1. Introduction

Foot Pedals, or Stomp Boxes, come in many different flavors. Some mix in other sound sources with the pass-through signal, but most enable effects on the signal. Pedals can be daisy-chained so that the main audio source passes through all of them. Each pedal has monophonic or stereo input and output ports. Those that mix in another signal have a third port for the second source input.

The inputs and outputs may be DC or AC coupled. DC coupling ties the midlevel bias voltage of the internal signal path to the bias voltage of whatever other devices are in the pedal board chain. AC coupling uses series capacitors to reference signals on the cables to a common reference, like zero volts (wherever that is in relation to earth ground…). If those capacitors are not carefully chosen, low-frequency signal energy can be filtered out, and phase distortion created.

Applying power to daisy-chained pedals can have issues, too. One is related to the DC coupling described above. Another is the potential for creating multiple ground paths, called ground loops, which can induce noise interference in the system.

Pedals do things to the signals that pass through them. That is obvious. What may not be is obvious are the unneeded things a pedal might do: add noise and delay, limit frequency and dynamic range, and signal inversion. The last disturbance may by inconsequential to many users, but to some it is critical.

Few souls like politics, especially when you are trying to keep your eyes on the prize. The prize in this case is simple, low-noise, great sound effects that add to your performance. However, being aware of pedal board politics can keep you out of trouble.

I have included crude, underlit sketches with this article to illustrate the concepts described herein. In some cases, I patched in scraps of redrawn figures to correct mistakes made in my doodles. I hope that they add a fireside hominess to the presentation. Seriously, I was tired of wrestling with Microsoft draw tools and was too lazy to spiff them up.

2. Pedal Bypass

Pedals having a bypass feature will maintain a passive input to output path when the pedal is not powered up. The figure below shows a bypass function implemented with a single pole, double throw switch or relay that defaults to the bypass path when not powered up.

The user may want to have a working signal path when power is not applied to the board. An input to output path that is “active” is only maintained when the pedal is powered up. A pedal with bypass usually has an active path with unity gain. One without a passive bypass path can have larger gain to bring a microphone (~1mV) or guitar (~10mV) input up to line level (roughly 0.5Vrms to 0.75Vrms). Wah-wah pedals also require high input-to-output gain to make the frequency shift effect work (check out Miller effect capacitance). These high gain pedals must go at the front of the pedal chain to prevent excessive line levels and clipping by downstream devices. A pedal with a bypass feature must have an active path that is non-inverting (see “What Happened to My Sound?” below).

3. Input and Output Port Coupling

DC coupling puts the internal DC signal bias out onto the “sleeve” or “quiet” or “shield” conductor of the cables that connect to the upstream and downstream pedals. This will have consequences, unless addressed by the Pedal Power, as will be shown later. AC coupling on an input or output port dissociates the audio signals from the internal “quiet” potential and references them to the sleeve/quiet/shield conductor of the cables at a common potential. Which “common” potential is it though? Is it the 0V terminal of the 9V power supply? Is it earth ground for the building’s electrical system? Does every pedal on your board reference to the same potential? Not likely. But these questions may be again addressed by the Pedal Board Power.

a. AC Coupling

For AC coupling of pedals, a good approach might be to apply AC coupling on the inputs, and DC coupling on the outputs. Since the line-level termination resistance is set on the inputs to the pedal, one may select a coupling capacitor value with a high-pass corner frequency below the desired lower limit to the frequency response. The designer does not know the impedance of the downstream pedal, so he/she must resort to over-specifying the component. Another downside to AC coupling both the input and output port signals is that the capacitance of the combined series combination of the two AC coupling capacitors is lower than either of them:

Cequivalent = Cout*Cin/(Cout+Cin)

possibly cutting off low-frequency signal energy.

b. DC Coupling

The “quiet” potential of every pedal may be different with respect to the 9V supply, so if the inputs and outputs are DC coupled, precautions must be taken when pedals are daisy-chained together.

c. Coupling Issues

For DC coupled inputs and outputs, these different potentials create DC currents on the signal paths between pedals if all the pedals on a board are run off the same 9v supply. These DC currents waste power and can induce noise in the signal paths.

Two steps are taken to resolve the problem of DC currents:

1. Use pedals that have AC coupling on both the input and output signal ports, and that these ports are all AC coupled to the same potential, like 0V. This method may be difficult to manage.

2. Run each pedal on a separate galvanically isolated power supply. This method is described further below. It is generally a more expensive solution, but almost worry-free.

Galvanic isolation is a principle of isolating functional sections of electrical systems to prevent current flow; no direct conduction path is permitted.[1][2]

1. John Huntington Show Networks and Control Systems: Formerly Control Systems for Live Entertainment 2012 ISBN 0615655904, page 98

2. ^ "Description of Galvanic Isolation". Schneider Electric. Retrieved 2019-03-29.

-from Wikipedia

4. Pedal Board Power

Power to most pedals is fed on a 2.1x5mm barrel plug to a 2.0x5.1mm jack on the pedal, at 9VDC, with the center conductor of the plug/jack being the negative polarity. Some pedals run on 12VDC, but the majority are at 9VDC. Each pedal creates its own internal DC signal bias, which ideally sits at the midpoint between 0 and 9V to permit equal positive and negative dynamic range for the audio signals processed by the pedal circuitry. The DC bias is not always 4.5V, and it can vary from pedal to pedal. The cables that tie the pedals together, and ultimately the amplifier system to which the last pedal is connected all have “quiet” conductors that force each pedal’s “quiet” node to the same potential.

In the figure below, the “quiet” node is the shield conductor of all the cables. The potential of this node is assumed to be zero volts. That zero volts needs to be in reference to some other known potential. The amplifier may have a grounding switch that connects its chassis and the Line In cable shield to Earth ground. If the switch is off, the chassis takes on a voltage close to Earth ground anyway, since (hopefully) no other power source feeds that node. But all cable shields, foot pedal cases, guitars, etc. connected to the daisy chain are at “zero” volts.

Pedal #1 has AC coupled Line In and Line Out. Pedal #2 has a DC coupled Line Out, and Pedal #3 has both ports DC coupled. The quiet potential of Pedal #1 is 4.5V, Pedal #2 is at 5V, and Pedal #N at 4V. Pedal #1 adapter power is from 0V to 9V only if the case is connected to the V1- power conductor, otherwise, V1+ and V1- could be any voltages as long as their potential difference is 9V. Pedal #2 power is from -5V to +4V because of the DC coupling on its Line Out port. Pedal #N power is from -4V to +5V, also because of DC coupling. Galvanic isolation of the individual foot pedal power supplies allows these potential shifts to happen.

a. Ground Loops

Along with the chance of overloading the power supply, using a common power supply for multiple foot pedals creates a ground path alternate to the path through the daisy-chained signal cabling. This alternate path creates ground loops which can induce noise from electromagnetic interference (EMI) energy passing through them. The drawing below attempts to show vectors of EMI passing through ground loops created by the signal cables and the cables from a common power supply. EMI has electrical (E) and magnetic (H) fields from radios, microwaves, cellphones, etc. The EMI induces noise that is mostly common to both conductors in each cable. If a connection to Earth ground is required, it should only be made at one point in the system. Any sound component worth its salt should have a grounding post or a switch to disconnect Earth ground from its chassis. The user can then make the connection wherever he or she chooses to.

b. Lousy Power Supply

A wall wart or AC to DC adapter may have the right voltage output and good load regulation (ie: it holds that voltage as the current demand varies), but it could have poor noise performance. The device to which it connects has a limited amount of noise filtering on its input. It’s just a small box after all. Whatever noise is filtered out on the AC adapter may be picked up again on the cable that brings the 9V power to the pedal. To suppress this noise, a common mode power choke may be used. The common mode power choke has appreciable impedance (mostly inductive) down to the1kHz frequency range, and pass DC currents of 100mA to 1A. It is more than an EMI filter, which usually specs its working impedance at 100MHz. This power choke passes differential DC current and kills common mode AC. The noise on an AC adapter output and the supply cable is predominately common mode, meaning that the same noise is present on both the positive and negative conductors.

5. What Happened to My Sound? (inverted signals)

Some pedals have circuits that invert the signal from input to output. Others do not. Think of signal inversion as a 180 degree shift in a 1kHz sinusoidal signal. If the inverted sine wave is added to the inverted version, the result is zero signal. Feeding both the inverted “effect” and the non-inverted “buffer” signal to a mixer creates an attenuated mixer output, as shown in the figure below.

A pedalboard that “processes” and audio signal may use several pedals to achieve the desired effect. The accumulation of inversions and non-inversions in the chain will result in a final inverted or non-inverted signal. Signal inversion is only an issue if the “processed” signal output is mixed with the “unprocessed” version of the same signal. Ignoring any other minor signal delays incurred in the chain, the good portion of the processed and unprocessed signals will be canceled out after summation. In cases like this, it is important to know the “sense” of each pedal. Buy some red dot stickers and affix one to each signal-inverting pedal.

6. Summary

Assembling a pedal board can be a political exercise, in the sense that there are many possible answers to the questions that arise when delving into the task, and the direction you ultimately take will not please everyone. It would be nice and quick if you could just connect all the pieces together like building blocks, and play on. Issues like the bypass function, signal processing sense, pedal power supplies, input and output signal coupling, and grounding strategy must all be dealt with to achieve desirable “effects”.

Jus'qu'a demain,

Ken Buttle

Pedal Board Politics