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Old 12th March 2005, 10:59 PM   #1
akunec is offline akunec  Canada
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Default active filters: which resistors to buy

I am about to buy a lot of resistors (3000+) for my active filters, and I was just wondering which values I should get. On linkwitzs site there are equations for hp, lp, notch and peak filters and I was wondering what range of resistor values would be optimal for me.

Would I get lower noise/distortion with lower resistors and higher caps or higher valued resistors and lower caps?

Is there a limit to how small or large of a resistor I can use?
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Old 13th March 2005, 01:02 AM   #2
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keep you resistor values in the 20K to 50K range. 1% metal films will do fine.
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Old 13th March 2005, 01:38 AM   #3
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Linkwitz tends to use values between 2.7 K and 10 k in his designs. Modern opamps tend to be rated to 600 ohm loads, so the above won't stress them overmuch.


Francois.
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Old 13th March 2005, 01:47 AM   #4
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Yes, but by using higher valued resistors you can use polystyrene caps in many places.
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Old 13th March 2005, 07:50 AM   #5
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To Bill : And whatabout noise ?
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Old 13th March 2005, 07:59 AM   #6
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Quote:
Originally posted by Upupa Epops
To Bill : And whatabout noise ?
What about it? For an electronic crossover, resistor noise isn't much of a factor. It's just another balancing act. Personally, I'd take the polystyrenes at the expense of a few microvolts of noise.
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Old 13th March 2005, 08:00 AM   #7
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In an active xover noise is a major factor and I'd say keep the resistors under 22k (or even better 10k) if possible to minimize noise.

\Jens
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Old 13th March 2005, 08:05 AM   #8
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Quote:
Originally posted by Bill Fitzpatrick
What about it? For an electronic crossover, resistor noise isn't much of a factor.
Maybe so, but stray caps will have bigger influence if higher resistor values are used. I agree with Jens here, use 10-22 k max but beware of minimum values also for caps. The "opamp" or opamp may get increased distortion with heavy loads.
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Old 13th March 2005, 08:54 AM   #9
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Well, horse pucky. I'll guess I'll have to toss out my crossover then. And my volume control as well. No, wait, there's more bad news - I just looked at the schematic of my pre-amp; that's gotta go too. I never realized how noisy my system is till this very moment.
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Old 13th March 2005, 09:26 AM   #10
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Well I will try to explain my point of view.

Active filters, pre amps, and other small signal carrying circuits will contribute relatively much to the overall noise in the signal chain.

Bigger value resistors are noisier than small value resistors. This fact along with the fact that small resistors require bigger capacitors than small ones to generate the same filter function, mean that the overall influence by parasitic capacitance will be smaller.

Anyway, the question was the range of resistors required to make active filters, I gave my view on the matter, it might be different than yours. After all we all have different experience with this, and mine tells me to try to keep the resistor values down.

\Jens
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