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Pedal Power

Stompbox Power Supplies By Paul White
Published June 2022

Pedal Power

Does it matter how you deliver power to your pedalboard?

Effects pedals aren’t only for gigging guitarists. Simon Small’s 'The Studio Pedalboard' article in this issue makes the case for having a ready‑to‑go pedalboard in your recording studio, and he has also written about the potential of pedals as mixing tools. Many keyboard players and modular synth enthusiasts make good use of pedals too. But, however you’re using them, the correct performance of your pedals depends on them receiving clean power of the correct voltage.

Battery Or PSU?

When pedals started to become popular, most were powered by 9V batteries. From a purely electrical perspective that’s a good system too. No special power‑conditioning circuitry is required inside the pedal (perhaps a capacitor across the supply rails) and there’s no risk of ground loops. Because battery power means that each pedal has its own power supply, mixing germanium and silicon pedals that work with power supplies of opposite polarities won’t confuse the user either. There’s even debate about what type of battery sounds best with certain pedals: some cheaper zinc battery types exhibit a voltage sag as more current is drawn from them, and that can alter the way a fuzz or drive pedal responds... but I won’t delve into that here!

A downside of batteries is that they eventually run out of power. While some analogue drive pedals’ batteries might power them happily for, say, 100 hours of use, sophisticated digital pedals can demand high operating currents, meaning much shorter battery life; a digital delay pedal can easily eat up a battery in a single gig. It’s usually better to use external power supplies, then — though having said that, there are now some pedalboard powering systems which run from a rechargeable lithium‑ion battery, similar to those we use for running power tools, and these now have sufficient capacity to power a pedalboard for several hours from a single charge (a lithium‑ion battery is usually good for up to 1000 charge cycles, but they should not be allowed to run completely flat).

With conventional 9V batteries, you’d be lucky to power some digital pedals through a whole set! But rechargeable lithium‑ion power supplies could be a practical option.With conventional 9V batteries, you’d be lucky to power some digital pedals through a whole set! But rechargeable lithium‑ion power supplies could be a practical option.External PSUs have traditionally been linear supplies based around a small transformer, a rectifier and some basic smoothing circuitry, but some modern ones designed to deliver a higher current are switch‑mode types — you can read more about those in the separate box. Boss set the template for much of today’s pedal powering by adopting a centre‑negative wiring scheme, which simplifies the automatic power‑on switching, using the jack contacts to connect the negative power supply when a jack is inserted. There are some unusual pedals that work with the opposite polarity of connector and, at some point, Boss increased the rating of their popular pedal PSUs from 9V to 9.6V — presumably to allow for the 0.6V drop caused by putting the safety diode inside their pedals that prevents damage when a reverse‑polarity supply is connected by mistake.

Some pedals might require a different voltage, 12V and 18V being typical examples, and a small number require AC current rather than the usual DC. So always check the requirements of your individual pedals when shopping for a power supply. But as long as your power supply is the correct voltage and polarity for the pedal, and it can supply at least as much clean current as the pedal demands, it should do a decent job. It doesn’t matter if the PSU is rated at a higher current than a pedal needs but one rated too low may cause excessive hum, failure to operate or even overheating of the power supply! The voltage, polarity and current should be marked on PSUs but using a label printer to label PSUs for specific pedals will prevent confusion.

It doesn’t matter if the PSU is rated at a higher current than the pedal needs but one rated too low may cause excessive hum, failure to operate or even overheating of the power supply!

One For All?

If you need to power a lot of pedals, an expandable power supply system such as this one by Strymon could prove more convenient than a single large PSU.If you need to power a lot of pedals, an expandable power supply system such as this one by Strymon could prove more convenient than a single large PSU.

When you use more than one pedal, should you use one PSU and daisy‑chain power, a separate PSU for each pedal, or a single PSU with multiple connections?

If the cables are kept very short, the risk of audible mains hum caused by powering two pedals from the same supply is very small. Using separate power supplies for each pedal does avoid the potential for ground‑loop hum, since there’s no connection to the mains ground, but if you have five or six pedals on a pedalboard, the space taken up by mains distribution strips and all the power supplies makes that approach impractical. Also, badly designed power supplies can inject noise back into the mains supply, affecting other connected pieces of equipment. So a more sophisticated approach to powering all your pedals is required if you want to guarantee you’ll get the best from them, with no unnecessary hum or noise.

If you add the current requirements of all your pedals and buy a single power supply capable of delivering that amount of current or more, a daisy‑chain cable can be used to power multiple pedals — if the pedals all operate on the same voltage and polarity, and the power supply is of good quality. But, again, unwanted noise may still be an issue because any pedal that doesn’t draw a consistent current may inject noise back into the 9V power line, and that will affect other connected pedals. There’s also potential to create a ground‑loop hum, since the pedals are connected both by the power‑supply ground and the signal‑cable screen. As I’ve mentioned, keeping the patch and power cables as short as possible will help avoid ground‑loop hum but, as a general rule, you should only use daisy‑chaining for low‑current analogue pedals such as overdrives, and chain as few pedals together as possible. If you have a multi‑output power supply and have run out of outputs, then it’s often acceptable to power two low‑current analogue pedals from one output using a splitter cable.

Regulation & Isolation

Many basic pedal PSUs are unregulated, and their output will vary with the mains input voltage and the amount of current drawn. Electrically, these are very simple, usually comprising a transformer followed by a rectifier and basic smoothing circuit. At the high end of their current rating they may suffer power supply ripple, as the smoothing capacitor is unable to level out the voltage between mains cycles, and this manifests itself as hum. Most analogue pedals can tolerate a little voltage variation, and if the current drawn is well within the spec of the PSU there should be no excessive ripple but, to ensure optimum performance, a regulated power supply is best. Regulated supplies can either be linear or switch‑mode types, the essential point being that their output voltage is stabilised at a fixed value, regardless of reasonable changes in input voltage or current draw.

Most serious pedalboard power supplies have multiple outputs that are completely isolated from each other, meaning they don’t share a common ground or power line. In linear PSUs, this can be achieved by using a mains transformer with several independent secondary windings, each feeding its own regulated power supply. With switch‑mode supplies, each output can be fed from its own power supply circuit because the necessary transformers are very small.

Some inexpensive pseudo‑isolated supplies, such as the one made by Caline, claim to have isolated outputs but aren’t completely isolated. They usually have a separate voltage regulator for each output or group of outputs, and may work for those on a tight budget, but the outputs share a common ground so ground loop hum could be an issue, and you can’t power opposite‑polarity pedals by using an inverter cable. Sometimes you’ll get good results, sometimes you won’t: much depends on the pedals you plug into them.

Space is often at a premium on a pedalboard, so it might be worth considering a PSU such as this one by CIOKS that slots neatly underneath a standard pedalboard.Space is often at a premium on a pedalboard, so it might be worth considering a PSU such as this one by CIOKS that slots neatly underneath a standard pedalboard.

Other Considerations

There are several other things to weigh up, including how the PSU connects to mains power. Some switch‑mode supplies use a laptop‑style external power adaptor, while others have a standard IEC mains socket. Physical size can be important if you want to mount your power supply underneath the pedalboard, and if you plan on international travel a universal‑voltage PSU will save headaches.

The number of outputs is obviously a factor, and if you plan on your pedalboard growing over time perhaps consider a PSU for which an expander unit is available. Strymon and CIOKS, for example, both make some such systems. It’s not just the sheer number of outputs that’s important, though: each group of outputs will often have a different current rating, and you must ensure that there are enough high‑current outputs to cater for all your high‑current pedals. Then there’s voltage: not all pedals run at 9V or 9.6V, so check that your prospective purchase has the right combination for your needs. Do you have one of those rare pedals that needs an AC power feed? If so choose a PSU that includes an AC output of the appropriate voltage and current. Some models provide you with a few outputs of switchable or variable voltage, and these can be very handy when you need to accommodate oddball pedals. Some even have switchable sag, to emulate the performance of those cheap batteries.

Don’t worry if you have a pedal that requires a centre‑positive supply, though: an isolated‑output power supply makes it possible to use a simple polarity‑swap cable to hook these up. It’s also possible to wire two 9V outputs in series to give you an 18V supply, should you have a pedal that needs that and a PSU with no dedicated 18V output.  

Switch‑mode Power Supplies

In recent years more manufacturers have opted for switch‑mode power supplies because they’re more compact than linear ones and much more efficient. Also, ‘universal’ switch‑mode designs can accept an input voltage from around 100 to 250 Volts, so can be used in just about any country.

A well‑designed switch‑mode supply can supply surprisingly high amounts of clean current in a very compact form factor but can be costly. A poorly designed one, on the other hand, can generate very high‑frequency noise! There are several types of switch‑mode power supplies, and the type used in systems designed to power multiple pedals, each from an isolated output, starts by rectifying and smoothing the mains input in order to feed a high‑frequency switch based on some type of high‑voltage transistor. When turned on and off at a very high frequency, this outputs a chain of rectangular waves that feed the primary of a high‑frequency power transformer.

The main benefit of working at a very high frequency is that the transformer can be made very small. The secondary coil of the transformer is rectified to provide the necessary output, but to maintain a constant output voltage there must be a feedback system from the output to the switching circuit. In order for the output to remain fully isolated from the input, this feedback path has to be made with no direct electrical connection, usually achieved with an opto‑isolator.