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Old 20th September 2007, 01:08 AM   #1
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Default Filter creation using a decade capacitor.

I just purchased a variable inductor (decade inductor), and I want a decade capacitor box, to create and experiment with the resulting LC filter created by the two.

My question is,....

For practical high-end use, should I use a capacitor "decade box" type,
or a variable "air capacitor" type.

I'm looking at older Gen Rad stuff for possible purchase.
I am new to filter design, so forgive any pertinent omissions.

thanks,
=RR=
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Old 20th September 2007, 03:33 AM   #2
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GenRad made a nice decade inductor. If you've got that or similar, it should be great for testing crossover ideas. Ideally you need a couple, but they tend to be very expensive and quite rare on the used market. They also made a ton of decade cap boxes in both mica and polystyrene. I'd get a couple of the polystyrene boxes; They aren't rare but still tend to be offered for too much money. Air caps are pretty much useless, as the maximum values are on the order of 1100pF from GR. I've made up some ganged units to 3-4000pF, but still not that useful for audio. With some switches and inexpensive polypropylene caps from Digikey, you can also build your own. Though the GR boxes are highly accurate, peoples claims that they were for calibration aren't really true. Back in "the day" they were just the benchtop decade RL&C boxes of choice, used for everyday prototyping.

edit/addition- you didn't say crossovers, so certainly you can use whatever values are suitable for what you're doing, and if air fits, it's the highest quality cap you'll find. Remember that the GR boxes are large, and the resistors are wire wound, so inductance won't be super low.
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Old 21st September 2007, 05:15 AM   #3
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Thanks Conrad.
Yes, I have a Gen Rad 1491-B with 3 dials....1H, 100mH, 10mH.

Maybe I will make my own cap box....hmmmmmm.....

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