6-band graphic eq project component substitutions / improvements

[dtleahy]

Hi I just joined, and this is my first post here.

I'm pretty new to electronics and see that there are some truly advanced engineering wizards here.

I want to build a small graphic equalizer to be installed in acoustic guitars that I'm building. (The EQ units commercially available are not ideal for one reason or another, such as only being 3-band, or only mounting in a soundhole, use their own preamp, aesthetic issues, or being too expensive, etc.)

I found a free set of project plans here: http://www.generalguitargadgets.com/projects/18-eq/46-6-band-graphic-eq

This set of plans is for an outboard ("stompbox"?) unit, not meant to be installed within the guitar, but it seems like making a smaller circuit board and replacing the "heavy duty foot switch" with a tiny toggle switch will allow it to work inside a guitar. EQ on a guitar is not something that gets tweaked a whole lot. Once a player finds a setting to work well with a specific guitar pickup, that setting will probably typically stay put.

Pots:
6 or 7 pots is a lot of pots, visually, and could look really ugly sticking out of the side of an acoustic guitar. I'm looking at aesthetics. I don't want round pots (6 or 7 knobs sticking out of the side of an acoustic guitar doesn't sound too cool, and they would need a lot of space.) Slider pots were my next thought, but the travel is too long to work well on curved guitar sides. So, I thought of thumbwheel pots. (I could just make little slits in the guitar side wood to have the thumbwheels erupt through - a good aesthetic option.)

(I really need something like these, vertical mount, so I can stack 6 of them next to each other.)

However, oddly, readily available thumbwheel pots don't seem to come in a 1/4 watt or higher rating. But, trimmer pots do. So, unless someone can help me find 5K, linear, vertical mount, thumbwheel pots with .25W or higher rating, I'm looking at trimmer pots. Do you think these Bournes 3352W will work? 3352W1502 These are by no means ideal, because the thumbwheel is so small, and because trimmers are usually hidden components and not really made for show.



If someone would look over the list of components (here) and make any suggestions for components that would audibly, noticeably, improve the sound beyond what these components will do, that would be appreciated. For example, the unit uses a pair of TL074 quad opamps, but the bill of materials does not specify anything more about the TL074 components, and I see a large list of alternatives that (beyond the physical soldering leg configurations) I don't understand at all. And, in searching around trying to find out what the difference is between designations like TL074ACN and TL074BCNE4, I find engineers debating about other (more expensive) quad opamps that may be better - but then again, may not be noticeable in this application.

Any help will be greatly appreciated!

I guess the best outcome would be to find a source for bigger vertical mount thumbwheel pots, but at least I'd like to know if the trimmers will work, and finally if any replacement parts or specific variants of the listed parts should be considered.

Thanks!

Dennis

[wakibaki]

It's quite a demanding application. Trimpots are not ideal, because they are not built with the expectation that they willl be reset frequently, and they are of open construction, so contaminants can get into them easily. You will see that the rotational life of the ones you have linked to is 200 cycles (end-to-end and back).

I'd be inclined to go for a solution using digital pots and push switches. A push switch for up, one for down, and maybe two to cycle backwards and forwards through the bands. Then you need an indicator for the setting, although this is a problem with the trimpots, you may not be able to see the setting with them, unless you have markings printed on them. A single LED per band to indicate the active band and a bargraph to indicate the setting.

You probably want a recessed switch to lock the settings so they don't get changed while playing.

w

[dtleahy]

Originally Posted by wakibaki

It's quite a demanding application. Trimpots are not ideal, because they are not built with the expectation that they willl be reset frequently, and they are of open construction, so contaminants can get into them easily. You will see that the rotational life of the ones you have linked to is 200 cycles (end-to-end and back).

I'd be inclined to go for a solution using digital pots and push switches. A push switch for up, one for down, and maybe two to cycle backwards and forwards through the bands. Then you need an indicator for the setting, although this is a problem with the trimpots, you may not be able to see the setting with them, unless you have markings printed on them. A single LED per band to indicate the active band and a bargraph to indicate the setting.

You probably want a recessed switch to lock the settings so they don't get changed while playing.

w

wakibaki,

Thanks for the reply! Wow, quick observation on your part, and underscores how little I know.

Trimpots are out!

For this project, I would prefer an analog solution.

So, I need to find either sliders with short travel, or source the thumbwheel pots that I just cannot find.

Would it be safe to say, that for a small project like this where I need to find small quantities, if Digikey and Mouser do not have them, I won't find them?

Maybe someone makes very small round pots, and that would be a final consideration. (5k linear, 1/4Watt or higher rating)

Dennis

[DF96]

Might be helpful to ask the mods to move this to Analogue Line Level forum?

[dtleahy]

Yes, DF96, thank you. I stumbled into the wrong sub-forum after seeing "equalizers" in the sub-forum description.

Mods, could you please move this to Forums / Source & Line / Analog Line Level

Thanks!

Dennis

[Cal Weldon]

Moved.

[dtleahy]

Thanks, Cal!

So, does anyone have any other advice or input for this proto-newbie on this project?

Maybe a source for 1/2W thumbwheel pots, or short travel slider pots?

Thanks!

Dennis

[mickeymoose]

Bourns, Inc. have slide controls from 10 to 100mm.
Rather than a 6 band EQ I would go with a industry standard 10 band though.
E

[dtleahy]

Originally Posted by mickeymoose

Bourns, Inc. have slide controls from 10 to 100mm.
Rather than a 6 band EQ I would go with a industry standard 10 band though.
E

Thanks, mickeymoose!

This is going to be mounted inside an acoustic guitar (with just the knobs visible.) Most acoustic guitar EQ is 3-band, and 5 or 6 is really plenty (and should be a nice improvement in controlling midrange frequencies!) 10-band would be a bit of overkill right on the guitar, (visually and space requirements), but it makes sense if I ever make an outboard unit.

Wattage:
Can you, (or anyone) tell me: if a project calls for 0.25W (1/4 Watt) minimum rated pots, can I safely get away with 0.20W (1/5 Watt) rated pots? (I'm also curious what happens when the signal exceeds the rating...audible distortion? Or, part failure?)

If 0.20W pots can be used, then there are a few more options in linear taper slider pots that I can select from. (I notice that the length of travel affects the power rating, so the very short travel length slider pots won't work.)

Thanks!

Dennis

[TomHinton]

The pots in the circuit you are building use maybe a fraction of a watt - Any wattage pot will work for your application.