Figure 1. The complete schematic of the buffered volume knob.
The top half of the schematic shows the power supply circuit, while
the bottom half of the schematic shows the signal chain. Let's
discuss the power supply first.
The power supply is simple, it receives an approximately 9V input at
"V_bat" and creates two voltage levels: One voltage to power the active
components (9V DC), and one that is halfway between this voltage and
ground (4.5V DC). The need for the 9V DC supply is obvious, and the 4.5V
DC level is to bias the AC signal between the supply rails (9V DC and
ground) to prevent distortion. A more complete discussion on why a
"mid-supply" bias voltage is required for single-supply circuits likes
these is available in any decent electronics textbook.
The only interesting thing about the power supply is the inclusion of
a minor improvement over the typical "voltage divider creates a
mid-supply bias voltage" technique used in guitar pedals everywhere.
Figure 2. The power supply of the buffered volume knob.
Simple voltage dividers are common in guitar pedals as means of
generating mid-supply bias voltages, but this technique has problems.
Mainly, that such a circuit's output voltage will vary with output
current. This variation is bad, as it represents unwanted noise. The
circuit is not able to differentiate between the desired AC signal from
the guitar and the unwanted variation in the 4.5V DC level! In the
majority of cases, adding noise is bad.
An easy way to get around this problem is to buffer the voltage
divider with an OpAmp wired as a voltage follower, which is the technique
employed in this circuit. The OpAmp (one half of an RC4558) ensures that
the output voltage remains at 4.5V DC, despite changes in current into or
out of the 4.5V node.
One point to consider about the power supply is the role of D1. This
diode serves as reverse polarity protection - it prevents the flow of
current if the power supply is connected backwards. The drawback of this
design is that the "9V" node will always be one diode voltage less than
9V DC (therefore the 4.5V DC node will be less than 4.5V as well),
but this is not a problem for the circuit we are considering.
Figure 3. the main signal path of the buffered volume knob.
Now onto the meat of the design. The input stage (everything left of
C2) is a blatant ripoff of the input stage on the famous mojo-filled
Ibanez Tube Screamer.
Other people have discussed this circuit at length, so I'll not
repeat their work here. The crux of it is that guitars sound best when
loaded with ~500K impedance, which is exactly what this input stage
does (besides acting as a buffer, of course!).
The heart of the design is the potentiometer lazily alluded to by R3
and R4 (my schematic drawing software of choice does not include
a nice symbol for potentiometers, so one must draw two
resistors instead). Before we consider it, let's turn our attention
to U1 and its role.
The OpAmp, (the other half of an RC4558) is configured with R5 and R9
to provide a gain of two to the guitar signal, regardless of where the
potentiometer is set. To see why this is useful, it's helpful to
consider what would happen in the circuit with the potentiometer set
in three different ways: all the way counter-clockwise, at the center,
and all the way clockwise.
1)
In the first case with the potentiometer set all the way
counter-clockwise, R3 will be nearly 100k, while R4 will be nearly
zero. This means that the OpAmp's positive input will be directly
connected to 4.5V DC and no guitar signal will get through. The
4.5V DC node acts as an "AC ground" in the circuit - remember that
this node is being driven by an OpAmp! Therefore, the output of
circuit will not contain the guitar signal at all.
2)
For the second case with the potentiometer set in the middle, R3 = R4
= 50k. With the 4.5V DC node acting as ground for the guitar signal,
the potentiometer works as a voltage divider for the guitar signal.
The RC4558 then sees the guitar signal at half of its original
amplitude, multiplies that by two, and you get an output amplitude
equal to the input amplitude for the circuit.
3)
In the final case with the potentiometer all the way clockwise, R3
will be nearly zero and R4 will be nearly 100k. This means that the
OpAmp will be directly coupled to the right-hand side of C2 and 100%
of the guitar signal will be present there. The RC4558 then amplifies
this signal by two as before and the output amplitude becomes double
the input amplitude.
Considering these three cases should give the reader a good picture of
how the circuit behaves. Of course, this functionality is predicated
on the "volume" potentiometer having a linear taper. Logarithmic
tapers are more common for potentiometers that see use as volume
controls, but using one in this circuit would ruin the nice "unity
gain at the middle position" behavior of the potentiometer.