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Now, the last topic we are going to talk about is the Wah Pedal, that many of

you've probably maybe you've played with them.

Or I'm sure you've heard them. you may not have known that you heard

them. now in a minute here, I'm going to cut

into just give a little bit of of illustration.

I found a couple of different YouTube videos from people demonstrating how

their Wah Pedals work. if you just go on YouTube and, and put in

Wah Pedal, you'll find hundreds of illustrations of how these work and how

they sound. This was a couple of them that I found

particularly illustrative. >> Okay.

This is the GCB-95 CryBaby Wah Wah by Dunlop.

[MUSIC].

And now, on the dirty channel on the amp.

[MUSIC].

>> [MUSIC].

Okay. So now, let's take a look at this circuit

in a little more, a little closer up and try and understand what's going on in

this circuit. This just has to look like a pretty

daunting challenge to you right now. But if we really study this, we can

understand what's going on. Now, the first thing I want to do is

let's identify the key components in this thing.

So, there is a a, and this is part of the complexity of building things with

bipolar transistors. There, there's a lot of, you have to set

the DC bias points on these devices so that they work in the way that you want.

And so, there's lots that, that gives you adds to the complexity.

There's a lot of these resistors in this schematic are really nothing more than

the set DC bias voltages on the terminal so the transistors.

And then, once you do that you have to bypass those with the capacitors, so the

signal can go through the capacitor, but the DC point as determined by the

resistor value. But that's in a side that we're not going

to get into. But, the essential thing here, is you've

got, what amounts to a voltage divider. There's a resistor, and then there's

another resistor here, and then there's a parallel combination of an inductor and a

capacitor. So here's the LC in the circuit.

Here's one R and there's another R. And so these are the essential

components. Now this capacitor is a very large

capacitor, and it's grounded. And so, essentially, the inductor as far

as the signals are concerned, looks like it's grounded at this point.

But this capacitor is not grounded. It's connected to this transistor output,

and so this circuit is a very clever use of feedback to make this capacitor look

like it has a variable value. And so, this, this first transistor

output signal goes through here. Here is a variable potentiometer.

This is the thing that moves when you move your foot up and down.

You're moving the position of the wiper. Then, there's a second transistor

amplifier stage, whose output goes back to the far side of this capacitor.

And so, this capacitor is not grounded. It's attached to the output of this

transistor. And so, that's the essence of this

circuit. Most of the rest of this are just

resistors to set bias points like this one.

Plus the bias point on the emitter of this bipolar transistor.

And then, various coupling and bypass capacitors.

But here's the input. It goes in here through this voltage

divider with this capacitor that has this funny ground connection, feedback ground

connection. And then, the output let's see, here's

the, where the output comes from. And right here, this is, this is the

output line, comes from the output of this transistor, right here.

So, this goes to the amplifier, okay. So, there's the input, the output.

It's just a voltage divider with this feedback thing going on.

Now, there's a lot to that, and we're going to talk about that now.

Now, what this is is a variable bandpass filter.

And so, you have a resistor followed by another combination of a resistor and a

RLC circuit. And so, this first resistor, is right

here,that's the R0. The second resistor, this one, is the R1.

Now, this implementation here does not have another resistor down in parallel

with the L and C, but you can add one so you can adjust the Q of this LRC circuit.

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So, here is the essence of it. So, it's, it's, this resistor followed by

this combination of these components. Here is V in.

There is V out. V out is measured across the series

combination of R1, and all of that. So, that is just a voltage divider.

So, here's the input voltage. Z1 is, that's R0.

Z2 is this combination of components. And the output is taken at the midpoint.

Now, typical values from and you can fine, there's number of websites that

dedicated to Wah filters and building them and, and modifying them.

But typical values are maybe 68 kilo-ohms here.

1 or 2 kilo-ohms. This may be on the order of 30 kilo-ohms.

The inductors are very large. Maybe half a henry.

And then, the capasitance value changes from around 0.01 microfarrad, that's 10

nanofarrads up to about 1 microfarrad. Now, here's the, the clever part in this

circuit, is that actually it's, it's very difficult and expensive to build variable

capacitors. And it's also difficult and expensive to

build a variable inductor. But variable resistors are relatively

easy to build the potentiometer. And so, this is a way to make the

apparent size of the capacitor change using a resistive potentiometer, that's

the, the secret of the Wah Pedal.