State of the Stomp: Power Corrupts, and Absolute Power Corrupts Absolutely

What are voltage, current, and power, and why would a guitar player care about them? Because unless you play all acoustic with the lights off, you have to harness their properties.

Let’s start with power. Regardless of how knowledgeable you are about electricity, you probably have some intuitive sense of power. How does the horsepower of a car affect the driving experience? What does it feel like to operate a powerful drill?

You can think of power as how much work can be done over a period of time. Work is defined as applying a force through a distance. If I push on the wall, I’m applying a force, but not doing any work. If I pick up a weight and lift it over my head, I’ve done work. The faster I lift it, the more power is required.

In electrical terms, power equals voltage times current. (Engineers chose the letter “I” to represent current because “C” already meant something else.)

Driving on the freeway, you might see power lines. These have a high voltage and carry a high current to deliver lots of power. When you get a static shock on a doorknob, the voltage is very high (maybe 10,000 volts), but since the current is very low, it doesn’t kill you. There’s not much power there.

Current Affairs
What is current? Engineers define current as the amount of electrons that flow over a given unit of time. Current always flows through something. If we compare electricity to water, current is the amount of water that flows through a pipe in, say, gallons per second.

What is voltage? This is the tricky one. Voltage is the electrical potential between two points. I know, that’s probably not very helpful, even though it’s correct. Voltage always exists across or between two points. (Never say “voltage through”—it’s always “voltage across” and “current through.”) Back to our water analogy: Voltage is like the water pressure. The pressure is there, waiting for the valve to open, allowing the water to flow. Similarly, a battery’s voltage is there, waiting for current to be drawn by connecting it to something.

Pedals and Power
What does this have to do with effects pedals? Say you bought a Strymon Orbit flanger with a wall-wart adapter that reads “Output 9VDC 660 mA.” What does this mean?

If I plug the adapter into a wall socket, but don’t connect the output to anything, then we'd measure 9VDC at the plug. But since nothing is plugged in yet, the current would be zero. The power is zero, since nine times zero is zero.

What does the 660 mA mean? The amount of power the adapter can provide is nine times .66 A (660 mA is the same as .66 A), or about six watts. If we try to draw more than 660 mA, the adapter will protest and shut down.

Now let’s plug our adapter into the pedal. What’s the current draw? As an example, the spec sheet for the Orbit flanger says 250 mA, so it must be 250 mA, right? Well, not exactly. The real number depends on what the pedal is doing, so there is no way to know without measuring. Ideally, it should be less than 250 mA, because it’s always a good to have some “extra” current available from the power source to ensure that everything is stable under all conditions.

Let’s look at it again: We have a power source (the wall adapter) and a load (the pedal). The power source provides the rated voltage (9V) as long as the current drawn by the load is less than the rated current (660 mA). The load draws whatever current it needs.

What happens if we plug the pedal into a different power source? As long as the power source can provide the current required by the pedal and maintain (but not exceed) the recommended voltage, everyone is happy. Things get murky if the power source can’t maintain the voltage.

Troubleshooting

If you’re using a third-party power source (such as those sold by Voodoo Lab, Ciocks, and Dunlop) but are having trouble powering your pedal, try running through these steps:

1. Double-check the current and voltage requirements recommended by the pedal manufacturer.

2. If your pedal shipped with a wall adapter from the factory, try powering with that. If it powers up, it’s probably a setup issue with your third-party supply.

3. Check the current. Make sure the supply is capable of providing more current than the pedal requires. For example, if your pedal requires 200 mA, it’s probably best to provide it at least 250 mA to ensure it works properly under all circumstances.

4. Check the voltage. Some pedals are very specific about their voltage requirements. If the pedal requires 9V, be sure not to send it more than 9V, because excessive voltage can irreparably damage a device. On the other hand, some pedals, such as fuzz boxes, are perfectly happy running at anywhere from 9V to 18V. Just be sure you know what the manufacturer recommends. (See step 1!)

5. Finally, check the polarity. Most pedals have center-negative power jacks, though there are a few with center-positive ones. Polarity is often indicated by a diagram on the pedal, but you may need to check with the manufacturer.