Attempting to design an inductorless wah pedal

[jrc4558]

Good day to you gentlemen!
I want to try to design my own wah, without inductor, purely on capacitor-resistor networks.
I found this: http://www.morleypedals.com/clwes.pdf
It seem to use two caps in series and a resistor to ground from the middle junction.
What are the formulas for calculation of these filters and what are they called? (sorry, I still haven't learn all those terms in English)
Also, how exactly does this filter work?

Thanks in advance. I promise ot post the design once done.

[JimRayden]

It's called "twin T", I hope the following links offer any help.

http://www-k.ext.ti.com/SRVS/Data/ti/KnowledgeBases/analog/document/faqs/tt.htm

http://www.radio-electronics.com/info/circuits/rc_notch_filter/twin_t_notch_filter.php

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Jimbo

[col]

There are several circuits in the Babani books that don't use inductors but use simple IC based designs. One of the early Coloursound wahs was a also simple transistor circuit and the schematic has been posted here in the past. I've not built one yet but If I can get a broken wah pedal I will try one . Has anyone built one of the Babani wahs? I built the one of the autowahs but I couldn't get it stable.

[jrc4558]

I never ever saw the Babani book. Do you have an ISBN number so I could look it up in the local library?
Or maybe you could just post some scans?

[gez]

Actually, it's a 'bridged T' with a few little additional networks to boost and cut.  I've got the formula for it somewhere or other, but mostly it's just easier to use one from a schematic and tweak it empirically.  There might be something at GEO, but I'm pretty sure I got the formula from an app note on oscillators.  Anyway, the MORLEY is a good place to start.

I breadboarded one of the Babani wahs once.  Didn't like it, but it wasn't as well designed as the circuit you posted a link to.

OTA filters are very, very nice! 

[cd]

There might be something at GEO

There is:

http://geofex.com/Article_Folders/wahpedl/wahped.htm
http://geofex.com/Article_Folders/EQs/paramet.htm

[Mark Hammer]

Keep in mind the basic premise that op-amp gain is always a function of how much, and what kind of negative feedback is applied.  For example, if a capacitor is placed in the feedback loop, then there is a low-impedance path for the negative feedback specific for frequency content above the limits set by the cap.  That means there is more negative feedback for high freqs compared to lower freqs, which means the high-end won't have nearly as much gain applied to it.  As I like to say, think about op-amp feedback as "stepping on the brakes".

The Morley pedal you link to uses a kind of notch filter in the feedback loop.  This is a common approach adopted by autowahs as well.  So lets think about this for a moment.  What does a notch filter do?  It shunts frequency content within a specified range to ground, but permits content above and below that range to pass through relatively unaffected.

Okay, if we had an amp that let lots of negative feedback go from output back to input, but only for high and low frequencies and not for those in between (because that content is "dumped" to ground), what are we doing?  Simple, we are providing LESS negative feedback for that band that is notch filtered.  Using the "foot on the brakes" analogy, we are taking the brakes off for that band, while stepping harder on the brakes for higher and lower content.

The sonic quality of that resonant boost will be a function of how tight or broad that notch is.  Most such notch filters will use a symmetrical arrangement, where the two caps involved are the same value, but they don't HAVE to be.

Clearer?

[jrc4558]

Hey Mark! I understood the basic idea of how it functions, but thanks for the discourse. You have a magic ability to phrase things i na very understandable manner.
So, if I understood you right, the morley Wah taht I linked to is a selective frequency booster and the center frequency at which the boost is occuring is being shifted up and down. Is that correct?

[G.Neyrey]

Babani book:
Electonic Music Projects
ISBN 0 900162 94 5
Published 1980

It's got a wah that uses a single LF351.

[Transmogrifox]

Hey Mark! I understood the basic idea of how it functions, but thanks for the discourse. You have a magic ability to phrase things i na very understandable manner.
So, if I understood you right, the morley Wah taht I linked to is a selective frequency booster and the center frequency at which the boost is occuring is being shifted up and down. Is that correct?

You understood correctly.

The twin T or Bridged T (there is a difference between the two) filters are more simple builds, but are generally more prone to distortion, noise and in my mind, a "weaker" sound.

I have become a big fan of the state variable filter, such as found in the Neutron.  The Neutron can be easily modified for operation with a treadle.  You can get some incredibly rich sounds out of the state variable filter if you add a mixing stage to allow mixing of the high pass, low pass, and band pass filters.  It really makes for a sweet wah.  You can also make the state variable filter using OTA's if you wish.

[jrc4558]

You understood correctly.
The twin T or Bridged T (there is a difference between the two) filters are more simple builds, but are generally more prone to distortion, noise and in my mind, a "weaker" sound.
I have become a big fan of the state variable filter, such as found in the Neutron.  The Neutron can be easily modified for operation with a treadle.  You can get some incredibly rich sounds out of the state variable filter if you add a mixing stage to allow mixing of the high pass, low pass, and band pass filters.  It really makes for a sweet wah.  You can also make the state variable filter using OTA's if you wish.

Jus one more question: In a bridged T filter, (two caps in series and a resistor in parallel, resistor to ground from the center connection of caps) is the resistor to ground the resistor that will shift the frequency? I just cant sit down with pencil and paper to calculate it just now..

[JimRayden]

You understood correctly.
The twin T or Bridged T (there is a difference between the two) filters are more simple builds, but are generally more prone to distortion, noise and in my mind, a "weaker" sound.
I have become a big fan of the state variable filter, such as found in the Neutron.  The Neutron can be easily modified for operation with a treadle.  You can get some incredibly rich sounds out of the state variable filter if you add a mixing stage to allow mixing of the high pass, low pass, and band pass filters.  It really makes for a sweet wah.  You can also make the state variable filter using OTA's if you wish.

Jus one more question: In a bridged T filter, (two caps in series and a resistor in parallel, resistor to ground from the center connection of caps) is the resistor to ground the resistor that will shift the frequency? I just cant sit down with pencil and paper to calculate it just now..

Yup, that's the LDR3 in your Morley Classic Wah you posted.

----------
Jimbo

[Mark Hammer]

Transmogrifox is actually spot on.  There is so much advantage to be gained by making a footswept state variable that I don't know why anyone would look elsewhere.  Years ago, I stuck a dual-ganged 100k pot and an EPFM Super Tone Control in a pedal, and it was a joy to work with.  Great sounds, tremendous variety (don't forget that a footswept notch filter is the same as a footswept P45), more resonance than what you get from a wah, simple range-switching, low cost.

The chief drawback is the two resistances that have to be changed in synchrony.  Some, but not all, wah shells have space to accommodate a dual-ganged pot, and some (but not all) dual-ganged pots hold up to being moved by arack and pinion system.  Perhaps a much better approach is the use one of the dual vactrol units and simply vary the voltage illuminating the LED with a single pot in the wah shell.  That also lets you eliminate any risk of scratchy pots because the LED pot is not in the audio path.

NOTE: There are dual vactrols where the LDRs share a common connection, and there are dual vactrols where the LDRs are completely independent.  You want the second one.

[John Lyons]

Hey Mark
I've been thinking about building the Super Tone Control in EPFM and building it into a wha shell.does this anything like a wah with more flexibility or is it it's own beast... with flexibility? WHat I mean is, can you get the wah queack out of it as well as the main function of tone controlability?

Thanks

John

[Mark Hammer]

Think Mutron.  If you like that sound, and you wouldn't mind controlling it with your foot, that's what you get.  The big caveat is really the same as any attempt to make a wah, and that is that pot taper and sweep range can be an issue.  Well, maybe "issue" is a strong word to use.  Let's just say that how well something"works" when you're twiddling it with your finger can beseparate from how well you feel it works when foot movements need to convert into the amount of filter sweep you were intending.  There may be some tinkering involved post construction, but ultimately you should find it pleasing to use.

Take a peak at the filter for the McMeat/Meatball.  It's the same as the Mutron, more or less, but incorporates an interest discrepancy between the two resistances that need to be changed to sweep the filter.  From what I recall, this adjusts the resonance a bit as you sweep.

[John Lyons]

So would you reccomend the Super tone control?

I'll look into the Mutron/neutron as well as mcmeat/meatball. 
It's just that the Super tone control is more realistic in the meantime as far as simplicity.