The Eurorack modular ecosystem is ideal for single-string guitar processing due to the large variety of modules available, allowing for essentially an infinite number of effects combinations.
The majority of Eurorack modular systems today are dedicated to synthesis, sequencing, and percussion applications, to name a few. All modules in such applications are processing signals at full Eurorack levels, namely 10Vpp or higher. Modular synthesizer noise originates from mainly two sources:
Switching power supplies
Digital modules
Both of these sources are of even greater importance when processing guitar signals; in addition, there's a third source of noise to deal with:
The first stage of amplification from low-level string signals to Eurorack levels
In this section we will take a closer look at each source:
Switching Power Supplies
The most popular power supplies for Eurorack modular systems are either a 15-18V AC-DC switching adapter coupled with a power distribution board (busboard) that generates +12V, -12V, +5V
or built-in AC-powered closed frame triple output power supplies with passive power distribution boards.
In both cases, a significant amount of power supply ripple (80-120mV) and noise is present, even though some power distribution boards employ modest filtering.
In modules with amplification stages the switching noise is manifested as a high-pitched whine that gets worse with increasing circuit gain.
Digital Modules
Modules that contain microcontrollers or microprocessors can inject clock noise back into the busboard ground planes and spread noise throughout the modular system unless care is taken by the designers to apply proper filtering. This can be mitigated by employing separate power supplies for digital modules, as well as power distribution boards with low-impedance ground paths (note that the latter cannot mitigate switching power supply whine).
High Gain Modules
At around 30-40dB of amplification (which is the range required to amplify guitar string signals to Eurorack modular levels) even the slightest amount of noise in a guitar signal becomes an obvious nuisance. Depending on noise-prevention techniques utilized in the guitar the noise can be a 50-60Hz hum, massive amounts of hiss, or even radio station reception! The entire ground path from modular power supply through the amplifier module and cable to the guitar pickups plays a critical role in suppressing guitar noise.
Recommendations in Order of Effectiveness
Use linear power supplies for lowest possible ripple & noise; the good ones are typically around 1mV RMS. I recommend Acopian Gold Box 51212T Triple Output supplies as the best you can buy.
Other good options include the Konstant Lab ZDROJ
and L-1 PSU.
Use power distribution boards that are designed to have the lowest possible ground impedance. My recommendation is the Genus Modu LIBB.
For a lower cost yet good performance solution, look to the Intellijel TPS80W Triple Power Supply Eurorack Busboard.
Don't get the switching AC-DC adapter - instead use a 120W HiFi Ultra Low Noise Linear Power supply to provide the 15V needed.
Don't forget the Guitar!
Use a single-point "star" grounding system inside the guitar with the largest AWG wire possible
Shield any active electronics using the copper foil method, including the inside of the pickguard
Use multiple conductors in the guitar cable for ground connection; do not rely on the shield as the only ground - it is there to shield the conductors and does not have a low enough impedance.
For more details on our noise studies, please see the following thread on VGuitar Forums.