Tons of electronic gizmos made for musicians, from interfaces to effects, use external power supplies—those cube-like enclosures that plug into the wall or power strip and then deliver electricity to the device via a cord and a plug.
We have to put up with the external versions, because internal power supplies are both more expensive (meaning more expensive for the maker to implement, who would then pass the expense on to you, the consumer) and bulkier (meaning some smaller effects would have to be made larger). So external power supplies, like death and taxes, are unavoidable facts of life.
Most external power supplies (sometimes called “AC adapters,” though the word “adapter” indicates a physical, and not electrical, transformation, as discussed in Tip #2 below) still come in the form known derisively as a “wall wart,” because the prongs that go into the outlet protrude directly from the box. This results in awkward placement both in a wall socket as well as a power strip. Often, wall warts will make inaccessible adjacent outlets, which is, frankly, rude. In recent years, some manufacturers have tried to vary the form factor, opting instead for a “line lump,” where the box that contains the power supply’s guts is further down the line and not right at the outlet prongs (similar to laptop power supplies). This is a more desirable option, and some designs even allow for a detachable AC power cord for even more versatility. Below is an example of the respective form factors of a wall wart (left) and a line lump.
Beyond the physical wrangling of an external power supply, the most critical attention must be paid to its specs: the type (AC or DC output), polarity (for DC; whether the tip is negative or positive), voltage, and current (in amps, or often milliamps).
Try as we might to keep them together, power supplies and the devices they belong to can become separated, leaving you scratching your head as to what supply you can safely use as a substitute to power your interface or effects pedal. And it's a reasonable expectation, assuming you know what you're doing.
First and foremost, locate the device's manual to confirm its power requirements. Most devices will have their specs posted somewhere online, perhaps even in a downloadable pdf manual. Once you know what the unit needs, you can start rummaging amongst your collection for a suitable replacement. And so that you know what you're looking for, here’s a brief explanation of the key features, terms, and symbols that appear on external power supplies.
Input: These requirements will always be listed in AC terms, because you’re plugging into a wall. If the supply is intended for use in the U.S., it will be marked from 110V to 120V, 60Hz; Europe and elsewhere is 220V to 240V, 50Hz.
Output: This is tricky, as most external power supplies (especially smaller ones, like the cube-shaped wall warts) output DC, but there are some that output AC. Make sure you distinguish which is which. You don’t want to mix up AC and DC. Usually the power supply will label its system using the letters “AC” or “DC,” but sometimes you’ll see the symbols instead, which look like this:
Polarity: Polarity is not an issue with AC, but since most power supplies are DC, it’s important to identify the plug’s polarity with regard to the tip and sleeve. This is almost always identified schematically, using two concentric circles with plus (+) and minus (-) signs and a line going to the center (tip) or outside ring (sleeve). Here’s how it looks graphically:
This is in no way standardized, but the majority of power supplies use the scheme negative tip/positive sleeve. Some pedals (such as ones by Tech 21) have sensors to gauge the polarity of a plug and adjust itself accordingly, but not all manufacturers are as accommodating as Tech 21, so take care in noting both the power supply’s and the pedal’s polarity orientation.
Voltage and Current. These are measures in Volts and Amps (using the letters “V” and “A”). Voltages will vary from about 3V to 18V. Amps are often measured in milliamps (mA), because they don’t usually exceed 1,000. A typical current rating is 500mA (0.5 amps). If you don’t have the original supply and are in a pinch, try to match as close you can the Voltage and Current to the device with the best choice among power supplies you have at hand. Generally speaking, it’s better if the supply has higher numbers than those required for the device. The device has protection circuitry to ward off higher values, but a mismatch where the supply's numbers produce lower current or voltage is neither good for the device nor the supply.
Plug size and fit. The power supply’s plug has to fit the jack (socket) of the unit, and be able to mate with its positive and negative terminals correctly. If you’re trying to find a replacement power supply for an effect, it doesn’t matter if all the specs match up exactly if you can’t fit the plug in the hole. Fortunately, there are just a handful of plug sizes to choose from, though this aspect is also disappointingly non-standardized (just like cellphones).
Now that you know the terms and symbols, take a look at the image below, which is a photo of the actual faceplate of a Yamaha power supply. Note the red text that tells you what each significant number means.
Here are two tips for dealing with power supplies and the units that use them.
Note that this supply can handle both U.S. and European input sources (110 and 220V, 60 and 50Hz). Also note that it doesn’t use two concentric circles to indicate polarity (see lower left of unit), but a more literal rendering of the plug outlines. You can’t quite read the text above the horizontal voltage switch (lower right), but notice that these voltages are also listed above in the text (1.5 to 12V). The Power Output doesn’t read “DC” but instead relies on the symbol (two parallel horizontal lines, one solid, one dotted), which you can see placed just to the right of the "V." Finally, this unit will supply current on demand (depending on the needs of the device), but has a 1,000mA (or 1 Amp) maximum ("MAX.").
The best thing for keeping power supplies straight is to never have them part company with the unit they power. But this just isn’t practical, as they’re very different shapes. It’s not uncommon to have a drawer or a box full of power supplies and a shelf of stacked effects. In a perfect world, there would be a one-to-one correspondence of power supplies to devices, but somehow you will always end up with more supplies—sometimes with ones with of inscrutable origins. Remember too that power supplies can become lost or damaged. So it pays to be able to read the specs, know the functions, and decide how you can furnish an alternate power supply if you need to.
Jon Chappell is a guitarist and the Senior Editor of Harmony Central. He has contributed numerous musical pieces to film and TV, including Northern Exposure, Walker, Texas Ranger, All My Children, and the feature film Bleeding Hearts, directed by actor-dancer Gregory Hines. He is the author of The Recording Guitarist: A Guide for Home and Studio (Hal Leonard), Essential Scales & Modes (Backbeat Books), and Build Your Own PC Recording Studio (McGraw-Hill), and has written six books in the popular Dummies series (Wiley Publishing).
My experience has been that matching voltage is more important than matching current. For example, if a device is designed to use 5VDC and 800mA, the device will work properly if I connect a 5VDC (match) and 1000mA (greater) power supply. The devices I have used will draw the current they need, which is why I am able to use a supply whose current output is greater than what the device is designed for. I try to avoid using a power supply whose voltage output is greater than what the device is designed for. I am not aware of any device that will only draw the voltage needed if too much voltage is applied.