expensive - but well designed ones do.

And oh, by they way, this long and short train idea? There's another name for this trick you may be more familiar with: Class D.

Yup, we've just described a Class D amplifier and the technology of longer and shorter "cars" is called PWM or Pulse Width Modulation.

PWM sounds realm technical, eh? Well maybe just call it CLM instead.

Car Length Modulation. Same thing, easier to understand.

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Paul McGowan

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