Paul McGowan: Our Hi Fi systems depend on power to operate and the power they need is DC – yet we feed them AC. Most of us are familiar with battery powered stereo products as they were all the rage for quite some time, but interest in them fizzled out and we plug our equipment into the AC wall socket instead.
If what came out of our wall socket was DC (battery voltage) then we would never need to convert AC to DC to make music. But imagine for a moment that DC was coming from your wall; what would the voltage be? 12 volts to run all your computers and mobile equipment? Fine, but you can’t then have enough voltage to run a power amp. Power amps need at least 100 volts DC and many bigger amps need 150 volts DC. AC is the only practical way to get different voltages to equipment in the home.
So AC it is but what kind of AC? Remember a few posts back I suggested that if you were to flip a battery around quickly going from + to – again and again, 60 times a second, you’d have AC? Well that simple explanation was correct but would make for horrible AC for our stereo systems. Flipping the battery like that would make a square wave. On, off, on, off. Looks like this:
If you put this into your stereo system it would sound awful. You’d hear a huge buzzing noise and be very unhappy. The buzzing noise happens because there are faster changes from + to – than 60 times a second and our power supplies aren’t equipped to handle these faster changes so they get added to the music we hear.
Instead what we want is a sine wave. A sine wave is the same thing you see in the picture of the square wave only with all the faster changes removed. A sine wave looks like this:
Notice how it is very gently curving up and down? That’s what our power supplies in our stereo systems want.
Just to make sure we all understand this very important fact, here is a picture showing a sine wave and a square wave laid on top of each other to make the point.
Notice how the two are the “same” but different shapes? The difference is important to understand because when we convert the AC to DC to make music, one works better than the other. So shape matters.
How did this sine wave come to be and why is everything we listen to in audio based on receiving this nice, gentle waveform? Was it because of some super smart guy like Tesla invented the sine wave with purpose in mind?
No, actually it was the other way around. The nature of how AC is generated creates the sine wave and our technological society adapted everything to it including our stereo systems. Sine waves weren’t invented, they are a result of a process. Soon I’ll explain how and the importance of the sine wave to making music.
