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Old 28th March 2004, 09:16 PM   #1
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Default Active lowpass filter problem

I have built a 100Hz lowpass filter using a TL081, and the diagram below.

I used the now non-existent online filter design on the Texas Instruments website.

I have built the circuit, and it works, but there is hardly any output from my amplifier when it is installed.
I THINK that frequencies above approx. 100Hz are being filtered out, but I haven’t checked it yet.
Before I added the circuit, my amplifier (OPA541) was very loud, but with the LPF, the output is very low.

I assumed that, as the filter was active, I wouldn’t lose signal like with a passive filter, but maybe I was wrong.

Help appreciated.

Matthew
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File Type: gif 100hz lowpass.gif (14.9 KB, 285 views)
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Old 28th March 2004, 09:40 PM   #2
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Well, what would you expect to hear when you've filtered out pretty much everything but the fundamental of the lowest notes on a guitar?

Rune
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Old 28th March 2004, 10:07 PM   #3
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Quote:
Originally posted by runebivrin
Well, what would you expect to hear when you've filtered out pretty much everything but the fundamental of the lowest notes on a guitar?

Rune
Its not that there is no high frequency output, but that there is almost NO output at any frequnecy.

Will the schematic work as I expect, or have I built it incorrectly?
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Old 28th March 2004, 10:37 PM   #4
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I'm afraid the answer must be that te schematic should work, but if you've built it correctly is a different issue I'd start by checking component values, and then wiring.

Rune
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Old 29th March 2004, 01:28 AM   #5
Optical is offline Optical  New Zealand
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You built this for a sub right? just makign sure you havent got things confused.. only bass notes will pass through the cct..

anyway, ive built sallen-key ccts before and found i needed a gain control cct after teh filter to amplify the signal as either it was attenuated or 100Hz and below wasnt what the speaker did best (if not a sub)

so my advice is to add a varible gain cct after that one to boost it up a bit
or increase the gain on the cct you have..
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Old 29th March 2004, 06:22 AM   #6
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Quote:
Originally posted by runebivrin
I'm afraid the answer must be that te schematic should work, but if you've built it correctly is a different issue I'd start by checking component values, and then wiring.

Rune
i have checked it, but i will do again.

Quote:
Originally posted by Optical
You built this for a sub right? just makign sure you havent got things confused.. only bass notes will pass through the cct..

anyway, ive built sallen-key ccts before and found i needed a gain control cct after teh filter to amplify the signal as either it was attenuated or 100Hz and below wasnt what the speaker did best (if not a sub)

so my advice is to add a varible gain cct after that one to boost it up a bit
or increase the gain on the cct you have..
yes, it is for a sub.
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Old 29th March 2004, 07:18 AM   #7
Nuuk is offline Nuuk  United Kingdom
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I recently built a controller including an LPF for a sub and thought that it wasn't working because I could hardly hear anything coming from the woofer.

However, when played with the main speakers, it had perfectly adequate volume. As has been said, you don't hear much below 100 Hz.
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Old 29th March 2004, 02:04 PM   #8
macboy is offline macboy  Canada
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Sounds like the opamp may be dead (or wired up wrong), so that the only signal that gets through the filter is essentially leaking through the feedback network to the output.

With a correctly functioning low pass filter with gain of 1, the deep bass should be the same intensity as before, midrange (e.g. voice) should be very faint, and treble (e.g. cymbals) should be virtually non-existant.
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Old 29th March 2004, 04:24 PM   #9
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Quote:
Originally posted by macboy
Sounds like the opamp may be dead (or wired up wrong), so that the only signal that gets through the filter is essentially leaking through the feedback network to the output.

With a correctly functioning low pass filter with gain of 1, the deep bass should be the same intensity as before, midrange (e.g. voice) should be very faint, and treble (e.g. cymbals) should be virtually non-existant.
I hadn't though of this possiblility, i will need to check this.

Thankyou for the suggestion.
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