3-band parametric EQ

Here’s my first attempt at a parametric equalizer. I combined parts of the graphic equalizer I did previously with information on how to make gyrators from R.G.Keen at Geofex. There’s a schematic and a vero layout, and I haven’t verified either yet. Updates to come soon.

Update (13.01.2011): Correctly labeled C1 as 150nF and not 150pF.

Update (24.02.2011): Built this circuit yesterday and I can hereby verify it as working.

Update (29.03.2011): Boxed this effect last night with a friend. This is my second take at using a transparent film for artwork and labeling, and it came out a lot nicer now. Only thing missing is a few (many!) knobs.

Update (20.10.2011): Fixed dead picture links.

Updated: 21 December, 2012 — 20:56


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  1. dave, you might be right. I haven’t looked at any of this for quite a while. Though the build I did seem to work quite well with the stated cap values.
    I’ll keep this in mind next time I revisit the EQ effects.

  2. The equation being f= 1/(2*pi*(SQRT R1*R2*C1*C2))
    Where r1 is the 470ohm and R2 is either end of the potentiometers, meaning 51k and 1.05M including the 51k resistor. If you leave C1 and C2 as nF they give quite outrageous values but when adjusted to pF they give realistic frequency values.
    Has any one built this circuit with the given values if so did it work.

  3. Hi there I was messing around with equations for the frequencies and found some surprising things with your frequencies. Are you sure your capacitors aren’t supposed to be in Pico farhads. When I changed them to Pico I got frequency ranges like 3-124 hz for the low range and 394-1789hz for the mid and finally 1029-4627 hz for the high range.

  4. Sorry for not getting back to you earlier, dale.
    First the pop; make sure you’ve got C14 oriented the correct way (rev.1 layout should be correct). Also, try connecting a 1M resistor from circuit input to ground.
    As for the 1044 I couldn’t say with any certainty, but I’ve heard several of these voltage ICs having high frequency noise, and supposedly the “S” prefix fixes this (do check the datasheet for the IC in question though). You might also be able to use a 7660 or an LM1054 chip.

  5. Hi Harald.
    Is the 1044 chip a special suffix. Other pedals say ‘only’ the 1044scpa on some builds.
    will any 1004 chip work or is there a good sub.
    Trying another eq…..fun fun fun

  6. Hi Michael,

    Boost/Cut pots set to the middle don´t change the sound significantly, but it did color the sound slightly (like most graphic eq when engaged). It is quiet in that position, and you can tell that moving up or down will add noise (again, just like any graphic eq…) – it is subtle and greatly benefits from tidy wiring.

    Q value – This pot is really interactive with Freq. When both are fully ccw or cw, Q value is low, so it will work on a restricted bandwidth. The more the pots are opposed in direction, higher the Q value, so it will work on a broader bandwidth.

    The maximum gain is relative… If the Q is low, it will boost/cut the given frequency with more power. If the Q is high, it will boost/cut with less intensity, but it will affect more frequencies. The perceived volume will be then, pretty much the same.

    Final note is that it won´t give you a really strong boost, but it can taylor your sound in really good way.

  7. Thanks André and Harald!!

    How does the Q pot affect the Q value? I assume using the calculator on http://www.muzique.com/lab/gyrator.htm is when the Q pots are set to zero? How do i work out what my Q range is?

    Also, roughly, how many db boost/cut does the gain pots provide? Am I right in saying setting the pot to the middle virtually ignores that band? I want to try and boost (0 to 8db’ish)


  8. As you say it should be pretty straight forward to figure out what sections to remove, but you may have to do a little bit of tracing. Both the schematic and layout is available, however, unfortunately for you I drew these before I saw the value in having the component labels match (I do stupid things all the time).
    Alternately, build all three bands, and just wire the pots for the one band you want to use. Leaving the other two there shouldn’t hurt.

  9. Oops, you mentioned the website already… Thinking of the low frequencies, r1 is 51k, r2 is 470 ohms, c1 is 150nf and c2 is 33nf… You figure the rest.
    The pots are there to give variations over the response, you can choose the values using the calculator, or just add some trimmers to help you tune things, which i highly recomend! Good luck

  10. Hi, great site!!

    I would like to use this circuit as a filter for one of my projects with a few alterations, but am a bit stuck.

    Will this work on line level inputs?

    I would like to change (remove sections) so that it becomes a single-band parametric equaliser, which seems simple enough as the circuit seems to be modular.

    I would also like to hard set the “frequency” and “Q” so that only the boost/cut is adjustable. I had a look at http://www.muzique.com/lab/gyrator.htm but am not sure which components link to what in the circuit diagram.

    Any help is much appreciated.

    PS: vero board is awesome.

  11. I’m still not comfortable with the theory side of things, but I’m thinking the input impedance is going to be IC || 1M || 100k, so less than 100k. But going a bit further I’m not so sure that 1M is needed at all since the 100k also subs as a bleed resistor. How about removing the 1M and just replace the 100k with 4M7? This would give you 4M7 || IC ~= 4M7 input impedance? Correct me if I’m wrong, please :)

  12. Hi Harald! Thanks for the great page, so many great stuff! Wish i have met you when i lived in Norway. Regarding this great project, i want to adapt it for using it with a harp mic. Crystal mics requires quite a high input impedance in order to keep the low end. The first 1 meg resistor is a pulldown, so i believe that the impedance is defined by the 100k resistor next to the 10uf cap. If i changed this to 4m7, it would probably be more adequate in my case. Do you think there would be side effects to the whole effect after it? Thanks a lot!

  13. Jimmie,
    “boost” on the board connects to lug #3 on all the “boost/cut” pots, same as “cut” on the board connects to lug #1 on the same pots.
    As for crosstalk etc. treat it as any high-gain effect when wiring it (FAQ for tips).

  14. Im currently wiring this one up and im wondering about J17 on the layout that says only “Boost”, where is that one going?
    Also, you mentioned to be careful with wiring this to avaid crosstalk, any specific things to think of, wiring certain wired togheter etc?

  15. I guess theres no demo of this one? :)
    Any idea what area the mid section sweeps?
    I would need something that i can boost around the 1khz area without boosting a range to wide.

  16. Hi Alex. Thanks a lot for the great offer, but I’m going to regretfully have to decline. I already have too much going on as it is and I likely wouldn’t be able to do it justice. But it would be a great idea having a collective space where everyone could add layouts. Hmm. Again, where’s the time…

  17. Hi Harald,

    It was for a friend and I don’t know much about bass either.

    I really love your website and I can’t thank you enough for it.
    I’ve been drawing some circuits using the same DIY Layouts program you use.
    Some are those that some people have requested you and I’ve already verified them (Marshall ShredMaster, Death By Audio Robot,…).
    If you want I can send you the original layouts files and, once you’ve double checked them and possibly corrected them, you can put your name and add them to the website.
    I wouldn’t want my name to appear on them.
    It would be just a way of thanking you for your great work.
    Please, let me know if you could be interested.

    Thanks Again


  18. You will probably need to swap out a few caps to shift the frequencies down. I don’t know that much about bass guitars. What frequency spectrum does a bass cover and what’s the center frequency?

  19. Hi,
    This site is amazing!
    I’m planing to build this for tone-shaping.
    Only one question: Could you please me about the Low/Mid/Hi Frequency Range?

  20. You should probably adjust the frequencies for bass, it’s set up for guitar as it is now. Let me think about it for a bit and I may add some modification instructions to the post.

  21. hi Harald,

    can i use this eq for bass guitar..?? how about the frequency range, is that dedicated for bass guitar too..??

  22. Hi, Stefan. Thanks for the input, but you have to excuse me. I’m not entirely sure what you mean. Do you mean the polarities or the fact that I mislabeled almost all of the components compared to the schematic?

  23. Hi,
    I noticed that C12 and C13 are interchanged in the vero layout.
    Best regards and thank you for this great site.

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