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Thread: EQ circuit help - width?

  1. #1
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    EQ circuit help - width?

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    Hi Guys

    I'm just wondering what determines the width of an EQ band. In my desk (an old live girl), my EQ is at 150hz, 2.2khz and 8khz, +/- 15db. Behold the schematic:



    If someone could talk me through this. I know the bigger value caps are for the bass side and the tiny ones are the treble; so fiddling these values will move around the frequency (that said I'm not really down with the relationship between the two caps, but still)...

    My question is, I guess, what determines the Q or the width of the notch/boost? If it's the 10k or 220k resistors, could I replace these with pots and have a kinda semi parametric Q thing going on? For that matter, if I replaced the caps with adjustable caps, could I sweep it...?

    I've tried reading many an article but it's hurting my brain and I just need someone to spell it out a bit easier using what I see above if possible? Any help much appreciated!

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    I'm no expert but...

    I would imagine that a pot would sweep the ranges but you'd need some sort of control for the width of that Q sweep as well otherwise you'd end up with 3 exact replicas. Now, different valued pots (and the values I have no clue) would be my guess but I'm absolutely, by no means an electronics expert, but I've done my share of tinkering.

    A very interesting prospect through. I'm eager to hear how this works out if you go through with it.

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    I continue to be tired this AM. That topology is called Sallen-Key unity gain, which is a second-order high-pass filter. Except in this case, the output is not taken from the filter opamps; instead, they act as a variable load on the input signal, which is taken across a 10K resistor, thus forming a frequency-dependent voltage divider that is the opposite, or a high-cut filter, when the pots are on the noninverting side of the opamp. What I don't get is how the various bands act as they are labeled; it looks to me as if each band is the same, rather than the low-shelf/mid-notch/high-shelf you'd expect.

    I don't think Q can be made variable in this circuit, but I could be wrong. Given that you only have three knob holes . . . if I am right about the controls, you can swap resistors and caps (well, probably with different values) in the low control. Or maybe the high control. I'm tired Notch, I have to think about . . . I think you can swap one resistor and one cap in that circuit . . . anyway, you can probably tune it for three bands that suit your preferences, and use some "character" caps if you are so inclined. Or swap everything to nice films and better opamps than 4558 for clean.

    But first, do me a favor and plot what this filter does at min and max settings of each knob--send white noise to the board, and record the board's output.

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    Check here: http://www.ethanwiner.com/spectrum.html
    Scroll down towards the bottom to "Graphic Equalizers" and figure 5. There's your circuit. Pretty good details and a bunch of formulas for you. I'm too lazy to do any more. Good luck.

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    Yeah, Ethan's graphic has a cap in the output amp's feedback path and also on the noninverting input, which makes all those filters peak/notch. I didn't see that in the OP circuit, but I can't see the whole schemo either. That contributes to my confusion . . .

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    OK, awake now. I see that Ethan's caps are only 100pF, I think this AM I read that as 100nF. So never mind my earlier comment. I'm going to have to build this circuit and play with it--I don't quite get why a virtual inductor makes a notch filter, but I suspect it's because that first cap is a cap (duh!) and the rest of the opamp circuit hanging off is the virtual inductor. That makes a resonant filter! I was thinking it was just acting a second-order inductor . . .

    Anyway, my point about the type of filter: do you really want three bands of notch? I would say no, but that's just me. Essentially, what you have is the same thing as Ethan's graphic; you can play with the frequencies and the width, but I don't know if you really want to do that. For example, if you make the 220K resistor a 250K pot, you can change parameters in a single control, but that would not be a typical Q or frequency control, I think it would be both at once. Interesting, perhaps, but not straightforward.

    You know my vote is for shelf/notch/shelf and stick with three knobs. You could dump the opamps and replaced with real inductors for more character, but the required inductor values will be large.

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    Ethan?

    Is slowmotion another Ethan? If so:

    Hi Ethan

    regards, Ethan
    In Sunny Vancouver ..... (Washington)

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    Quote Originally Posted by evm1024 View Post
    Is slowmotion another Ethan? If so:

    Hi Ethan

    regards, Ethan
    I was referring to Ethan Winer . . . OP, I dunno.

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    Quote Originally Posted by mshilarious View Post
    I was referring to Ethan Winer . . . OP, I dunno.
    Ah, Of course - Now it is my turn to wake up!

    -Ethan
    In Sunny Vancouver ..... (Washington)

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    Thumbs up Cheers!

    Quote Originally Posted by evm1024 View Post
    Is slowmotion another Ethan? If so:

    Hi Ethan

    regards, Ethan
    Thanks, but I'm a Matt (one of the millions sigh)...

    Thanks for that link too guys, very handy. I do only have 3 holes to play with but I'm also happy to drill small ones for switches and such; if the resistors are... well both freq and Q, switching in other values with something like an on/on/on might be cool (rather than a pot)...

    Let's just be clear here though: I have no grand plan. Perhaps I should, but I'm also keen to bend it until it breaks, so to speak, and see what happens.

    Oh and no, I don't really want 3 bands of notch either, but then I'm not using this thing for much other than a big clumsy noisy patchbay so it doesn't
    *actually* matter. I'm a long way from building a whole new channel, much much more learning, reading and digesting!!

    Thanks again guys, much appreciated.

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