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Capacitors

— By Paul McGowan —
I wrote in yesterday's post about Direct Coupling audio circuits; a practice that started in the late 1970's and continues through today. Direct coupling describes a direct connection between two pieces of audio electronics without an intermediary component such as a capacitor or transformer. Why should that matter if there's a capacitor in the signal path? Because less is more in audio, certainly if you're not giving up anything in the process. We all like listening to shorter paths with fewer components between the music and our ears. Most solid state audio products manufacturers direct couple many of their products today without much fanfare; pretty much a given for high end audio products. Tube audio manufacturers are the opposite; they routinely require either a capacitor or a transformer to couple the tube's input and output to whatever the user wishes to connect to. The main reason a designer would want to include a capacitor is to block DC and allow only AC to enter or leave the audio device. As you will remember music is AC, meaning it must continuously move between + and - to force the loudspeaker cone back and forth so we can hear sound in our room. Put DC into a power amplifier and the woofer cone of your loudspeaker will jump forward and stay pushed outwards as the voice coil of that speaker heats up and then burns out; not something we're interested in. Capacitors will not pass DC and will only pass AC, which is a nice feature for a musical amplification device - since we certainly want nothing to do with DC going anywhere in our musical systems. So, what is a capacitor? Two conductors and an insulator. One conductor is on the input of the capacitor and the other forms the output of the part. The two conductors are separated from each other with an insulator, sometimes called a dielectric. For those of you following these posts this arrangement might sound familiar - and that's because it is - we call the other device a transformer. Neither the capacitor nor the transformer will pass DC but both will pass what we want, musical AC. So far so good. If you remember, a transformer is two coils of wire separated by an insulator (insulators won't conduct electricity - think wood, plastic, paper). Just like our friend the capacitor, there's an input side and an output side. Place a signal on the input side of a transformer and it becomes a magnet - generating a magnetic field - that is then sensed by the output coil and converted back to its original electrical form. The key to this device is that we removed the physical connection in our signal path - and replaced it with a magnetic connection that travels through air. Were you just to have a wire between a source with both DC and AC present at the same time, you'd get both passed right on through. But with a transformer or capacitor between two pieces of equipment - only AC makes it through while DC stays behind. Capacitors are similar but instead of generating a magnetic field, they generate an electrostatic field. Place a signal on the input conductor side of a capacitor and a small electric field is developed (think of the charge you generate and store on a hot dry day when you get static zapped touching something). The field hangs out in space (just like it does in you when you're all charged up) until the second conductor is attached to a different potential on a battery (think of the difference between + and - of the battery) and then the field jumps across the insulator and goes to the output conductor completing the transfer of energy, Here's a picture: Again, just like the transformer, we've eliminated the physical connection (the wire) between input and output and our music travels merrily through the air leaving the unwanted DC behind in the dust. The amount of energy storage and the frequency that can excite this electrostatic field is dependent on the size of the conductor and insulator's surface area; more surface area, lower frequency and great energy transfer/storage. Tomorrow I'll explain why we need this at all, then we can discuss the various types of capacitors that you've no doubt hear of like film caps, electrolytics, oil and paper, etc.

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