Capacitors, or caps, of course, are widely used in electrical circuitry, and range from the big metal canister variety containing an electrolyte (that sometimes leak in older amps and receivers) down to tiny ceramic disk types commonly employed in a variety of electrical circuits. Analog tuners employ variable capacitors as well.
In the case of amplification devices, normally the circuit is using direct current. In general the first thing that happens in an amplifier is the AC current from the wall outlet is converted to DC current. So the direction of current flow is generally critical in most electrical equipment. However, in order to drive a speaker, this DC current has to be converted back to AC current, not 60 cycle AC but a lower voltage AC where the wave of the AC varies with frequency and mimics the sound that was amplified. You can think of a speaker as simply an AC motor designed to pump air. If the current feeding a speaker werenít alternating, the cone would not vibrate and the speaker would simply go to the limits of its excursion and stop. Thatís what happens when you hook a flashlight battery to a speaker. The cone travels to its excursion limit (in or out depending on which way the battery is connected), a click or pop is heard and the cone stops moving. †Connecting DC current to a speaker voice coils is not a great idea, because you can easily burn the thin wires, but 1.5 volts of a flashlight battery is usually not sufficient to do any harm. When old amps and receivers go bad, often something goes bad wrong in the final stages of the circuitryóan† electrolytic cap in a big can fails, an output transistor goes bad or a similar catastrophe. When this happens itís not uncommon to blow a speaker, as high level DC current rather than AC gets shipped right out the speaker terminals. An amplifier when it clips sends out a current flow that looks much more like DC than AC current to the speaker coil, and thatís why even low wattage amps driven to clipping have been known to fry speakers.
OK, well inside any electrolytic capacitor is an electrolyte. An electrolyte is a liquid that conducts electricity. I havenít actually cut into one, but I am told that this stuff is more like a gooey gel than like water, in part so that it doesnít leak out. Old radios often used paper capacitorsólayers of a substance like wax paper along with layers of thin metal. The paper is soaked in the electrolyte and the whole thing encased in a waxy cardboard container not unlike a milk carton. Over time these things often deteriorate and leak, and are a major headache for restorers of old radios.
Obviously, Polarized capacitors are commonly employed in electrical circuits as the current is DC and the direction of flow is critical. But in a loudspeaker, NON-Polarized capacitors are required, because the current flow is in both directions so the speaker can move back and forth.
There are several different types of capacitors found in speakers. The traditional one, common in speakers built in the 70s and earlier, are NON Polarized electrolytics or NP caps. These are comparatively cheap and work ok in a lot of applications. They are especially cost-effective in instances where you need a large-value capósay 8 or more uF
I was playing around with speaker tweaking and decided one Saturday I needed some large value caps. So I pulled up the Radio Shack Web site and found their Electrolytic caps. There was no designation as to whether they were polarized or non-polarized, but from the photo they looked similar to the NP caps in the PE catalog. I went down to the RS store where the guy pulled the numbers I needed, I made my purchase and went home. As soon as I got I realized I had goofed. These were clearly marked for the direction of current flow with an arrow, and had a groove on one end but not the other. I had inadvertently ended up with some small POLARIZED . In retrospect I should have known because the rating on them was for 35 volts DC current, something you would find in a amplifier circuit but certainly not in a speaker!
The far right of the photo shows how Radio Shack packages theseóthey simply say Electrolytic capacitors not POLARIZED Electrolytic capacitors. Note on the small cap next to the package the arrow and the groove on only the one end.
Next to them in the plastic bag are NP Electrolytic capacitors of the variety commonly used in vintage speaker, thoough these came from Parts Express. Note that PE does NOT call these Electrolytic capacitors but rather simply Non Polarized. There are deep grooves on both ends.
Apparently, Polarized caps do strange things when fed AC. Aside from not working I am told they are a safety hazard. Whether they heat up, blow up or both Iím not sure.
I have been told that you can make a NP Electrolytic cap out of two Polarized Electrolytic caps, if you hook them end to end with the grooves to the outside, but I have not tried this as its way cheaper to just buy the NP cap. Iíve kept these Polarized caps around simply to do things like explain the differences here.
Modern plastics have had their impact on capacitors. The other types of non polarized capacitors commonly used in loudspeakers employ a Mylar or Polypropylene film. Mylar, I believe, is a Dupont-invented plastic that has other uses in audio. It came in big in the early 60s as a backing for audio tape. Its advantage over the backings that were used before was that it is much stronger and could be made into a sturdy tape from considerably thinner material. 1.0 mil and 0.5 mil was common compared with the old backings that were typically 1.5 mil. This meant that you could put a lot more tape on a reel for much longer recording times. The new 120 minute cassettes were 0.5 mil Mylar.
The disadvantage was that before it broke it often stretched. Before Mylar, tape backings deteriorated and often broke. But they broke cleanly without stretching first. As a result gre clean breaks could be simply taped back together Mylar was strong but it stretched before it broke, ruining the music on either side of the break. The fact that very thin pieces of Mylar can be made that are still sturdy would make it useful for making capacitors too
I tend to treat Mylar and Polypropylene NP caps as pretty much interchangeable in building loudspeakers. But this is one of those topics where Iím sure people have different opinions on. No doubt the properties of these two materials differ in some respects, tho both are plastics. The yellow capacitor on the photo is a 3 uF one I picked up on a Madisound surplus page. Iím almost certain itís polypropylene not Mylar. Madisound refers to this as ďT.I AxialĒ whatever that means. †I think itís pretty similar to a Dayton or Solen Poly cap. I donít ever see Mylar caps in the PE catalog, but Madisound always seems to have tons of them for sale usually at prices under a dollar. The greenish and brown caps are shown here are Mylars. The Mylar caps I have seen all are squashed and are oval in profile. Often they are not labeled very clearly as to value. The green one here is 2 uF, the larger brown one is 1.5 uF but rated for 400 volts. I paid 35 cents each for the green one and 30 cents each for the yellow and brown onesóDavid L. Debertin.