Power supplies are everywhere. A large percentage of the electronic devices we use have power supplies, whether they live in or outside the chassis. Some of these are large rack-mounted units that power consoles, but let’s focus on the more troublesome denizens, a.k.a. wall warts and line lumps. Unless something blows up, we generally don’t have much interaction with power supplies of the internal variety, but as to those wall warts and line lumps? That’s a different story. They tend to be a constant source of aggravation, disappearing into the void and showing up when you least expect them.
The Hows and Whys
Power supplies convert local AC line voltage into the lower AC or DC voltages required by audio circuits to operate. The audio circuitry of your average wireless receiver would be pretty shocked (ha!) if you plugged it directly into an AC outlet.
Manufacturers use external power supplies for a variety of reasons, such as isolating audio circuitry from noise generated by power transformers, reducing heat by getting the PS out of the equipment chassis, reducing size of the device and annoying you. There’s also the issue of cost. Any electronic device sold on the mass market in most countries must pass regulatory certification, and it’s easier to avoid the cost of doing so by sourcing an off-the-shelf power supply that’s already been approved.
One advantage of an external power supply is that, if it should fail, it generally won’t damage the audio device itself, and you have a pretty decent chance of replacing a blown supply with a spare that’s in your work box (if you’re smart), or by sending a runner to the local music store to purchase one before sound check. External power supplies also allow a manufacturer to build one device that runs on a specific power requirement (12 volts DC, for example), and then furnish wall warts for use in specific regions. In fact, some manufacturers (certainly not all) package their gear with “universal” wall wart power supplies that accept voltage inputs ranging from 100 to 240 volts AC, and employ slide-on adapters to mate with various local AC sockets.
Solving the Mysteries…
So, how do you know you’ve got the right power supply for a given device? Well, you might be lucky and find that the manufacturer has put their name on the wall wart and identified it “for use with [insert the name of your favorite guitar pedal here].” Good luck with that. More likely it will show the name of the company that built the power supply, not the manufacturer of the audio device. That’s why it’s a good idea to label your wall warts with the name of the device they’re used to power. A small piece of white tape and a quick Sharpie scribble now may prevent later hassles.
Labels notwithstanding, you can start by making sure that the connector is the correct fit for the jack. Most — not all — wall warts and line lumps use barrel connectors (as shown in Fig. 1), and of course these things not only come in different lengths and diameters, but also with different internal diameters to ensure a snug fit with the pin inside the jack.
The next thing to do is check out the power requirements for the device. You’ll usually find this info on the rear panel near the power jack, as shown in Fig. 2.
If you look carefully at Fig. 2, you’ll find some useful information. First, you’ll see that the device requires 12 volts. AC or DC? See that little graphic with the straight line and three short dashes underneath it? That means the device needs DC power. If you see a little sine wave, that means the device requires AC. Or maybe the manufacturer has made it easy and actually printed “AC” or “DC” next to the value. Note than the supply in Fig. 3 outputs 9 VDC and cannot be used with the drum module shown in Fig. 2.
The diagram in Fig. 4 indicates whether the center pin or the barrel is + (hot). We’re talking about DC, so polarity matters. In this case, the pin is hot and the barrel is negative; the power supply must match this arrangement. If not, the device won’t work (best case) or you will damage the unit (worst case).
You’ve got the right size connector, the correct voltage and the correct polarity, so now you’re good to go, right? Not so fast. You need to ensure that the power supply can deliver the correct amount of current that the device requires. You may not find this on the rear panel so you might need to consult the manual. Look for the power requirement spec (duh), specifically the current draw which refers to how much current the device needs in order to operate. Current is measured in amps (A) or milliAmps (mA). The current draw for an audio device with an external power supply can range anywhere from a meager 15 mA for a Boss CH-1 Super Chorus pedal to 600 mA (or more) for an outboard microphone preamp.
It’s Complicated…
The next question is: Does the current capability of the power supply need to match the amount of current required by the device? The answer is: No, it should be higher for safe operation.
Here’s an example. Boss manufactures a power supply that can be used in lieu of a battery for many of their guitar pedals, the PSA-120S. It’s rated to supply 9 volts, 500 mA. It can easily power the CH-1 pedal because the CH-1 requires only 15 mA.
In fact, the PSA-120S would have a lot of headroom when powering the CH-1, so it won’t be taxed. Given the correct cables, the PSA-120S could also power a few more Boss pedals. Take for example, the TU-3 Chromatic Tuner Pedal, specs shown in Fig. 5.
The TU-3 uses the same voltage as the CH-1, and requires 30 mA (minimum), or 85 mA when used in High Brightness mode. To determine if the PSA-120S can power both pedals we need to know if the voltages of the power supply and the pedals match (they do), and then add their current requirements: 15 mA + 85 mA = 100 mA, a value which is still well below the capabilities of the PSA-120S. Note that we do not add the voltage requirements of the pedals.
We could continue adding more pedals to the chain as long as they run on 9 volts DC, and their total current draw doesn’t exceed 500 mA. If we ask the PSA-120S (or any power supply) to deliver more than its rated current, one of several things will happen: (1) the power supply may shut down to protect itself; (2) the power supply may overload and burn out; (3) some or all of the devices work, but they don’t perform the way they’re supposed to (for example, increased distortion, higher noise floor, displays showing weird characters; or (4) none of them work.
The same possibilities apply when you attempt to power a single device from an under-rated power supply.
One thing to remember is that wall warts multiply like rabbits. If you put one or two in a drawer, the next time you open it, there will be a half-dozen in there — and if they’re labeled, you life will be a lot easier.
Steve “Woody” La Cerra is the tour manager and front of house engineer for Blue Öyster Cult.
