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Power supply terminology part 1 – volts, amperes, polarity and watts

When dealing with supplying pedal effects with power two terms are very important. These terms are:

  • Voltage (volts; V)
  • Current (amperes or amps: A; milliamperes: mA)

Even though voltage and current are interdependent units in natural sciences, you shouldn’t get them confused when it comes to power supply units.

Voltage

A volt (V) is the unit which measures the electrical potential a pedal needs to function properly. The components inside the pedal have been chosen, and its circuitry designed, to operate at the certain voltage. This means you should always check pedal’s specifications list – often printed somewhere on the effect or found from it's manual – to make sure you’re supplying the correct voltage.

The most common voltage used in effect pedals is nine volts (9 V), while a few pedals require 12 or even 18 volts. Always feed the correct voltage to a pedal, as specified by the manufacturer. If the voltage is too high a pedal could be damaged, while a too low voltage can keep the pedal from functioning at all. Your power supply won’t be damaged if you’ve accidentally connected an effect with the “wrong” voltage.

      DC voltage

      Ninety-five percent of all effect pedals run on DC voltage. This means that the negative and positive poles never change, and the current runs through the circuit from start to finish in a single direction.

      What is polarity in DC-voltage?

      In DC-voltage the electricity moves directionally between two poles – positive and negative. These poles are represented in the most common DC plugs as one centre (tip) contact and an outer (sleeve) contact. In majority of pedal effects the negative pole is carried by the centre and the positive by the sleeve.

      Look for a similar symbol on your effect pedal, which will tell you the correct polarity. If you can’t find this information on your pedal, read the owner’s manual (or look on the Internet) and look for the term “centre/center negative”. This will tell you that the effect in question is run with “normal” polarity.

      Only a few, mostly vintage-based transistor-driven designs and a modern Eventide digital pedals need reverse polarity with a positive tip and a negative sleeve.

      AC voltage

      In AC voltage the current changes direction at regular intervals. This means the positive and negative poles are switched by a 180 degrees at a frequency of 50 or 60 hertz (Hz), depending on the country you live in.


      Should you want to use an AC-powered pedal you’ll need a power supply that can supply AC voltage – either a dedicated device or a PSU with a special AC section. AC pedals are much less common than DC effects, but you should always make sure what type of power each pedal requires, before connecting it to a power supply. Please note that polarity is not an issue with AC-pedals.

      Current

      The current is measured in amperes (A) and – in the case of most pedals – milliamperes (mA). The current defines the “amount” of current (mA) an effect always needs of to function properly, and should not be confused with the voltage (V).

      A pedal will try to draw the current needed from the PSU’s output. In contrast to the voltage supplied, an output providing more amperes than the pedal requires is not a bad thing.

      A power supply output might be marked as 400 mA. This means the output will supply a current of 400 mA at the set voltage. If you’re running a nine volt effect that only required 10 mA, the output will have an unused surplus of 390 mA, which nonetheless doesn’t have a negative effect on the PSU or your pedal.

      You will be running into serious issues, though, if you try to run, say, a digital pedal requiring 300 mA off of a 100 mA output. The output won’t be able to produce the required current, even though the selected voltage may be correct. Abusing a PSU in this way will invariably lead to buzz and hum, and overheating, as well as a pedal that often won’t even switch on.

      Always make sure you know the correct current draw for each pedal you connect. If you can’t find the value for current printed on the effect itself, visit the manufacturer’s homepage, or contact your trusted retailer. Every pedal manufacturer is keen on supplying the correct data for their products, so make sure you know it before you select a PSU for your pedalboard.

      Watts

      A watt (W) is the scientific unit for power or energy transfer. Watts are commonly used to define the power of amplifiers, but some power supply manufacturers list the total power consumption of their devices in watts. Luckily, this is quite rare, because watts are very difficult to relate to pedals, voltages and currents in such a way that a normal musician would understand. At Custom Boards we try to steer clear of mixing volts, amperes and watts for the sake of clarity. Don’t worry about watts when you choose your power supply, besides this example below.

      Voodoo Lab’s Pedal Power 2 Plus and its additional AC output

      On the back of a Voodoo Lab Pedal Power 2 Plus you will find an additional IEC-connector that allows you to connect a second PSU or hook up a Schuko-socket to power additional effects. This AC output is hardwired to the 2 Plus’ AC input – bypassing the actual transformer circuits – yet there’s a marking next to it, giving the output a rating of 200 watts. This means the additional electronic devices can be powered with up to 200 W.


      What does this number mean for us? If our pedals run on nine volts (9 V), and now we know that there are 200 W of power on offer. Next we have to find out how both numbers are reflected in the total current (A) that can be provided by the AC output. Otherwise we won’t be able to calculate which pedals (and how many) we can connect to the additional output.

      USE THIS ONLINE CALCULATOR

      The nifty online calculator gives us a whopping 22.22 amperes. Even the most greedy Strymon and Eventide effects only use 500 mA each, which means the additional output could power up to 44 additional pedals. This means that the additional IEC-output included in the Pedal Power 2 Plus adds massive amounts of flexibility in configuring your pedalboard, without the need for an additional AC-cable.

      *****

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