PEDALBOARD POWER BASICS OF DC VOLTAGE
by Mark King
Pedal effects and stomp boxes were primitive in the 1960’s. An external power supply input jack was a rarity and there were no standards for how it should be wired. Most effects were distortion, wah or compression and these typically use so little current that a pedal can run on a single internal 9-volt transistor radio battery.
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| CLASSIC EHX 9-VOLT BATTERY |
In the early 1970’s MXR was a manufacturer and early market leader in stompbox production but their products did not feature external power inputs. In an effort to power their pedals from AC wall current MXR produced a four-output power supply which featured 1/4” guitar jacks to supply the power.
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| NOTICE 65mA MAXIMUM POWER |
MXR introduced replacement metal backs for their existing line of effects which included fixed cables that terminated with a 1/4” plug into their external power supply product. It was a clunky system and was not well accepted by the industry.
Many pedal manufacturers began to pop up but it was Boss that set the standard for the industry. In 1977 they released six compact effect pedals. A year later Boss released the DS-1 distortion and the year after that they released the CE-2 Chorus pedal. 1977 was my first NAMM show in Chicago and I remember seeing the colorful Boss pedals. While MXR was floundering with their cheapie “Commande” series in plastic enclosures Boss was beginning its ascent to greatness with colorful little metal boxes of sonic firepower.
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| CLASSIC BOSS PEDAL PACKAGING |
Boss pedals introduced the first viable external DC power system and it has become the standard virtually every manufacturer uses today. The little power plug that is used for connecting pedals was chosen because it does not short-circuit the power source when inserted into the socket.
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| BOSS TUNER AND DAISY CHAIN POWER CABLE |
The coaxial DC power plug is a two-conductor connector, it does not have mechanical differences between source and load ends, the connecting cables feature the same size, electrical connector on both ends so it is reversible. Often times cables come with a right-angle plug on one end and a straight plug on the other, this is purely a matter of convenience, the connectors are both 2.1mm in the critical area where power from the cable mates with the input connector on the pedal.
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| TYPICAL PEDAL POWER CONNECTION CABLE |
The hole in the center of the power connector is either 2.1mm or 2.5mm. Visually it is difficult to distinguish the difference. Boss created the industry standard by selecting 2.1mm for use on their stompboxes and effects pedals.
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| 2.5mm left red plug, 2.1mm right black plug |
The 2.5mm is used by some manufacturers (Voodoo Lab and Eventide) to distinguish their higher current demands. You can’t plug a 2.1mm plug into a 2.5mm socket because the center pin hole is too small on the 2.1mm.
NINE-VOLT DC POWER
It is very important to correctly wire your pedalboard power system, if you get the voltage wrong you can easily end up with a dead pedal.
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| A LITTLE SIGN ON WALL IN MY SHOP TO REMIND ME OF CORRECT WIRING |
Direct Current or DC power has polarity, it flows in one direction. The term “voltage or volts” refers to how high the voltage is while “amperes or current” refers to how much power a source has to lift the voltage to its intended height.
POLARITY
A battery provides the most pure and basic form of DC electrical power. A battery has terminals labeled plus and minus to indicate correct connection. The plus and minus DC voltage relationship is called polarity. It is critical in DC circuits that the plus source is connected to the plus terminal on an effect and it is equally important that the minus source is connected to the minus input. It sounds simple but I’ve seen many blown pedals in my repair shop caused by wrong connections.
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| NOTE (+) INDICATOR FOR POLARITY |
Some distortion pedals utilize reversed power connections, hopefully the pedal manufacturer makes it obvious and clear that you need reversed power. The Fulltone ’69 pedal is their recreation of the FuzzFace which Jimi Hendrix played with, it uses reversed polarity power. In addition to non-standard polarity this pedal needs to be at the beginning of your string of pedals because of the Germanium transistors used in the pedals circuits. After owning a Fulltone ’69 and dealing with its power supply quirks I recommend players look for pedals that use standard power supply connections.
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| MIKE MATHEWS OF EHX FAME |
Nine-volts is the standard for DC power to stompboxes and effects pedals. There are other voltages such as 12 or 18 volts which are used in more obscure instances. I personally avoid higher voltages for one reason, if you put 12 or 18 volts into a pedal designed for 9 volts there is a 98.7% chance the pedal will be destroyed.
I have experimented with 18 volt buffers and preamps from manufacturers who claim better performance running their analog designs on higher supply voltage (aka 12 or 18 volts). For me there is too great a danger of accidentally injecting 18 volts somewhere in my pedalboard where it should not be. In a big installation power supply cables can get mixed up and the chances for a deadly accident go up. The way to avoid over-voltage problems is to just use 9-volt power distribution.
Modern circuit design has something called DC-to-DC conversion which means you can put 9-volts into a box and internally the circuit can step that voltage up to whatever it wants, 12, 18, 24, 48 or more. As long as you have plenty of mA available DC-to-DC conversion offers pedal designers any internal voltage they desire. Buy gadgets that use advanced technology rather than forcing you to provide dangerous alternative power.
One last important point about AC conversion to DC power is called “regulation” and it has nothing to do with government or laws. A power-supply must convert 110 volt AC down to 9-volts DC, that’s a big step down. We’ve already stated that pedals run on 9-volts DC, the term “regulation” applies to how well or how closely the power supply maintains 9-volts regardless of the number of units connected. A single power supply may have enough power to drive six pedals but it may only be connected to one. Inside the power supply a “voltage regulator” maintains the output at 9-volts no matter if you have one, two, six or even more pedals connected.
Part One, What is 9-volts DC? <- You are here
Part Three, What are Pedalboard Power Supplies?
Part Four, Pedalboard Power Cables and Connectors
Part Five, Pedalboard Noise, Hum, Hiss and Whirrrrr
This article is ©2021 by Mark King. It's not ok to copy or quote without written permission, Thanks for reading High on Technology.
Good Music to You!
