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Audio filter

Regarding capacitor selection for an audio filter, I discussed polyester capacitors with a friend and she showed me a book on the subject. It points out that a problem with polyester capacitors is their poor temperature stability.

In a filter, to some extent, you don’t want the frequency shifting with temperature changes. According to that book, polypropylene capacitors are about as good as polystyrene and they reckon they’re better in all three categories than polyester (accuracy, temperature stability and leakage).

Any comments on this? I would like to know whether there’s any reason to pay the extra that polypropylene costs or should I stick with polyester in most applications? (N. V., via email).

• We don’t think temperature stability is particularly important for audio circuits, with the possible exception of notch filters.

To get an idea of the significance of temperature stability, have a look at the likely capacitance variation. Polyester has the highest temperature coefficient for a plastic dielectric capacitor but if you look at the variation from 0°C to 70°C, it is likely to be less than 1.5%, ie, not worth worrying about for the vast majority of circuits. Yes, such a variation will shift the corner frequency of a filter but not by much.

On the other hand, we would avoid using any type of ceramic capacitor (disc, tubular, monolithic etc) for filter circuits as they are definitely inferior to plastic dielectric capacitors.

USB charger
for motorbike

I need to charge a GPS device from my motorbike via a Mini-B USB plug. I was thinking I would get a normal cigarette lighter charger with the cigarette plug on one end (with a built-in 12V-to-5V converter) and USB on the other end. I would then cut off the USB end and solder on a waterproof female USB A plug, which comes with a cap for keeping it waterproof when not in use.

At the other end, I was thinking about taking the cigarette lighter part apart, waterproofing the lot within some resin and adding a small fuse before it and connecting it directly to the battery. This avoids any cutting of wires (voiding the bike warranty) and makes the whole thing easy to remove later.

My question is, will connecting one of these 12V-to-5V converters directly to the battery always drain a small amount of power, even when there is nothing plugged into the USB cable? If it will, what is the likely current drain when not in use? I do not want this thing to drain the battery when the bike is left in the shed for two or three weeks.

The only other option is to put a switch inline before the 12V-to-5V converter or cut into the power cables after the ignition switch (cannot do due to voiding warranty). (D. E., Ainslie, ACT).

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