Feasible to make a line-level 3rd-order infrasonic filter? - Page 2 - diyAudio
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Old 13th September 2005, 07:53 PM   #11
Did it Himself
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I have just looked and the PCB is a 3rd-order filter with input buffer and output drive capability. It's a board I made a while ago, so you would have to be able to follow a hand drawn component overlay on a seperate sheet of paper as there is none on the board. A PSU is needed, but this could be very simple.

Probably a bit ineffective to send to the US, but you're welcome to it if you pay postage.
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Old 13th September 2005, 07:56 PM   #12
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Oops, you replied while I was editing...
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Old 13th September 2005, 08:52 PM   #13
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Default filter

I think you may find it hard to achieve much attenuation of 16Hz if you want to operate at 18-20Hz. I think you are looking at a clocked digital filter. I seem to remember using a very simple design using an LTC clocked filter device. They are easy to use and could be battery powered. You can obtain a 7th order design quite easily. Worth taking a look at the LTC website (Linear Technologies) I think the part I used previously was an LTC 1083 or something similar. I used this as a sub filter and it worked well.

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Old 13th September 2005, 09:15 PM   #14
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Steve,

7th order, wow.

I went to LTC site, 1083 is a regulator.

OK, found lots of filter IC's.

Looks pretty straightforward, I just hook up the external resistors, caps, and PS, right?

In the spec table they list F(0) max, I assume there's no low freq limit, correct?

Thanks

Noah
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Old 13th September 2005, 09:56 PM   #15
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Replying to your edits:

The passband will be droopy. This means that the roll-off curve will be relatively gentle because the Q is very low, with the effect that the passband will see some attenuation as you get nearer to the actual cut-off frequency.

You can reduce insertion loss by making the impedance differences smaller, but then the filter sections interact. Going active is really the only sensible thing to do with 2nd-order or above.

By all means try digital/chips, but a 3rd-order filter with Q around 0.8 might be just fine and is simple.

For best integration you would build a stereo infra-sonic filter and run it before your main amp, e.g. through a tape loop. That way the group delay/phase effects happen to all speakers so it's not so noticeable.
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Old 13th September 2005, 10:39 PM   #16
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Quote:
Looking at the schematic for 2nd-order HP at t-linespeakers.org, the amp impedance can be merged with R2, but there's nothing directly in parallel with R1 to merge it with.
I meant on a lone first order filter sorry, where R1 would lie in parallel with the amp input impedance.
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Old 14th September 2005, 05:18 PM   #17
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I'm pretty sure there is no low frequence limit and I apologise for the incorrect part number. Yes you are right about the external Rs and Cs. It's worth a go. As I said, I have used them as low pass filters for sub use, set as 50Hz high pass without problems. Some people may say they are a little noisy, but at these very low frequencies I doubt that it would be noticable.
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Old 14th September 2005, 08:45 PM   #18
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Richie,

"By all means try digital/chips, but a 3rd-order filter with Q around 0.8 might be just fine and is simple."

I'm confused by the "but"; you mean a 3rd-order w/the digital chip, right?

Steve,

What did you do for PS?

A wall wart on its own would have 60/120 Hz hum, right? I guess I could just put a big filter cap on it.

Thanks guys
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Old 21st September 2005, 10:22 AM   #19
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No I meant go digital if you want with whatever filter order you want,,,,,,,,but,,,,,,,the analogue filter you can build which is 3rd-order is simple and easy to build and will probably do the job fine.

Quote:
Some people may say they are a little noisy, but at these very low frequencies I doubt that it would be noticable.
Actually that assumption is not correct in theory. Precisely because the sub should only emit low frequencies makes the (higher frequency content of) noise even more audible, because it is not masked.

Quote:
A wall wart on its own would have 60/120 Hz hum, right? I guess I could just put a big filter cap on it.
I think you misunderstand how to build a PSU. You can't just use an AC wall wart 'as is' -- it needs to be rectified and filtered to get the DC. A very nice yet still fairly simple one is on my website if you can make your own PCBs. Or maybe I can find a spare board for that as well Note though that you can get away with a simpler PSU if needs be for this particular application.
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Old 21st September 2005, 06:35 PM   #20
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I meant a DC wall wart, but no matter, I bought an Alesis PEQ450 parametric EQ with HP filters built in.

Thanks
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