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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:

complete schematic of the buffered volume knob

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.

power supply schematic

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.

main signal path

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.

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 explanation 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 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?