HOMO ELECTROMAGNETICUS
A Buffered Volume Knob
Part I: Introduction
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.
Part II: Design
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:
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 recieves 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.
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.
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. Logarythmic
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.
Part III: Closing Remarks
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 explaination here is simply that
the RC4558 has a lot of mojo in the guitar world!
Appendix M: Mojo
What is this mojo thing twice alluded to here i nthis 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 sythesizer. 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 definately 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?
