Seems like life was a lot easier when all your effect pedals could be powered by batteries—except when one goes dead in the middle of a gig and finding out your spare is also dead. With modern, power-hungry pedals, you often have no choice but to use a power supply. Fortunately, plugging in can go smoothly if you understand a few of the basics of powering your pedals with a power supply.
Voltage is measure in volts (V). If your power supply reads 9V, then it is a 9-volt power supply. Use a power supply with the same voltage rating as the pedal—not more, not less.
Current is measured in amps (A). A milliamp, or mA, is one-thousandth of an amp. The current rating on your power supply is the maximum it can supply. For example, if your power supply reads 100 mA, then it can supply up to 100 mA of current. Less is okay, but not more.
Use a power supply with a current rating greater than the current rating of the pedal connected. If you are powering multiple pedals from a single power supply (daisy-chaining, more on this later), then add up all the current ratings of your pedals. Your power supply must provide more current than this total.
Most pedals are DC (direct current). Pedals requiring AC (alternating current) usually come with a dedicated power supply, and it is recommended that you use only this power supply with the pedal.
DC voltage has polarity, which simply indicates which conductor has positive voltage and which has negative. With a barrel-type power plug, this is usually specified as center-positive or center-negative. A center-negative plug will have the negative voltage on the center conductor and the positive voltage on the sleeve (outside) conductor.
A 9V DC center-negative power supply is the de facto standard for pedals.
A side note:
Most electronic devices and their power supplies have center-positive polarity, so why do pedals have center-negative polarity? Because many pedals work with either a battery or a power supply; and, to preserve the life of the battery, it is disconnected from the circuit when either (1) a power plug is inserted or (2) the input plug is removed. This is accomplished by connecting the battery's negative terminal to the input jack's ring contact and the battery's positive terminal to the sleeve shunt on the power jack. When the input plug is inserted, it completes the circuit for the negative battery terminal. When a power plug is inserted, it breaks the circuit to the positive terminal of the battery. Power jacks do not have a shunt for the center (pin) contact, so a center-positive power supply would not work in this type of circuit.
Many off-the-shelf non-pedal power supplies are unregulated. These will provide the specified voltage when the specified current is drawn, but the voltage will likely be higher—often by several volts—for lower load currents. These power supplies sometimes have high voltage ripple, which can result in hum or buzz in the audio signal. For these reasons, it is highly recommended that you power your pedals with a regulated power supply.
Instead of the bulky transformer in a conventional power supply, a switching power supply uses a high-frequency switching circuit with a small transformer to generate a regulated power supply. This type of power supply is smaller and lighter, can produce higher currents, and can accept a wide range of line voltages—hence the name universal-input.
The disadvantages of switching power supplies are higher cost and a potential for high-frequency noise interference (from the switching circuit). However, the noise usually is not an issue with a properly designed power supply.
A daisy-chain cable may be used to power several pedals from one power supply. Generally, this works without incident when all the pedals are negative-ground referenced, meaning that they internally connect the negative terminal of the power supply to ground (i.e., the signal cable shield). Rarely, a pedal will be designed using a positive ground or split supply; and these pedals should not be daisy-chained with negative-ground pedals. Almost always, a pedal requiring a standard 9V center-negative supply will be negative-ground referenced.
Some people may be concerned about ground loops with a daisy-chained power supply; however, this is usually not an issue when your interconnecting cables are short.
Rechargeable Battery Packs
Rechargeable battery packs are becoming more popular and are a great alternative for those who want to cut the cord.
Rechargeable batteries have a current capacity specified in mAh (milliamp-hours). Add up the mA ratings of the pedals you will be powering with one battery, then multiply that number by the number of hours of run time you need before recharging. This is your minimum current-capacity requirement: choose a battery pack that can provide this capacity or more.
If your requirement is greater than what you battery can provide, you'll need to split your pedals between two or more batteries.