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]

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]

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.

 

[mickeymoose]

Hm....if it is an acoustical guitar: why mess with the sound?
E

 

[TomHinton]

These small conductive plastic pots do not need a knob, they can be easily turned by hand with just the shaft. There are variations of the 3310 series, check them all to see which you might prefer.

3310C-001-502L Bourns Panel Mount Potentiometers

Also, you do not have to implement all 6 bands, just the ones you think you will need.

 

[dtleahy]

Hi Tom, and thanks for the reply.

Does it matter which acoustic guitar pickup and preamp are feeding the circuit? Some of them claim to be higher output than typical pickups, in fact I have a Shadow NanoMag and a DTAR Wavelength and they are both extra-hot output. Or, maybe the pickups go directly to the EQ circuit first, then to their preamp?

Dennis

 

[TomHinton]

Signal from pickup should go to the preamp first, then output from preamp should go to EQ unit.

If you begin altering the internal cavity of the acoustic guitar to mount electronics, won't this screw up the sound character by altering the resonant properties of the body?

 

[dtleahy]

Signal from pickup should go to the preamp first, then output from preamp should go to EQ unit.

If you begin altering the internal cavity of the acoustic guitar to mount electronics, won't this screw up the sound character by altering the resonant properties of the body?

Hi Tom,

Are you saying ideally that's the signal path? Pickup --> Preamp --> EQ

All of the active pickups come with (or need) a preamp, so they are often installed as just a pickup/preamp right to the output jack, with no EQ at all in the guitar. Then, players can plug into external EQ or effects. So, I know that often, the EQ is later in the signal path. However, I don't know what's ideal, or why. (I'd appreciate any insight.)

Yes, reducing the size of the guitar's interior will alter the resonant chamber. But in reality, if you threw 2 tennis balls into an acoustic guitar, and A/B compared with and without, to a blindfolded player, they'd have no idea. Acoustic guitar sides don't resonate. The top/soundboard does (that's maybe 90% of the guitar's sound) and depending on construction technique, the back may resonate as well. But the sides are stiff (like a speaker chassis or a banjo or drum.) Luthiers often report guitars are louder when the sides are extra stiff. Some even laminate the sides specifically to make them extra-stiff. So, attaching an EQ module to the sides will not audibly alter the sound.

Dennis

 

[wakibaki]

You might be interested in this:- Make your own piezo pickup | making guitar pickups

w

 

[TomHinton]

The signal path configuration I previously stated is optimal.

If you install equipment inside the guitar body, you decrease an already small internal volume, and the resonant properties will change, even if all sides were perfectly stiff. There is a resonate mode based solely on internal volume + opening area.

 

[dtleahy]

The signal path configuration I previously stated is optimal.

If you install equipment inside the guitar body, you decrease an already small internal volume, and the resonant properties will change, even if all sides were perfectly stiff. There is a resonate mode based solely on internal volume + opening area.

Thanks, Tom.

True, it is the Helmholtz frequency that will change. Still, I'd bet a sack of potatoes you and I could not pick out when a guitar had a couple of tennis balls thrown inside. There are some left-brained luthiers that would argue the point, but I'd still be confident I'd not lose my taters.

I think you'd need sensitive analysis of the waveform to determine the difference in Helmholtz frequency with and without the addition of an EQ module (that I would hope wold take up no more than a few cubic inches of interior volume, out of the available 600 to 800 cubic inches.)

Thanks for the info about the optimal signal path, as it does help to justify putting the EQ in/on the guitar rather than in an exterior box.

Dennis

 

[dtleahy]

I guess I did not refresh my screen, and several posts were made that I missed. Thanks for the replies!

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

Tom, I apologize for being thickheaded, but I'd like to revisit this. These pickups are active, and (as you mentioned about optimal signal path) will go through a preamp first. The D-TAR Wavelength even has a voltage converter to run at 18V rather than 9V (I don't know the wattage out, maybe just miliwatts?) So, even with active pickups going through a preamp, I still don't need to worry about the rated wattage of the pots? If I use pots with a lower rating, do I need to adjust any of the other components, and what would be the indication of a problem - distortion, or part failure?

Hm....if it is an acoustical guitar: why mess with the sound?
E

Mickeymoose, that's always a good question, and it has to do with how hard it is to mic a guitar on stage - especially if the player wants to move around at all. That's the major reason people want pickups in acoustics. Then, the success of the pickup is how much it sounds like a guitar with no pickup, only louder.

These small conductive plastic pots do not need a knob, they can be easily turned by hand with just the shaft. There are variations of the 3310 series, check them all to see which you might prefer.

3310C-001-502L Bourns Panel Mount Potentiometers

Also, you do not have to implement all 6 bands, just the ones you think you will need.

Tom, thanks, those might provide a better aesthetic on curved guitar sides than sliders. Most manufacturers that use sliders make (what I consider to be) a huge, ugly unit that drops into a rectangular hole in the guitar. Like this: http://www.chrisguitars.com/iban-aw20ce-used2.jpg

Whichever way I go, I want just the knobs or slits with sliders to be exposed. (I have given up on finding thumbwheel pots, as they have too low of power rating and vertical mount is difficult to source in small quantity.)

Yes, 5 bands may be plenty. I find 3-bands are inadequate at handling midrange (with just one pot.)

You might be interested in this:- Make your own piezo pickup | making guitar pickups

w

Thanks, wakibaki

I have a friend that has made his own for just a few dollars, and swears they sound the same as the $80 commercially available piezo pickups. I might try to make some sometime.

Piezos are the most frequently used solution on production acoustic-electric guitars, but every manufacturer of acoustic guitar pickups seems to fiddle around with circuitry in the preamp or EQ to try to kill the harsh, "quacky" sound of piezos. Supposedly, the D-TAR Wavelength solved it with an 18V preamp.

Thanks!

Dennis

 

[TomHinton]

The output of the preamp has nothing to do with the power requirements for the EQ pots - it is determined by the EQ circuit topology, and the one you have choosen uses a tiny fraction of a watt; you can use any pot you like.

The control pots for th EQ do not have to be mounted on the PCB, they can mount on guitar side with only shaft slightly protruding, then have short wires to the PCB. The link I sent for 3310 series have several mtg. options. Search Mouser for Bourns 3310 with no suffix.