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Old 14th February 2012, 08:37 PM   #1
Elbert is offline Elbert  Norway
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Default Parametric EQ values?

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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Old 15th February 2012, 05:31 AM   #2
alexf is offline alexf  United States
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Multiply the values of the C1 and C2 by 4 and the frequency range will shift accordingly.

P.S. That's a nice typo: "deducated". Must mean both educated and dedicated
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Old 15th February 2012, 05:40 AM   #3
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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.
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Old 15th February 2012, 06:54 AM   #4
sesebe is offline sesebe  Romania
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Hello rrrremus,

I'm interested to view this schematic. Can you post the schematic or provide a link for it?
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Old 15th February 2012, 07:00 AM   #5
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Yes Sesebe. You would have found it easily with a search, but here it is : ESP Projects Pages - DIY Audio and Electronics - Crossovers and Effects
I recommend project no. 84
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Old 15th February 2012, 02:35 PM   #6
Elbert is offline Elbert  Norway
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Hehe! well spotted! Deducated.. or educated by deduction (or assumption sometimes?) Sounds like I'm talking about my self here really.. 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..
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Old 16th February 2012, 12:34 AM   #7
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Quote:
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
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Old 16th February 2012, 08:32 PM   #8
Elbert is offline Elbert  Norway
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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!
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Old 17th February 2012, 12:05 AM   #9
alexf is offline alexf  United States
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>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.

Last edited by alexf; 17th February 2012 at 12:21 AM.
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Old 17th February 2012, 06:49 AM   #10
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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.
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