Parametric EQ values?

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Once again, I turn to those more learned and better deducated than my self here in this forum.

I found a schematic for a full parametric EQ. This is good news, as this is what I was looking for.
Even better news still, I came across the schematic in Douglas Selfs book on small signal audio circuits, which means it should be a good circuit. I have the distinct impression that this guy, as opposed to yours faithfully, knowas what he's talking about.

The bad news is, that the schematic is for an EQ with a sweep range of 70-1200HZ, and I'd like a range of, say, 25-80 Hz for subwoofer equalization.

Even worse news still, I have no clue which components set the sweep range and how to calculate for the desired range.

any assistance in this matter will be greatly appreciated as I have a couple of room-modes ruining my otherwise pleasant DIY Hi-Fi experience..:(
 

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I remember i built a subwoofer EQ following a schematic from Elliot Sound Products. It worked very well and it had a modular construction. You might want to give it a try. Elliot also documented the schematic very well, you will be able to built it with your exact needs.
 
Hehe! well spotted! :) Deducated.. or educated by deduction (or assumption sometimes?) Sounds like I'm talking about my self here really.. :D Good one, I must remember that! :)

Anyways, thanks for looking in to this and chiming in!

Just to make sure I get it... if I multiply C1 and C2 by four, giving me 40nF and nF188, will this change the range to what I'm after?

Sorry if that's a stupid question by the way..

Rrrremus, thanks for the tip abourt that parametric constant Q EQ, actually looked in to that one allready, and I agree it looks promising, unless I can couple together a working parametric EQ, that's the next obvious candidate!

It would probably do a very good job of it, but I can't help thinking that being able to target the exact right frquency and adjust the Q would give the best posibility for targeting a room mode exactly..
 
Even worse news still, I have no clue which components set the sweep range and how to calculate for the desired range.
I too have been hoping someone with the knowledge and skills would tackle your request. It seems these circuits are usually provided with set component values, or (as with ESP) a table of values is given - even Rod Elliott admits calculating these values is difficult. I'm not sure myself of the difference, if any, between a multiple feedback filter and a state-variable filter. With the latter, my understanding is that the center frequency is approximately equal to the cutoff frequencies of the integrators, but Self's circuit has different values for each and I don't know what to do with that. Except babble:)
 
Well, babble is prety much all I can do as well, I don't even half know what an integrator is.. really hope someone can clarify the component value issue. If so, I'll get back with a PCB layout, I suspect I'm not the only one that's looking for a good tool for tweaking subwoofer response! :)
 
>if I multiply C1 and C2 by four, giving me 40nF and nF188, will this change the range to what I'm after?

It should. The center frequency is 1/(2 * pi * sqrt(C1*C2*R1*R2))
R1 and R2 here are the sums of the potentiometers and the resistors in series.


Do yourself a favor: download a free LT Spice program. Model your circuit and see for yourself what the values do.
 
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Ah yes, I was hoping for some formulae. That one seems to work - I get center frequency (fo) = 68.7Hz or 1080Hz with RV2-RV3 at the extremes. Pretty close to the 70-1200 stated.
Yes, multiplying the caps by 4 should shift the freq down by the same 4 factor, ie 17.5 & 300. Q should remain the same since that circuit portion isn't changed, The bandwidth = fo/Q so it will narrow, which is desirable.
 
Cool!

Now we're talking!

I do have a demo-version of LspCad Which I used when tweaking the active X-over for another project, I'll see if that will do the job. And now I know which values to fool around with as well!

17-300 Hz is not bad for my intended application, allthough I don't really need to go as high as 300 Hz.. lets see what we can do about that...
 
Yes! Got the simulation working!! At first, it only simulated noise. but then i changed the values of the two resistors emulating the cut/ boost pot from 50/50 to not qite 50/50 and then it suddenly worked!

Now I'll see if I can change the values of R11 and R10 to see if it is possible to narrow the Q a bit further at the narrowest setting.

I just love these simulators...
 
Oh.. I'm on a roll now...

The first thing I'll definitively change, is R11 this limits the maximum Q. changing this from 470 to 220 Ohm should be good, any narrower than that is probably too sharp for any practical use. I then changed C1 from 10 nF to 47 nF and C2 from 47 nF to 220 nF. In order to limit the range upward, I changed R3 and R12 from 6,8k to 13k. According to the simulations, this EQ should now have a sweep from about 15 Hz up to just over 100 Hz. which should be quite good for subwoofer fine-tuning! :)

The only problem now is to find a 4-gang 100k log pot as I would like to build a stereo version since I Have two subs! Something tells me that's not comming right of anybody's shelf..
 
Now for some results, did some screen captures of the transfer functions from different "potmeter settings" I tried in the simulation model.

As I simulated with various settings, there were no strange results, so in theory, the circuit should work with the modified values.

Now I have to make a circuitboard layout.
 

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