In this final week of lectures, we're going to use all of the techniques that
you've learned so far to discuss a few common audio signal processing circuits.
And then we're going to take another look at the amplifier and spend some time in
the last lecture talking about exactly how the amplifier works.
And give you a few pointers on how to build it and demonstrate the prototypes
that we built. So in the first lecture today though, I'm
going to talk about three specific things.
The first one is how a guitar volume and tone control circuit works, which is
relatively straight forward. Then the second topic we're going to talk
about are band-pass filters and graphic equalizers.
So this is like the graphic equalizer that you see in iTunes or something like
that, but we'll discuss how those are designed, how they work, and we'll show
you an example that I programmed. And then the last thing that we'll talk
about is the guitar effect called a wah pedal.
So if you've never heard a wah pedal or don't know what that is, we'll, I'll show
you that and we'll look at a couple of videos.
But it's a nice illustration of a circuit that uses resistors, conductors,
capacitors. And op amps, altogether to give you a
very interesting audio effect. The first circuit we're going to look at
is the volume and tone control circuit for a simple electric guitar.
So we want to get an intuitive feel for how that works.
And then do an analysis of it using all of the AC circuit analysis techniques
that we've developed and then predict what the frequency response of the tone
control should look like. So, we're going to start by looking at
about the simplest version of a volume and tone control circuitry on an electric
guitar that you're going to be able to find.
And that's something like a telecaster design.
This goes back to the 1950s from Fender. And it's a very prototypical type of
electric guitar. With only two simple pickups, there's a
neck pickup, and a bridge pickup. This is the bridge, and obviously the
neck. And then it has a volume and tone control
knob, one for volume, one for tone. And then there's a selector switch that
lets you select which pick up, one or the other, or both of them together.
And if you've ever had the nerve to open up the control control cavity on the
guitar. And take a look at the wiring inside.
It's kind of a, a mess of wires. And it's very confusing to figure out
what's going on. Now, a lot of this complexity is just
from the selector switch. And let's take another look at this.
This is from a, a, a data sheet. On the four position selector switch that
you can modify your telecaster with. usually they come with the three position
selector switch. but you can use a four position switch to
give you a little more tonal variability. But you've got the selector switch, the
volume pot, potentiometer, and tone potentiometer.
There's one capacitor in there. This is the output jack that you plug
your guitar cable into and these are the leads.
One of the they call the lead pickup. That would be the bridge pickup.
And the rhythm pickup would be the, the net pickup, and so these are the and this
is to the, the cover. This is just the, the grounding
on the plate that's used to shield. That's this plate, and you see inside the
cavity is painted with a metallic paint. To provide electromagnetic shielding of
the electronic, so you can try to reduce 60 Hz pickup.
Now, or 50 Hz if you're not in the US. Now the what we want to do now is try to
make some sense out of this circuit and diagram it in in a rational way and then
do an analysis to figure out the frequency response of this circuit.
So, let's just say a little bit about the selector switch.
Now, that's the part that we're not going to worry about.
and it turns out it's probably the most confusing part of this when you try to
dupe out what's happening in this circuit.
really, the best thing to do if you get your hands on one of these, is to just
sit down with an ohm meter. And connect it to, between various
terminals for different switch positions and see how the switch connects what to
what. And it can be very confusing, but this
wiring here. And if you want to take the time and try
and trace out what's happening, please by my guest.
But you have the two pickups that are connected to various terminals of, of
this switch. And then it's set up so that the switch
has four positions. In one of the positions only the bridge
pickup is sent off, the signal is sent off to the volume and tone control.
And another position, you're coupled only to the neck pickup.
And then the two other positions lets you combine the neck and the bridge pickup,
either in parallel or in series. So you get your choice.
The impedence is going to be lower in the parallel combination than the series
combination, and the tonal properties are somewhat different.
So that's the part of the circuit that understanding it just boils down to
tracing symbols, and there's no real AC circuit analysis in that.
But let's redraw this circuit and take a look at what's going on.
So, I'm going to treat this with just a single pickup.
So we'll say that your selector switch is set up, is, is at one of the positions,
so you're coupled to one of the pickups. Doesn't matter which one it is for the
analysis. Now, there are two potentiometers.
There is one in the tone control circuit. And there's one in the volume control
circuit. So here they are.
And the one in the tone control circuit is used only as a variable resistor.
You don't use the third terminal. So you see the third terminal of this is
just floating. It's not connected to anything.
But what happens when you move the wiper. Your changing the effective value of the
resistance coming down this branch and then through the capacitor so if I move
the wiper all the way to this end. Then I have the entire resistance of the
tone pod if a wiper is moved all the way up to this end.
Then the resistance gets small and approaches zero.
So you're really using this potentiometer just as a two-terminal device, which is a
variable resistor. Now, the volume control circuit is just
the prototypical voltage divider circuit. It's nothing more than one resistor of
resistance RO for R output. And that's the total resistance of the
volume pot. And then the center wiper here, is where
you get your output signal. So let's take and identify a few points
that are in common between this drawing and this circuit diagram.
So point A here, this is the ground point.
So this is the point where the, one side of the capacitor is attached to point A.
And also you see that's actually soldered to the the the casing of the
potentiometer meters. So this potentiometer is grounding and
this is the ground point. So there's point A.
point B, here, that is this point. This is the line coming from the pick up.
So this is actually this terminal. And the other terminal here, this is the
output of the multi-position selector switch.
And so here's the, you've selected one of the pickups and it.
It's connected to this point B. But that's also connected to the top of
the resistor In the tone control part of the circuit.
So here's point B. So it goes through the tone control
resistor, through the capacitor, to ground.
And so here's, you go through this resistor, through the capacitor to
ground. And it also is attached to the top.
Of the potentiometer and the volume control circuit.
So there's point b. Then point c.
That's the wiper in the tone control pot. And that's connected to the other end of
the capacitor. So there's that point.
And the last point is the output. So here's the wiper and the volume
control potentiometer. And that goes to your output, right here.
So this is just a redrawing of this diagram of what's inside the, the cavity.
And we're ignoring all of the messiness over here in the selector switch.
for now. That's something you could figure out if
you were so inclined. Okay, so now let's go on and analyze this
circuit in a little bit of detail.
