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Old 24th June 2005, 04:21 PM   #1
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Default Active Filter

I've already posted a thread on the car audio, but I couldn't get my questions answered. I plan on making an active filter to filter a signal below 250 hz. for my front speakers. Someone suggested I use a passive filter, but I can't find the capcitance of the high pass on the two-way speaker. First question, will the opamp serve as a buffer between the RC circuits, meaning the highpass in the speaker and the Rc circuit before the opamp? Second question, I want a gain of one on the filter, so will I need to place a positive/negative DC signal on VCC & VEE of the opamp? I appreciate your assistance.
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Old 24th June 2005, 04:48 PM   #2
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Not sure what you are asking. Are you trying to make a high pass to send everything above 250 Hz to your two way speaker or a two way active crossover?

Unity gain does not depend on rail configuration. Suitable op amps and circuitry can have unity gain from a single rail.

patience is a virtue - You'll get better results if you don't get insulting when your question isn't immediately answered, as you did in the car audio forum.
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Old 24th June 2005, 05:20 PM   #3
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Bob, I'm sorry if you take me saying I hope I didn't stump anyone as insulting, but if you noticed I added a second question to the thread. This was the goal of the second post.

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Hope I didn't stump anyone, the thing I have a hard time believing is that with all the semi-conductors in an opamp that the one cap could possibly overcome that. One other question, that hopefully will get answered. If I want a gain of one on the Opamp do I need to apply the positive/negative DC to VCC & VEE.
You jogged my memory, correct me if I'm wrong, I need to use the correct resistances to make the correct unity gain.

Also, all I'm looking for is a filter. Not an active crossover. I know the basic configuration I want, but I just would appreciate any knowledge on the subject.

Thanks guys.
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Old 24th June 2005, 05:31 PM   #4
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take a look at Rod Elliot's site www.sound.westhost.com for some filter info - this is an active crossover, of sorts, just that you only have the high pass section. Now you need to decide what order you need, and build on Rod's building blocks.

Unity gain opamp filters are easy - output ties directly to to the inverting input. an RC to the buffer non-nverting input gives you a second order filter. Rod has a handy dandy LR filter calulator on his site to help you choose the component values.

Just remember that for single rail operation you'll need capacitor coupling and a virtual earth.
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Old 24th June 2005, 05:35 PM   #5
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Thanks.
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