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Old 20th November 2008, 12:29 PM   #1
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Question Filters - adjustable gain

Despite decent searching skills and access to the internet as well as a good university library, I have significant difficulties finding any information at all on this subject. What I'm looking for is not a graphic equalizer, that's easy enough to find info on, but rather a parametric one similar to those on mixing consoles. In other words, a low-shelving filter, one or more mid-range notch filters (low Q) and a high-shelving filter (or perhaps another notch filter), all adjustable to, say, +/- 10dB.

Now, the low-shelving filter is pretty straight-forward, but the rest of them have me stumped. I can easily enough design a notch filter that gives +10dB at a certain frequency. I can also design it's inverse (in other words -10dB at the same frequency), and use a blend pot and a buffer or two to get either 0dB to +10dB or -10dB to 0dB (though this doesn't feel like the optimal solution). But I can't figure out how to get the entire range -10dB to +10dB. Not without duplicating the filter and blending it with it's inverse anyway.

So, the question is: I want to sweep between a filter and it's inverse, is there a way to do this for any given (active) filter? Is it completely depending on the filter topology (if so, I'd especially appreciate an example of how to do it for a wien-bridge notch filter)?

Many thanks,
Stefan
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Old 20th November 2008, 12:43 PM   #2
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Will this do the job with some changes in values?

http://instruct1.cit.cornell.edu/Cou...Afinaldemo.htm
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Old 20th November 2008, 01:31 PM   #3
AndrewT is offline AndrewT  Scotland
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http://www.diyaudio.com/forums/showt...794#post837794
gives lots of info if you go to his website.
or
search for moamps as name and MOX for design info.
or
search for Jens as partial name and filter for a long list of info.
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Old 24th November 2008, 07:25 AM   #4
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Quote:
Originally posted by Frank Berry
Will this do the job with some changes in values?

http://instruct1.cit.cornell.edu/Cou...Afinaldemo.htm
I've seen something similar being used by Albert Kreuzer. It looks like three notches rather than actual shelving filters for the bass and treble (though I'm not entirely sure about the treble, perhaps some calculations are in order), but other than that it looks useful for better understanding active filters, if not useable straight away. Also, since the frequency is set by the caps, it would be tricky to make it sweepable (I have an intense and completely unjustified dislike for mid filters with switchable frequency), should I want to.

Quote:
Originally posted by AndrewT
http://www.diyaudio.com/forums/showt...794#post837794
gives lots of info if you go to his website.
or
search for moamps as name and MOX for design info.
or
search for Jens as partial name and filter for a long list of info.
Since the MOX is a crossover it's not what I was looking for. The first link however, might help some with the info on gyrators and active inductors, thanks.
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Old 24th November 2008, 07:41 AM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by StefanTH
Since the MOX is a crossover it's not what I was looking for. The first link however, might help some with the info on gyrators and active inductors, thanks.
sounds like you looked at the titles and could not be bothered to read the Moamps' and Jen's documents.
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Old 24th November 2008, 11:39 AM   #6
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Quote:
Originally posted by AndrewT
sounds like you looked at the titles and could not be bothered to read the Moamps' and Jen's documents.
I'm not quite sure what you mean by documents, but if you mean the actual contents of the threads, I skim through the first parts and if I can't find what I'm after, I leave it. Being a bit more pedantic this time, I found moamp's quasi-parametric eq on page 8. Because of the boost/cut switch though, it's really a rather poor solution (except perhaps for the particular application that EQ was intended for (loudspeaker design, as I understand). The interdependency of Q and f0 that Jens pointed out would be rather annoying too in any application outside the of the test lab.

Jens' parametric EQ, or rather the links provided in the thread about it, looks more promising. The gain part of it also works by pretty much the same principle as this adjustable wien-bridge notch filter I managed to find after much ado (note to self: need to find a better app than Illustrator to draw basic schematics ). Between these two, I definately have something to toy with. I'm not sure how to apply it to a high-shelving filter though, but if things were easy they wouldn't be as fun.
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Old 27th November 2008, 08:27 AM   #7
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Quote:
Originally posted by StefanTH
I've seen something similar being used by Albert Kreuzer. It looks like three notches rather than actual shelving filters for the bass and treble (though I'm not entirely sure about the treble, perhaps some calculations are in order), but other than that it looks useful for better understanding active filters, if not useable straight away. Also, since the frequency is set by the caps, it would be tricky to make it sweepable (I have an intense and completely unjustified dislike for mid filters with switchable frequency), should I want to.
My apologies, they are indeed shelving filters. A while ago, I did a rather sloppy simulation of a similar circuit I found which included some poorly designed surrounding circuitry that made the curves look a bit like notch/peak filters at a quick glance.
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