A friend of mine plays guitar and likes to experiment
with guitar pedals and guitar amps. He recently asked for
a quick way to change the volume coming from his guitar
without having to stop playing. Many guitar players use volume pedals from companies like Ernie Ball to
accomplish this, but these tools require that the user control the guitar
volume via the angle of his or her foot. This can be fiddly in a live
setting, so my friend asked for a simple on / off foot switch that would
change his guitar's volume to a predefined level and back again. This
friend of mine tried some DIY solutions of his own, but found that these
"sucked out his tone." He specifically asked for something that wouldn't
ruin his guitar sound.
This is a simple enough task, but I took the opportunity to have a
little fun.
The general idea here is first buffer the input signal, drive a variable
voltage divider (a potentiometer) with that buffered signal, and then
buffer that again to easily drive any circuit that may be present further
in the guitar-to-amp signal chain. The fun part was adding a little
"mojo" by employing some elements of famous circuits in guitar pedal
history. More detail on mojo is presented in Appendix M, at the end of
this article.
Figure 1, below, shows the complete design of the effect:
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.
R6 and R11, at first glance, seem to serve no purpose in the circuit.
While it's not immediately obvious, these resistors do in fact play
an important role in the circuit's working as a guitar effect. These
resistors are called "pull down resistors" by
people with a lot of experience in the guitar pedal world. They
serve to eliminate transients caused by the interaction of capacitor
leakage and hard mechanical bypass switching common in guitar pedals.
The naming of the "V_bat" node from where external power enters the
circuit is just that - a name. When the I built the circuit "in the
flesh," it was wired so that it could only be powered from of a 9V
DC "wall wart." 9V Batteries are lame.
A typical question that comes up often with guitar effects employing
the RC4558 looks like this - why did the designer choose the lowly
RC4558 for this circuit? A quick search on DigiKey will show that many
OpAmps exist with far better performance, so it might seem strange to
choose it for modern circuits. The explanation here is simply that
the RC4558 has a lot of mojo in the guitar world!
What is this mojo thing twice alluded to here in this article? The answer
might not be satisfying: Certain components are revered in the music
world for their use in famous circuits within that world, and those that
revere them attest that these parts are the keys to pleasing musical
timbres. These revered parts are said to have mojo.
For example, the RC4558 dual OpAmp was made famous among guitar players
for its use in the Ibanez Tube Screamer. The Tube Screamer is an effect
intended to emulate a smooth, "bluesy" distortion that one might get
by playing a Fender guitar amp very loudly. Its sound is adored by
guitar players everywhere, and many ascribe a large part of it to the
humble RC4558. As another example, the SSM2040 voltage-controlled filter
is famous among synthesizer nerds for its use in a number of popular
synthesizers from the late 1970s, most notably in the mighty Prophet 5
synthesizer. Many synthesists claim that the SSM2040 is responsible for
the pleasing filter resonance and powerful sound of the Prophet 5. The
list of mojo parts is quite long, as serious research into this matter
will reveal.
Are these claims with merit? It is fair to place the responsibility of
"good tone" on a single component in a larger system? Whether or not the
claims about these mojo parts are accurate or just, parts with mojo are
definitely useful as psychological tools to help musicians enjoy their
gear more and perhaps even to play better! This characterization is not
meant to be reproachful, I merely intend to highlight the importance
of one's mental state when playing music (or doing anything, for that
matter!). Besides, if choosing a certain electrical component makes the
end user happier, why argue?