Tuesday, 12 May 2015

Inductorless wah pedal using TL072 op amp

Having made a few guitar effects pedals lately (a fuzz face, tuner pedal, fuzz factory and so on) and spurred on by the relative success we've had to date, the next obvious project seemed to be a wah pedal.

A wah pedal is a great effect for a guitar. It has an instantly recognisable sound - and can make even the simplest of riffs sound really cool and "bluesy" - perfect for a live jam night, for example.

There are quite a few different designs for wah pedals, but usually break down into two distinct types - inductor based (like the classic Dunlop CryBaby) and non-inductor (or inductorless) varieties, like the ColorSound Wah. We even made a simple wah, a few years back.

While it worked, the wah effect wasn't very pronounced. Not really understanding the concepts behind analogue/audio electronics, it just meant this was something we learned to live with - i.e. we'd made a pretty poor wah pedal. But it was still cool - because it was made, and not bought (though in truth, our genuine, second-hand, Dunlop CryBaby pedal is the one that gets used if we want some of that funky 70s chukka-chukka sound, and the home-made pedal hasn't really been used much after being tested out just to see if it works).

Having recently discovered a load of power amp and op amp IC chips in a stash of stuff long forgotten in one of our many moves, the idea of making an inductorless wah seemed strangely alluring. After all, what better way to use our newly created pedals, than to daisy-chain them and make some really crazy sounds at the next jam night?

Most inductorless designs use the "classic T" layout.
In fact, this is the core schematic for the £159-a-time ColorSound wah pedals.

A while back we had a go at making one of these, but the effect was more of a poor tone controller than a "proper" wah sound. Instead of reducing the frequency "envelope" of the sound, it simply cut either the high-end (treble) or low-end (bass) from the guitar sound.

But a quick google search for "inductorless wah op amp" turned up a few interesting circuits.

This design uses a single TL072 dual-channel op amp.
If truth be told, we don't really understand how it works (yes, there are explanations on line about what each part of the circuit is doing, and the idea of creating an envelope and phase-shifting to create a peak of sound in a limited frequency range does make sense - just not in the way that explains why certain components are used, and why specific values are chosen).

But that's not going to stop us having a go at making one anyway!
After all, if it doesn't work, the sum cost of the components is probably little more than  a quid or so (if it doesn't work, we can always reuse the more expensive parts, the like jack sockets and footswitch in another project in the future!)

The nice thing about this design is that the wah control is a single 3-pin potentiometer. And from where it's placed in the circuit, it looks like it should be easy enough to isolate from the rest of the circuit. The reason?

Well, it might be cool to have an onboard, hand-operated, maybe capacitive sensing wah controller, instead of a footswitch. Or perhaps we could create a sequencer to vary the resistance between R2 and R3, to control the wah sound. Or use an ultrasonic sensor to vary a resistive load between the two resistors. Or an LDR... or any number of other ways of creating a variable resistance.

In short, being able to isolate the bit that controls the wah sound means we can use this same design for a number of different types of wah effect, rather than being stuck with the old-fashioned foot rocker switch (though in all probability, that's the most likely design we'll end up using).

(press-n-peel ready layout - the 9-pin footswitch is not necessary - it was just used during the design of the PCB to ensure we got all the connections on the board!)

This PCB layout is surprisingly small, just 30mm x 45mm, so it should fit in even the smallest of enclosures and still leave plenty room for the battery.

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