help designing a parametric eq pedal with molten voltage pedal sync midi

[lavastudios]

so after plenty of searching, i've concluded that nobody makes an eq pedal that fits my requirements, so i'm going to have to create something unique. i'm not an electrical engineer, and my soldering skills are pretty basic, so i will have to pay a tech to do the work, but i was hoping to get some help with the design from you knowledgable folks. basically, i want to have a 3 band fully parametric eq pedal, with hp and lp filters (pretty much like the pro tools default eq plugin in guitar pedal form), and it needs to be controllable via midi. the only way i know how to achieve this is with molten voltage pedal sync:

http://www.moltenvoltage.com/pedalsync/PedalSync_audio_control_MIDI_chips_modules_index.html

i emailed molten voltage, and they said i need the "four pots" modules:

http://www.moltenvoltage.com/pedalsync/documentation/PedalSync_MV-56_MV-56B_Four_Pots_Datasheet.pdf

most of the info on that data sheet is over my head, but i'm guessing one module can control four pots, so i'd need four modules to control 15 pots (high pass gain, high pass freq, high pass q, low gain, low freq, low q, mid gain, mid freq, mid q, high gain, high freq, high q, low pass gain, low pass freq, low pass q). can anyone recommend what parts to use that will be compatible with the "four pots" modules and be a great choice sonically? i could buy an existing parametric eq pedal such as the empress paraeq or wmd utility (if the parts are compatible?) and ad more components and rehouse it all in an appropriately sized enclosure, or go for a more ground up approach.

the most important thing is that it's a super high quality, low noise, musical eq. i'd be powering it with a cioks ciokolate, and i have a 4-15VDC (100mA) slot or a 9VAC (800mA) slot open to power it. this is all pretty confusing for me as i'm so clueless about electrical engineering, and the replies i've gotten from my emails to molten voltage haven't been very detailed or helpful (not a complaint, as i'm sure they're used to dealing with pedal designers, not clueless musicians). i'm not expecting anyone to design this pedal for me or tell me every little part required, i'm more looking for little suggestions or ideas that i might not think of and that my tech might not think of (he's mostly an amp repair guy), or even for someone to tell me that part of what i want to do isn't possible, if that's the case, so any thoughts like that i could get from you guys would be amazing!

cheers,

simon

[dschwartz]

What have you built before? Sounds like a very difficult project for a beginner

[lavastudios]

lego.  but as i said, i won't be doing the work myself, i'm just trying to figure out whether it's possible and what the considerations might be, and maybe get some idea about what the ideal components are so i can go in to see the tech with some idea of what i want.

[lavastudios]

uh... nobody?

[MrStab]

welcome aboard, Simon.

controllable via. MIDI? i can't really help you with that beyond conjecture, tbh.
parametric EQ with switchable shelving modes? i think i CAN at least help you with that part.

i can't go into it too much right now, but have a look at the basic parametric EQ design here: http://sound.westhost.com/articles/state-variable.htm, particularly Figure 5.

you may notice that in the actual state-variable filter stage, U1B inverts the signal (and is the HPF output), U2A inverts it BACK (and is the band-pass output), and U2B inverts it again (...and is the LPF output). so if the band-pass output has to be non-inverted, it follows that the shelving outputs have to be non-inverted, too.

take the output from either the HPF or LPF opamp, send it into a basic inverting op-amp setup, then feed that into the mixer (U1A) instead of the band-pass output. then, disconnect the Q pot from the main circuit at both sides.

i've tried it and it seems to work well, but i've not used it extensively. in summary: take shelving output, invert, send to mixer, disconnect Q pot. this can all be done with a 3PDT, assuming you only want one of the bands to be switchable to either the HPF or LPF, and not between all three on the same band.

i'll try to draw up a schem later if this is too convoluted.

[samhay]

>switchable shelving modes?
If you use gyrators for each band, you can switch between shelving and peaking modes by switching in/out the series cap like Figure 14 here:
http://sound.westhost.com/articles/gyrator-filters.htm

OP - It is fairly easy to design the audio side of this project using the approach shown in the above article, which is quite modular. It will probably pay to tweak components using a circuit simulator.
You can replace as many potentiometers with digipots as you like, but bare in mind that this will limit your headroom as these typically run off 5V rails. 
The midi side of thing might be able to be fairly easily implemented using the Molten voltage stuff. They should offer enough support to tell you whether this is feasible or not.

If you won't/can't build this, I suggest you find someone that builds large complicated pedals. This might be your amp guy, but many amp guys are not going to be comfortable debugging midi controllers, etc. You might find someone here that will do the design work for you, but this is not the sort of thing many of us are likely to want to build/invest the time in beyond the sketch stage.

[slacker]

What do you want MIDI to control? The Molten Voltage modules just let you save and recall presets, is that all you need?

[MrStab]

gyrators seem much more straightforward to implement, and for all intents and purposes, it's probably the best solution when weighing up ease of MIDI-control integration with functionality. however, AFAIK and IME, the parameters in gyrator EQs aren't independent of one-another and the Q changes as the frequency range changes. so that might be worth mentioning. loads of people use em and don't find it to be an issue, though.

[lavastudios]

thanks for the replies guys!

MrStab, i wasn't sure if switchable shelving modes were possible, mixing desks i've used in the past had separate LPFs and HPFs, i've only used switchable modes on plugin eqs. if it's simple enough to do it, it would be cool to have a one pot switchable between a low band and a HPF, and one pot switch between a high band and a LPF. but if it will add much cost (either in parts or in time for the tech) then i'm just as happy to have one HPF, one LPF (preferably both with gain, freq and q controls), and the 3 bands. and thanks for the link, i'll have a read of that tonight.

slacker, yep, i want to be able to save eq presets and recall them with my rjm pbc (a pedalboard controller with midi). i'll sometimes be running synth sounds through my pedalboard as well as guitars, so i want to be able to save all kinds of eq curves so i don't have to spend half my time on stage twiddling knobs.

samhay, the pdf from molten voltage says "Standard 5 volt and Extended 18 volt versions available" - should i be going for the 18 volt version? in the reply to my email the guy said "It's all very pedal-specific, but it's all do-able. It's that you need to figure out the appropriate technique for replacing each pot". still not sure which pots to use, but it definitely sounds feasible. my amp guy repairs pedals, i'm sure with the pdf sheet from molten voltage he'd be able to work it out, but if not i'll have to find someone else to build it. do you know what would be a fair price for someone experienced to work with me to do the design work?

cheers everyone, this is helping me start to wrap my head around the project!

[dschwartz]

You know what would be really cool?
A graphic eq with x bands, each band having a 10 step dual color led bar and a momentary button. Aside, a single pot and an edit/save button..

When you press a band button, the pot controls the boost/cut of that band, and the led bar shows the level set in one color..but when playing, the led bars shows the graphic response (like hi fi displays) in other color..

That would be a great programmable eq...

But sounds like a lot of PIC programming...

[lavastudios]

hey dschwartz, kinda like this:

http://payne-labs.com/joomla/index.php?Itemid=53&id=46&option=com_content&view=article

i was planning to get it, but when i emailed the company the guy told me it's not being made anymore. in any case, parametric eq is way more useful than graphic! but a visual display would be awesome, some sort of representation of the curve would be amazing, but i'm guessing that would make things a lot more complicated/expensive

[lavastudios]

update: i'm discussing the project with peter from vfe pedals. looks like it'll cost upwards of $1500. a lot of coin, but i'll be able to pay it off slowly while he's building it i guess!