Making effects wah controllable?

[mattpocket]

Firstly, someone has asked to build them a high pass filter with the filter range controlled by a wah pedal. This they want to use to fade in a song from high trebley fizz to normal sound. Toe down = more bass cut, Heel back = less bass cut

I though that a LofoMofo could be modded for this use. However, I am concerned that the effect will not allow for this. I mean that when the wah pedal is heel back the sound of the guitar needs to be unchanged. Is there a way to do this. What does the footswitch under the way do? Is it a toggle switch or momentary (i.e. only disengages effect whilst in heel back position)

Does anyone know if the grit knob of the lofomofo will allow sound through unchanged when turned all the way to the left.

Any ideas?

[mattpocket]

I found the above in arons FAQ, its a high pass filter (cuts bass) if I used a different cap value would this be able to go from no bass cut to really high bass cut. Also, would there be frequency change at all times? I want no frequency change to occur when in the heel back position.

Or would there need to be some setup like this:



Where one cap would greatly cut frequency and one would cut little to no frequency?

Any ideas?

[Mark Hammer]

I did this some 20+ years ago.  Took the Super Tone Control from EPFM I and built it into a wah/volume shell, with a switch that flipped the outside lug connections of the required dual-ganged 100k pot so that I could sweep down by moving my foot forward, or sweep up.  It made a fantastic swept filter.  One of my favourite tones involved sweeping the highpass filter downward, so that you start with a thin mosquito tone and quickly transform into full bandwidth.  The only recorded example I can think of off the top of my head is that old tune by The Fixx called "One thing leads to another", which appears to use a similar swept filter.

In truth, it doesn't have to be that particular circuit but can be any state-variable filter with low, high, and bandpass outputs.  A large proportion of parametric EQ sections are often built around such filters with only the bandpass output used and the low and highpass ignored.  Finally, as amply demonstrated by Craig Anderton, one of the neat side effects of using a state-variable filter is that yuo can combine the low and highpass outputs for a notch filter.  Note that a footswept notch filter is approximately equivalent to a footswept Phase 45 pedal.  Not teh best phaser ever, but a nice add-on for the price of a toggle and a couple of resistors.

Some qualifications, first.

Dual-ganged pots are not necessarily the ideal for footswept pedals.  Often they won't fit in the available space, or come with collets long enough that they can be properly secured to the bracket intended to hold them steady.  My gut sense is that they probably undergo more stress than single pots, but I may be wrong about that.  They also tend to be less available and in as many taper and resistance-value choices as single pots.  Finally, adjusting the range and sweep width or foot-to-frequency-movement ratio is complicated for dual-ganged pots.

Do you need to use a dual-ganged pot?  Not necessarily.  The Mu/Q/Neu-Tron amply illustrates that.  What is critical in sweeping the centre /cornerfrequency of a state-variable filter is the simultaneous adjustment of two resistances.  That could be done with a dual-ganged pot, but it can also be done with two LED/LDR combinations controlled by a single pot governing the current fed to the light source.  The nice thing about that arrangement is that you can stick trimpots in series with the two outside lugs of the foot-controlled pot and adjust how foot movement translates into some amount of frequency sweep.  You can make the pedal as "twitchy" or "twitch-proof" as you want.

State-variable filters can also be implemented using transconductance amps (CA3080/3280, LM13600/13700, CA3094, etc) instead of LDRs, where the single control pot governs the Iabc current going to the OTAs.  Basically, the sort of multi-mode filter module seen on your basic 1978 modular synth, but with foot-control only, rather than inputs from envelope generators, LFOs, keyboard tracking, etc.

[Fleetdog]

Since you said you wanted "high trebley fizz", it makes me think bass cut plus distortion of some sort.  So in my mind, that's like doing a high pass filrter and a fuzz and you want the wah to change the intensity of both of those effects.  What about making the wah type enclosure a foot controled mixer/effects loop.  You could put any effects you wanted in the loop and use the rocker pedal to mix dry signal with the wet signal comming back from the loop.  Heel down would be all dry signal (bypass) and toe down would be all wet (in your case a high frequency fuzz). 

Actually, I would probably reverse that so toe down is bypass that way you could use the button on the toe end of most wahs to do true bypass.

If you're just looking for a bass cut with no distortion added, the top schematic you posted should get you close.   

[Transmogrifox]

Do you need to use a dual-ganged pot?  Not necessarily.  The Mu/Q/Neu-Tron amply illustrates that.  What is critical in sweeping the centre /cornerfrequency of a state-variable filter is the simultaneous adjustment of two resistances.  That could be done with a dual-ganged pot, but it can also be done with two LED/LDR combinations controlled by a single pot governing the current fed to the light source.  The nice thing about that arrangement is that you can stick trimpots in series with the two outside lugs of the foot-controlled pot and adjust how foot movement translates into some amount of frequency sweep.  You can make the pedal as "twitchy" or "twitch-proof" as you want.

Speaking of manually sweeping the state variable filter....I have had some marked success feeding white noise into a wah pedal, then the wah into an envelope follower.  There just need to be a few controls on the EF output to adjust DC bias conditions so that you can "0" the pedal at heel-down minimum frequency.  The easiest pedal to mod for this kind of effect (sorry a little OT) would be the Maestro S/H.   You'd add an output jack for noise-out, and an input jack switched to the envelope follower for noise-in.  Probably need a trimpot to adjust the noise amplitude on the output to the wah.

[mattpocket]

Right. I think it will be best if I just do a low cut filter with no effect (distortion or fuzz) and leave that to any pedals that the person would have in his normal chain.

I dont really understand why I need a dual gang pot. My frequency sweep will go from full bandwidth to an extremely high treble.

Can anyone dumb this down for me?

Matt

[Fleetdog]

Would it work to start with a passive volume pedal (just a case,jacks and a pot really)  then put the input on the wiper of the pot, the output on one end of the pot, a cap from the input to the output, and the other end would go through a regular reisitor to ground?  At one end of the pot sweep, the wiper is touching the outside lug shorting out the cap and connecting the input directly to the output.  At the other extreme, the cap would still let highs through while the pot and resistor would act as a voltage divider on the lows cutting most of them out.

A schematic would probably serve us well here, but I don't have any way to draw it right now. 

[mattpocket]

Fleetdog, that sounds really good, but I am not able to say whether it would work (I dont have the knowledge). Someone else opinion would be great on this.

Matt

[Transmogrifox]

If I'm understanding you right, then what you have right now does what you want it to do with a pot, and you just want to make it controllable by using a wah pedal.

Questions:
Are you wanting to gut a wah shell and just use the pot that comes with a wah?
Do you want the original wah pedal to still be functional?
Or...
Do you want this thing in a box that you can plug the wah into, step on a button, and it's a wah, stomp the button again, and it's the effect you're describing?

It's hard to give suggestions since I don't feel like the objective is very well defined.

For example, when I think of wah controllable, I literally think "Wah Controllable"--meaning you plug a standard wah pedal into the box and the box does some "magic" to control the effect with relationship to the pedal position.

...but...you may simply mean that you want the effect to be controlled by a treadle, just like a wah pedal works...and this implies that you have a wah pedal that you don't mind gutting so you can replace the effect with what you're describing.

I hope I haven't typed too much so to confuse you.  What it sounds like you're asking to do appears to be pretty simple, but it can turn into something really complex if you let us geeks have our way with it.

[mattpocket]

Right, I will clarify.

I have not yet made the effect I describe, and I have not yet decided whether to put the whole thing in a wah, or buy a volume pedal and modify it to control the effect "remotely".

The basics are pretty easy.

I have never done anything with a wah/expression pedal DIY wise and I am unsure of a few things, but I am guessing a wah pot still has 3 lugs on it which I can connect to my chosen cap across it as in the first schematic I posted at the top.

What I am unsure of is if the high pass filter will work as I want it to. i.e. will I be able to get a really high treble/no bass sound at one end of the pot travel and be able to sweep back to full bandwidth at the other end. It is imperative that I get full bandwidth when the pedal is at one end of its travel. I think it will be pretty easy to get this by carefully choosing cap values, but will I get enough bass cut as I specify? Will it be too twitchy? Maybe the schem is too simple for my needs?

Maybe an EQ setup would be better, so that the pedal can be manually adjusted to set full bandwidth to a desired level and then just have some kind of setup to remove all the bass and mids with the expression pedal.

Maybe I should start another thread about creating a high pass filter?

Matt

[Transmogrifox]

I found the above in arons FAQ, its a high pass filter (cuts bass) if I used a different cap value would this be able to go from no bass cut to really high bass cut. Also, would there be frequency change at all times? I want no frequency change to occur when in the heel back position.

I  think this one would do what you want if you made the cap small enough.  You'll have a volume drop in the full-bandwidth state, so you'd want to do a gain recovery stage, most likely.

You can use a wah or volume pedal pot exactly the same way, so if you can come up with something that works with a 100k pot on your breadboard by hand twisting, it will work just as well in a wah shell.

Perhaps if I have time I'll post a schematic to some sort of active filter that would not require gain recovery.

[mattpocket]

Perhaps if I have time I'll post a schematic to some sort of active filter that would not require gain recovery.

That would be AMAZING if you could!

Otherwise would a clean boost do the job after the filter? I'd exchange the boost pot for an internal trim pot and play around with it until there is no volume drop when I A/B the pedal.

Also, I am thinking I am going to make the pedal in a hammond enclosure and have it be controlled via a volume pedal. You might not think that it needs this but it will make it possible for me to control other pedals that I may build at a later stage. How would I go about this. I would like to do it like on a boss auto wah. But these use a single jack out where you can plug an expression pedal in. Do you do plug a standard volume pedal in here or do you need a different kind? A volume pedal has an in and out, but these autowahs only have an expression out. How does it work? Would I need to modify the volume pedal for this use?

Matt

[Transmogrifox]

Try this one:

http://www.geocities.com/transmogrifox/LowCut.JPG

It's a dual op-amp deal, buffered, unity gain, in phase when the pot wiper is all the way to the last opamp output.

Sorry it's so large.  Follow the link below the image if you have trouble reading it in the post.

Your idea of housing this in a hammond box with an external espression pedal input is a good one.  For the pot, use a stereo jack on the wah or expression pedal or volume pedal (whatever you use).  Tip and ring should be the outer lugs, and the sleeve can be the wiper.  Make the ring the lug that connects to the op amp output so you are able to somewhat reject external noise radiated onto the connection between expression pedal and hammond enclosure.  DO NOT GROUND ANY OF THE WIRES FROM THE EXPRESSION PEDAL, NOR GROUND THE JACKS TO THE FRAME.   Keep the jacks electrically isolated from the grounded enclosure.

I actually just thought of a way you could ground the sleeve, and only use the tip and ring for signal/pot.  Short the wiper to one of the lugs and use it as a variable resistor.  That would have the same effect.

The theory of this thing is that the first opamp in the circuit doubles as a unity gain buffer and low-pass filter.  When the resistor "R2" (not pot R2) is shorted to the output, there is no gain on the low pass filter so the only signal that comes through is the unity gain signal on the noninverting input.

When the pot wiper goes the other way, you have a low pass filter being added to the clean signal 180 degrees out of phase, so the low frequency components cancel, leaving the high frequency stuff behind.

This combo leaves you a filter corner frequency of about 3 kHz.  If this doesn't leave enough treble bite, then make the value of the capacitor smaller.  If you can't make this work as you want by fiddling with that capacitor value, then we'll have to try something different.  This seems like a simple enough first attempt since it does not change the gain or frequency response of the signal in heel down position, and only cuts the lows in toe-down position.

I just fear that we may need to concoct something that significantly boosts the gain on the highs when the heel is down, instead of merely cutting the lows.    If this is the case, you can get more gain out of the circuit by lowering the value of R2 (not the pot).  For example, if the pot is 100k, and you have made the cap smaller and the signal level seems week on the high end, then change R2 to 10k.

If none of those ideas work, then you'll probably have to use a 2nd order filter.

You be the judge. . 

[mattpocket]

That looks great. So I would have to fiddle around with some cap values and whatnot?

Erm, do you think it might be easier to make a filter that would start with all bass and then sweep out to full bandwidth, as I spoke to my mate, and he said if thats easier that would be great too.

Thanks for the help too Transmogrifox! I really appreciate it, I dont really understand electronic enough yet to come up with such a design.

Matt

[Transmogrifox]

So I would have to fiddle around with some cap values and whatnot?

Do notice that I gave you some numbers to start with.  "R" is 10k, C=0.05uF.  The designation "2R" just means that the resistor should be twice the value of "R".  For example if R is 10k, then 2R is 20k. 

This is not to be confused with "R2".  R2 refers to the value that the pot measures.  The fixed resistor "R2" should be approximately the same.  The closer they are, the better out-of-band rejection you get.

This said, you may have to adjust the capacitor value, but then again, maybe not.  I just guessed that a cutoff of about 3kHz would sufficiently cut the lows without the signal getting too terribly thin.

Erm, do you think it might be easier to make a filter that would start with all bass and then sweep out to full bandwidth, as I spoke to my mate, and he said if thats easier that would be great too.

This would be similar to your guitar's tone control pot.  Actually, you'd probably use the identical passive circuit, merely using the pedal controlled pot, instead of the finger controlled pot.  If you think that your guitar's tone pot creates a desired effect when you manually sweep it with your hand, then this is what you may wish to build into a wah shell.  In this case, all you would need is the wah shell or volume pedal.  I would not recommend building something in a hammond box for something this simple.  If you wanted to control other things later, then you could add a switch to the capacitor to remove its effect from the volume pedal circuit.


I also came up with a circuit a while back to be used as a tone control for a distortion pedal.  In the center position it has a flat frequency response.  On one end of the pot, it boosts the highs and on the other end it cuts the highs.  It would probably work as a passive circuit, but would be best if it was buffered on the input and output to guarantee the frequency response.

[mattpocket]

I also came up with a circuit a while back to be used as a tone control for a distortion pedal.  In the center position it has a flat frequency response.  On one end of the pot, it boosts the highs and on the other end it cuts the highs.  It would probably work as a passive circuit, but would be best if it was buffered on the input and output to guarantee the frequency response.

If you have that lying around, that would be great!

If not, how would I employ this passive circuit similar to the guitar tone pot. A pot, and a cap, does the signal go on the wiper, the output on one of the outside lugs and a cap on the other lug to ground? I forget how I wired mine.

Matt

[Transmogrifox]

Here it is:
http://www.geocities.com/transmogrifox/PassiveTone.JPG


It would be best to add a booster to the output to recover the gain.  It divides the signal by 2.  A buffer on the input would also make it work better. 

1nF capacitor is a good starting value.  It may please you the first time, but I suspect you'll have to try a few different capacitor values to find the sound you like.  There's the beauty of DIY