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Old 26th March 2012, 02:50 PM   #1
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Default Help with an active (OP amp) filter design?

Summary... I need an op amp based filter that is the opposite of a band pass filter, but certainly not as severe or narrow as a notch filter.

Background, FYI: I'm trying to make one magnetic guitar pickup sound more like another..This is for a small guitar with a built in amplifier / speaker that worked out amazingly well. However, in trying to minimize costs to make this a marketable product, I'm attempting to use one of the many cheap e-bay guitar pickups in place of the one made by Tyler Mountain I originally started with. The latter sounded absolutely perfect, and is still reasonably cheap as pickups go. But the company won't even talk to me about wholesale prices, and even if they did I'm sure it would still be a lot more than the average ebay price of $1!!! So in analyzing the difference, I found that the only major issue I have is that the cheaper pickup has just a bit too strong a response in the frequency range from about 140hZ to around 300hz. But I DO need the frequencies below that, so this is not a matter of a simple high pass filter.. I'm not sure how to do this kind of "anti-bandpass" filter.

Any help appreciated!
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Old 26th March 2012, 03:33 PM   #2
RJM1 is offline RJM1  United States
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Download Active Filter Design Application – FILTERPRO from Ti.com
Active Filter Design Application - FILTERPRO - TI Software Folder
and run it for a band stop filter.
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Old 26th March 2012, 10:22 PM   #3
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RJM1: I guess I'm not working it right, or don't understand the parameters. I chose a band stop as you suggested, gain = 1, center freq 220hz. The passband ripple I left at its default 1db. The passband I didn't understand really, but I tried the default 1000hz and also 10000hz, because I really want everything not in the stopband to be left alone. For the bandwidth (of the stopband I chose 120hz, and tried -12dB for an attenuation. Anyway, I can't go further because the program keeps telling me it would require a filter order > 12 which it can't do. I'm pretty sure if I had a 1/3 octave graphic equalizer in the circuit and dropped the 220hz channel I'd have about what I want, so I can't believe its impossible. What am I doing wrong?
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Old 26th March 2012, 10:59 PM   #4
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I guess I've gotten the FilterPro to work... but no matter what I do I can't seem to really get a response like I'm looking for. In fact the end result always looks more like a notch filter than I want, and even when i select a max attenuation of -12 DB, the actual curves once a circuitry design and response type is chosen dips down to like -60 gain at the center frequency. In other words, it wants to be a notch filter, even though all I want is a -12 over a specific band, from about 120hz to around 300hz. maybe a "stopband" filter isn't the right name for what I'm looking for?
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Old 27th March 2012, 12:36 AM   #5
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Correct; you're looking for peaking EQ.
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Old 27th March 2012, 01:58 PM   #6
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Quote:
Originally Posted by twest820 View Post
Correct; you're looking for peaking EQ.
I haven't seen "peaking EQ" as an option in that filter design circuit.

Anyway, I guess I still need help designing my audio filter.
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Old 27th March 2012, 02:12 PM   #7
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Hi ,
just an idea .
using something like this
IC BAND-PASS FILTER SDIP18 - BA3830S
then do the difference operation in the feedback path of an ...operational.

(something like karaoke voice elimination )
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Old 27th March 2012, 04:04 PM   #8
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Thanks pico! I'll look into that! I seem to have had a little luck in the past hour making the following Rupe-Goldberg approach. At least the response curve looks good on my spice simulator, though it doesn't do phase plots, so god know what a mess it will actually sound like. What I did was make a high pass filter with a passband starting around (120hz), and a low pass filter with a passband starting around 300hz and below. So I feed my buffered signal into both of them, and then sum the two outputs together a summing OP-amp stage. It looks like just the response I want, basically an all pass with a 12 dB dip around 220Hz! Its 3 op amps, so with the passives its more parts than I'd hoped to use, but at least i have a working starting point! :-)

Of course once I start looking into these new nifty all in one filter chips, that might inspire a whole new round of ideas, but I appreciate a helping hand from technology!
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Old 27th March 2012, 04:23 PM   #9
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probably the thing is far simpler ,as the quality of engineering is seen when
the solution doesn't bring over complication.
A Jfet and bipolar buffer at the input then something like a loudness-contour
tone stage and another buffer .
Curiously , the shape of a loudness control with bass and treble at full boost reminds me of a band pass
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Old 27th March 2012, 05:00 PM   #10
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Acutally, I wrote that backwards... the HP filter starts around 300Hz, and the lowpass starts around 120 and below. Otherwise I'd have a boost instead of the desired cut in the band of interest.
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