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I'm one of those guys who doesn't use the tone pot ever so I decided to check into other ways to adjust the tone on my next project.

I'm building another all maple guitar (I love them) and wanted to use something similar to the EMG-VMC since I have one on my tiger guitar and love the way it works. The VMC is pretty expensive though so I wanted to see if I could build something similar. Lovekraft and Ansil have really inspired me to crack open the old electronics books and see what I could do (those guys rule!).

So, what I came up with was to build an onboard 5 band graphic EQ. It wouldn't make much sense to put it onboard if it were just a single channel EQ so I made it a dual channel. The idea being that you could then use one channel for each pickup (it's a two pickup guitar). That way, you could adjust the sound of each pickup without having to hit a pedal when switching between lead and rythm, ...

I got that working and was happy with it but I still wanted something similar to that VMC control. To accomplish this, I decided to build a simple parametric EQ with a frequency range of roughly 600Hz to 2.7KHz (you can boost or cut any frequency in that range). The frequency that I was really after was about 2KHz but having a range selectable was an added bonus. I plan to wire this into the bridge pickup only (I think).

The project isn't done yet but I'm so happy with the results, I wanted to share it with you guys and take the oppotunity to thank Lovekraft and Ansil for their inspiration.

Here's the pics:

The dual 5 band graphic EQ:


The parametric EQ:


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I haven't decided on exactly how I'm going to switch it yet. I thought it might be nice to just have a single toggle switch that completely bypasses all of the EQ's but I'm not sure yet.

I've also been thinking about being able to force a certain channel to be used (on the 5 band graphic EQ) via a toggle switch. Most likely, I'll end up keeping it simple with one pickup assigned to a channel and have one bypass toggle.

I have one more circuit to add - a boost similar to the EMG booster or the Seymour Duncan pickup booster.

It should end up something like this:

- Volume pot with push/pull for pickup selection

- Parametric EQ (dual concentric) assigned to the bridge pickup only

- Gain pot for boost circuit

- toggle switch for bypassing EQ's

- toggle switch for killing the boost circuit

3 pots, two toggles.

(I may eliminate the gain pot and just dial it in from the back - I don't know yet)

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umm - let's see - what I ended up doing was figuring out how most graphic eq's work and then scaled it all down to it's simplest form (to conserve space and keep the electronics theory at a minimum to save my brain from hurting :D). The basic element of the EQ (graphic or parametric) is a gyrator circuit. It's main purpose is to simulate an inductor. There's lots of examples of gyrator circuits out there but not many that really explain exactly how they work.

The gyrators I used were based on transistors instead of opamps. I'd like to try a board layout with opamps someday soon to see if I can still fit one inside a control cavity or not. The opamp gyrators are supposed to give much better performance so I think it would be worth checking into.

If you need schematics of existing eq's, let me know. I have a couple of them - one of them is the BOSS GE-7. I think I can fit that into an onboard unit but I don't know if I could make a dual channel unit fit.

The difference between a parametric and a graphic EQ is that the parametric is adjustable in it's frequency range. A graphic EQ has several fixed freq's that can be cut or boosted.

I think the ideal tone control for a guitar would be a mix of graphic and parametric EQ's which is what I've tried to do here. I'll let you know when it's all done if I've really succeeded or not. I've tested them individually and was honestly surprised that they actually worked as well as they did!

I'd like to make them available to PG members soon (if there's any interest). The price would be much less than what EMG sells their stuff for. I just need to make some adjustments such as adding a level control to the graphic EQ (and see if I can reduce the size of it) and some other small things. The parametric EQ is sweet but I really need to find a better pot for it. I need an exotic combination of 10K/1M which I cannot find anywhere. The one I'm using now is a 500K/500K which works but doesn't perform at it's maximum potential.

Thanks for the reply -


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Dave, you maniac, that's incredible! That parametric is just about the ticket - since you can't find the pot you need, you might consider scaling the circuit values to fit the one you have access to. It's not a trivial effort, and it can be a PITA, but it almost always can be done, at least with audio flters. Obviously, if you're planning on selling them you won't want to give up your schematics, but there's good info on impedance scaling in Lancaster's Active Filter Cookbook and on Randall Aiken's amp site in the Tech section under Tone Control Scaling. Alternatively, Alpha will make you just about any pot you can imagine for a minimum order of about 1000, at very reasonable prices - I got a quote on some push/pull pots this spring that was very competitive, but I couldn't figure out how to get rid of the other 985 pots. :D Gimme a shout if you need clarification on any of this - great work, can't wait to hear the results!

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Thanks lovekraft. I should have worded that a bit better - what I meant was that I would be willing to build these for PG members at a very reasonable cost (just price of parts plus time for soldering) - the way I worded it sounded like I was going to try to make a living off selling them which is not my plan.

I'll check into the info you just provided - thank you so much. Is impedance scaling the same concept as normalizing?

I did find a similar company that builds custom pots but they also had a $1000 minimum :D. If I ever come up with anything that really blows me away, I might consider doing that. In the mean time, I'll most likely be finding work-arounds.

Thanks for the inspiration - I've been away from electronics for so long. It feels great to be back in it again.

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"I could live without rock and roll, as long as i could still have the sex and the drugs" Nigel Tufnel, the mighty Spinal Tap

sorry about going off topic but..........dude your Spinal tap quote is incorrect. Nigel did not say that.

Mick Shrimpton : As long as there's, you know, sex and drugs, I can do without the rock and roll.


Edited by Lee
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Is impedance scaling the same concept as normalizing?
Yeah, you got it, normalizing is just scaling to an easy number, usually one, like 1k and 1µF, and then setting your chosen response by simple multiplication or division. You shouldn't have any problems doingthe scaling, but if you don't mind sharing your circuit, I'd be happy to help if I can - I've got a lot of time on my hands here recently, and I'm getting pretty good at using my simulator. B) Those homemade SM boards are pretty impressive! Wish I could do that!

I did find a similar company that builds custom pots but they also had a $1000 minimum  :D. If I ever come up with anything that really blows me away, I might consider doing that. In the mean time, I'll most likely be finding work-arounds.
That's why manufacturers spend so much money on engineering so they can use off-the-shelf components to keep production costs low - work-arounds are the American way! I just ask myself what Leo Fender (or Dan Armstrong, or Zachary Vex) would do, then check the budget and plow forward. When it works, it's great! Now stop, you're making the rest of us look bad! :D
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