voltage controlled wah pedal/bandpass filter

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MarbledMoog
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voltage controlled wah pedal/bandpass filter

Post by MarbledMoog » Thu Jul 23, 2009 5:29 am

Entertain me on this: Would it be possible to modify a wah pedal, say a regular Vox v847, to disable the pedal treadle from controlling the wah pot, and instead having a control voltage control it? Disabling the treadle only in temporary by means of either a switch or something. Has anyone done something like this?

I think of this because I've read someone say that the maestro sample/hold was actually a bandpass filter. I haven't listened to Ship Ahoy in awhile, but that would make sense to me. I could see how using a random bandpass filter could be very effective with a guitar. Thoughts?
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MarbledMoog
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Post by MarbledMoog » Sat Jul 25, 2009 1:54 am

so no one's got any idea about this really?
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Bryan B
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Post by Bryan B » Sat Jul 25, 2009 5:52 am

I have wondered this as well. I wanted to put CV in to control in different ways.
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DontBelievetheHype
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Post by DontBelievetheHype » Sat Jul 25, 2009 7:39 am

I think some pedals are harder to do than others-- and if they can even be converted at all, they may not be in an ideal CV range;

For instance, I contacted Howard Davis about adding CV control to some parameters on my EHX Polychorus, and the answer I got was that it would be a bit of an undertaking, and for CV over 'manual/sweep' I would need to provide 12 or 13+volts, and for the 'Resonance' I could only get an input for a potentiometer/volume pedal control.

For CV controllable filters in stompbox form with LP/BP/HP I would recommend the Robot Factory Photron (MS20 clone) and the Barge Concepts Grinder (meatball clone). I have and use both of these filters- the barge is mainly used for enveloping, and the photron covers what the MoogLPF can't. The Photron is nice and comby, very transparent, and has a very focused resonant range compared to the Moog- I prefer the Moog for lowpassing in most cases, but the Photron sounds great as well and has LP/BP/HP modes, a built in LFO, and the option for passive or active CV control over frequency cutoff.

The Grinder is a great filter, and has a super fat and defined envelope, but I find it a little too clean for non-enveloped lfo/cv filtering. It doesn't have an active CV input, but its input is scaled well for use with the MP201 for instance--- Its can be used with voltages, but I wouldn't recommend it for someone who would primarily be controlling it with voltages, its better as just an envelope filter, though it is sufficient as a CV filter.

Those are both expensive options, but from my searches I have found them to be some of the best. If you really like the MoogLPF and want something similar for BP/HP filtering, I would definitely recommend the Photron. You can also contact Mario and Robot Factory about custom work-- he does lots of custom stuff and has been building stripped down versions of the Photron filter (ie: No built in LFO or envelope),
Synth bass guitar rig including MS-20 filter clone, Paradox TZF, MF101, 102, 103, 107(x2), CP-251 and MP-201

Just Me
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Post by Just Me » Sat Jul 25, 2009 9:34 am

Why not buy a DotCom Q107 and case it to suit your needs? Could be done for under 225USD if you are frugal.
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Voltor07
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Post by Voltor07 » Sat Jul 25, 2009 2:36 pm

Adding CV to a guitar pedal is a massive undertaking if not an impractical one. As has been mentioned, the voltage required to make miniscule changes would be considerably higher than even modular voltages. I like Just Me's idea. :wink:
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Post by Alien8 » Sat Jul 25, 2009 6:25 pm

Theoretically its not that hard to do. In many cases you can simply "remove a pot, and replace it with a jack". There are many variables though depending on the pedal you wish to do it to.

Check out the DIYstompboxes.com forum, or start a post about it and you may just find what you seek.

Secondly, EHX has the new Q-Balls filters, one for Bass and one for Guitar. Both have expression inputs, so potentially CV is possible.
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Post by Voltor07 » Sat Jul 25, 2009 11:20 pm

Alien8 wrote:Theoretically its not that hard to do. In many cases you can simply "remove a pot, and replace it with a jack". There are many variables though depending on the pedal you wish to do it to.

Check out the DIYstompboxes.com forum, or start a post about it and you may just find what you seek.

Secondly, EHX has the new Q-Balls filters, one for Bass and one for Guitar. Both have expression inputs, so potentially CV is possible.
The thing is, there's a difference between an expression pedal and CV. Control voltage is applied to a circuit to change a particular parameter. Very hard to implement. An expression pedal is merely an external potentiometer which passively changes a parameter.
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Post by MarbledMoog » Sun Jul 26, 2009 12:08 am

well I don't have enough money to buy another filter or module...only parts and some labor costs. I'll stop by the DIY pedal forum.

Can we get into why it would be so hard to implement? What we are talking about is a standard run of the mill Vox wah-wah pedal. What are the specifics to why I couldn't disconnect the potentiometer from the circuit, and in its place have a jack where by an LFO controls what the pot would normally control?
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Voltor07
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Post by Voltor07 » Sun Jul 26, 2009 12:30 am

MarbledMoog wrote:Can we get into why it would be so hard to implement? What we are talking about is a standard run of the mill Vox wah-wah pedal. What are the specifics to why I couldn't disconnect the potentiometer from the circuit, and in its place have a jack where by an LFO controls what the pot would normally control?
Adding voltage where voltage isn't meant to be added could overload parts of the circuit. It may or may not cause harm, but it may or may not cause unpredictable results that aren't desirable. Now what could be implemented is an LED and photo resistor replacing the desired pot. Power the LED through a timer circuit, and that'll give you a similar effect to what you're going for. :wink:
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Post by Alien8 » Sun Jul 26, 2009 2:04 am

The thing is, there's a difference between an expression pedal and CV. Control voltage is applied to a circuit to change a particular parameter. Very hard to implement. An expression pedal is merely an external potentiometer which passively changes a parameter.
Technically no, there is no difference. Control Voltage is merely a term used to communicate a standard method of communication between pedals, and one that is not as well known in the pedal world. Synths yes, pedals no. This doesn't mean that you won't fry a pedal by trying to supply it's EXP IN jack with control voltage - if you don't know, just don't do it.

If a pedal has control over a specific parameter via a pot that you turn, you are in fact changing the voltage used to control that parameter. Depending on the pedal you are using, the voltage required to change or control the parameter may vary. A Vox wah may require 0.5 V to control it's entire range, while a Morley wah may require 1.5 V - hypothetically, I don't know the actuals. Rocking your foot opens and closes the pot (sends Voltage to ground, or its destination) for the purpose of opening and closing the filter (changing the frequency).

For specifics about voltage look here: http://en.wikipedia.org/wiki/Voltage
For specifics about potentiometers (pots / variable resistors) look here: http://en.wikipedia.org/wiki/Potentiometer
Can we get into why it would be so hard to implement? What we are talking about is a standard run of the mill Vox wah-wah pedal. What are the specifics to why I couldn't disconnect the potentiometer from the circuit, and in its place have a jack where by an LFO controls what the pot would normally control?
The EHX HOG has an expression in jack that is used to plug an expression pedal in, the function of the pedal is to change the parameter you have it set to change. This in fact is not unlike a moogerfooger, where you simply plug an expression pedal into a specific jack to change a specific parameter. You essentially create a remote knob to control the parameter. A pot is a variable resistor, sending a voltage through it will change the voltage, unless fully open. This happens regardless of the circuit being active (added power) or passive (self generating power like a magnetic guitar pick-up).

The jacks on your moogerfoogers (EXP IN if you will) are designed to default to the permanently mounted knobs in the moogerfooger to define the parameters voltage when not in use. When you plug in the expression pedal, it now becomes the default, or master control knob if you will. The pedal's internal voltage is supplied to the jack / pot and back again. When you plug a "control voltage" into the EXP IN jacks on the back of your moogerfooger you are doing the exact same thing, supplying the parameter with a specific control voltage, that when changed, changes the effect of that specific parameter.

Some effects are designed so that excess voltage is sent to ground, meaning you don't fry your pedal when you supply it with a control voltage in the EXP IN jack. Some pedals are not set up this way, and send too much voltage to the internal circuitry, thus overheating, or damaging components.

So, if you have a parameter already available to change, you can theoretically implement a jack to allow an external control source. Due to the design of some pedals, both physically and electronically, adding a jack can be quite difficult. However, in the case of a wah pedal, you have access to everything you need to make it happen.

I bring up the HOG, because it works with MOOG control voltage defaults at maximum and minimum. You can try it with any pedal - at your own risk - using a CP-251. Route an LFO to the attenuator - set to middle, or 0 - route the output of the attenuator to the EXP IN jack on your pedal. Slowly turn the attenuators knob and see if it affects the effect. To find the maximum, send a constant voltage through the attenuator in the same manner. As you turn the attenuator control, you will hear the effect parameter stop changing. This is the maximum control voltage required for that particular pedal. DO NOT EXCEED IT.

Do some reading about how pots work, buy a volt meter, and test your wah out. Then go to the DIY forums for assistance, but try not to go empty handed, you get more help when you have done some leg work, and can discuss the solution further than "I want to use CV to control my Vox wah, tell me how".
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Voltor07
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Post by Voltor07 » Sun Jul 26, 2009 5:24 am

Alien8 wrote:...If a pedal has control over a specific parameter via a pot that you turn, you are in fact changing the voltage used to control that parameter. Depending on the pedal you are using, the voltage required to change or control the parameter may vary. A Vox wah may require 0.5 V to control it's entire range, while a Morley wah may require 1.5 V - hypothetically, I don't know the actuals. Rocking your foot opens and closes the pot (sends Voltage to ground, or its destination) for the purpose of opening and closing the filter (changing the frequency).
First of all, I agree with the whole post here. Very well written, and much more in-depth than my hastily written explanation. :) Just want to clarify Alien's quoted statement above, though. The voltage that goes through the potentiometer which in turn controls the wah is a predetermined voltage. Let's say, 2V at maximum. In order to deliver a 2V signal from an LFO, one would need to disable the internal voltage.

The problem here lies in the fact that the internal voltage comes from a main power source, usually a wall wart or 9 volt battery. That battery (or adaptor) provides power throughout the circuit, not just to the main pot. Therefore, one would need to trace the power supply and determine how the whole circuit works before taking out pots here and adding jacks there. It could get really messy, really quick.
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Post by Alien8 » Sun Jul 26, 2009 1:27 pm

The voltage that goes through the potentiometer which in turn controls the wah is a predetermined voltage. Let's say, 2V at maximum. In order to deliver a 2V signal from an LFO, one would need to disable the internal voltage.

The problem here lies in the fact that the internal voltage comes from a main power source, usually a wall wart or 9 volt battery. That battery (or adaptor) provides power throughout the circuit, not just to the main pot. Therefore, one would need to trace the power supply and determine how the whole circuit works before taking out pots here and adding jacks there. It could get really messy, really quick.
Yes, you would need to disable the original voltage, however it's generally not that messy. Sometimes it is as simple as using the new expression pedal jack to short out the original pot when in use. It would have to be a switched TRS jack, and it's generally a good idea to use the plastic isolated type. You really need to go to DIYstompboxes.com and read up on this...

Moogerfoogers are really advanced when it comes to this CV option. It's near impossible to fry one... they have been designed to use TS and TRS cables to communicate CV, and they allow the use of the existing permanent pot to control the lower limit of the CV range, sometimes allowing +10V - going above and beyond what the pedal 'normally' does. As always, I'm impressed by how much thought they put into their product.

Here's a link to a modded DOD 440 filter that uses both an internal vactrol generated LFO and an expression pedal to control the filter:
http://www.diystompboxes.com/smfforum/i ... ic=68221.0 There is a youtube link a few posts down from the OP demonstrating it.

And the article I've been looking for this whole time:
http://www.geofex.com/Article_Folders/xprnped.htm

Converting a VOX wah to a fixed wah with expression option:
http://www.diystompboxes.com/smfforum/i ... ic=50376.0

Not all pedals expect control voltage, some expect only they resistance to be variable. This is where you need to investigate the wah's requirements.
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Post by Voltor07 » Sun Jul 26, 2009 2:27 pm

Excellent links, Alien! :D My previous post describes my findings with the Danelectro Dan-O-Wah. The voltage that goes to the potentiometer in that pedal is in series with everything else, so that disabling the control voltage disables the rest of the circuit and makes funny noises that are not wah-like at all. Therefore I stand by my statement that things can get messy. Luckily, not all wah's are created equal. :wink:
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MarbledMoog
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Post by MarbledMoog » Mon Jul 27, 2009 5:31 am

thanks guys for all the in depth information. I'm going to have to do some serious research regarding this...I won't have time to tinker with this for awhile, but thanks for the head start!
MF-101, MF-102, CP-251

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