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Old 9th June 2005, 12:20 PM   #1
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Default 5th order filter chip

One project I have been working on for a while now is a low power satelite system for my office using 3 LM3886s one for the Sub, and one each for the powered satelites.

I have been looking for an easy way to have an adjustable low pass filter for the sub. Does anyone here have a good example they can point me to?

I was thinking of designing a circuit based on this chip:
LT1062 datasheet

Thanks,
Russ
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Old 9th June 2005, 01:15 PM   #2
macboy is offline macboy  Canada
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The device you refer to may work for a subwoofer, since high quality isn't paramount for that application. I would think that 5th order is a little steep and you might find that the transition between the sub and satellites is too abrupt.

You can use a standard analog 2nd-order active filter with a dual-ganged pot for the two R's.

Two excellent resources to help you to design an active filter are TI's FilterPro, which is a Winblows program, and Analog Device's Active Filter Synthesis, which runs in your web browser. Active filters use Resistors and Capacitors (but not inductors for practical reasons). Keep in mind that to make the filter variable you have to vary the R's (not the C's for practical reasons). For an n-th order filter, there are n R's and n C's. Since it is hard to find a pot that is more than 2-gang (2 pots controlled by one shaft), you are probably limited to a 2nd order filter (unless you can find a 3- or 4-gang pot). But 2nd order is probably sufficient, especially in this application.
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Old 9th June 2005, 01:27 PM   #3
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Very low passband noise for this chip --

I have used the clock-tuneable filter chips from Linear in a low noise oscillator -- these have some "clock-through" but they are very easy to use. (the "clock-through" is taken care of with a final discrete filter.)

you can, perhaps, the GC chip itself as a second order filter -- using Texas Instruments "filter pro" freeeware, setting the gain at 20. You can even do the Linkwitz transform by setting the Q's equal at 0.707. I have seen this "discussed" , not yet implemented. Remember, that if you do "second order" the phase's will be 180 degrees opposite for HP and LP so you will have to connect your speakers accordingly.
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Old 9th June 2005, 03:16 PM   #4
Nuuk is offline Nuuk  United Kingdom
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Russ, is this any use to you?
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Old 9th June 2005, 03:19 PM   #5
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Quote:
Originally posted by Nuuk
Russ, is this any use to you?

Good stuff Nuuk. Thanks so much.
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Old 9th June 2005, 03:41 PM   #6
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Quote:
Originally posted by macboy
The device you refer to may work for a subwoofer, since high quality isn't paramount for that application. I would think that 5th order is a little steep and you might find that the transition between the sub and satellites is too abrupt.
The THD+N for the LTC1062 is very, very good -- but wrt transient response there is a graph in the product PDF
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Old 9th June 2005, 03:47 PM   #7
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How does such a sub/satelite system work? does every channel need a filter? sub: lowpass, all other: highpass?

I also want to build a home cinema system and what i need is a schematic for this project.

Normally a dvd player has all needed output signals/channels, right?

mic
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Old 9th June 2005, 03:54 PM   #8
Mr Evil is offline Mr Evil  United Kingdom
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Has anyone tried using VCAs or maybe voltage controlled resistors (JFETs perhaps) to make a variable filter? I thought about it some time ago when I was going to make such a filter for my subwoofer, but I didn't need it in the end so never actually tried doing it. In my search for information I didn't come across any such designs; most of what I found on variable filters was using dual-ganged pots, which is not much good for anything above 2nd order, whereas a VCA/VCR filter could be any order using only a single pot.
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Old 9th June 2005, 04:19 PM   #9
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See analog devices AN281 and also their application notes on digital potentiometers for oscillators and filters.

jack
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