More Pedal Power

This is a follow on from a previous Pedal Power post on power supplies for effects pedals. In this article we are going to take a look at how to match power supply features with your pedals, and choose the right supply to meet your requirements.

 

1. Input Power

First, you are going to need to check that the input power requirements of the power supply are compatible with the locations where you will be using it, i.e. Can you plug it into the wall in the country where you live?

Power from a  wall socket is almost always AC (alternating current). The power is delivered at different voltages and with different connectors (plugs) around the world. Wikipedias Mains electricity by country maintains a useful table with voltages and plug types by country.

Check the specifications of your intended power supply to make sure it can support the voltages where you intend to use it. In the United States we use 120VAC wall power. Most devices rated at 120V will normally work with a range of voltages around that number, say 100V – 130V, but check the specs to be sure. Most European countries supply something between 220VAC and 240VAC. Again a power supply rated at 220V will normally support this range.

A power supply that is switchable or automatically supports both 120V and 220V ranges is very useful if you tour or travel around the world with your rig. Otherwise you will have to use a separate transformer. You will also need the country specific power cables, or suitable adapters. The Pedaltrain Powertrain 1250 is switch selectable between 120V and 240V and comes with a set of power cables for worldwide use.

Powertrain 1250 with selectable voltage switch
Powertrain 1250 with selectable voltage switch

 

2. DC Output jacks

The majority of effects pedals use a 2.1mm co-axial, aka ‘barrel’, connector for their DC power input connection. This is often referred to by musicians as a ‘Boss type’ connector due to it’s use on the popular Roland Boss range of effects pedals.

The outside diameter of the jack plug is 5.5mm and the inside diameter is 2.1mm. Unfortunately there is another connector sometimes used for pedals where the inside diameter is 2.5mm. It’s  hard to tell these apart just by looking as the difference is less than half a millimeter, but you’ll need the correct cable or an adapter if your pedal uses a 2.5mm connector. The 2.5mm connector is often used by devices that use AC rather than DC such as some MIDI controllers and multi-effects devices, but there’s no standard that says it has to be that way. There are a few pedals that require DC inputs and use a 2.5mm connector, so read the manual or check with the manufacturer if you are not sure.

2-1mm
2.1mm barrel connector

 

Make sure the power supply has enough outputs for all your pedals, and a few spare for future upgrades. For very large rigs you may need more than one power supply. If you need to use multiple power supplies, look for a units with a through power connector that lets you connect two together. The power supply may have several different DC voltage outputs which we cover in item 5.

3. DC Polarity

Most effects pedals use a ‘center pin negative’ DC polarity where the pin in the center is the negative conductor, and the outside of the jack is the positive. On rare occasions you may come across a center pin positive pedal. To power these pedals from a power supply you need two things. Firstly you need to use a reverse polarity DC power cable such as this from Voodoo Lab. Secondly you need to make sure that you only connect the reverse polarity cable to an isolated output on the power supply so that it does not cause a problem with the other center pin negative pedals connected to the same source. We cover isolated outputs in the first Pedal Power article.

4. AC Output Jacks

Some devices such as multi-effects and MIDI controllers may require a AC output. Power supplies such as the MXR MC403 feature 2.5mm 9VAC as well as DC outputs.

MC403PowerSystem
MXR Power Supply with 9VAC outputs in red.

 

5. Output voltage

The vast majority of small effects pedals require a 9VDC supply. This is due to a history of effects pedals being designed to run on a 9V battery. The 9VDC from an external power supply simply replaces the 9V battery without any extra circuitry required in the pedal. Some pedals though, may require a different DC voltage: pedals requiring 12V, 15V, 18V and even 48V can be found.

Some pedals can be run on a range of inputs, in particular op-amp driven pedals will often support 9-18V, with the higher voltage providing more headroom. Check the voltage requirements of all your pedals, it’s normally listed in the user manual, and make sure the power supply has outputs for each of them. You may have to run unusual pedals from their own separate wall power supply.

6. Output current

Pedal power supplies commonly use current draw as a method for rating. A power supply will be rated for a maximum current, either per output, or per group of similar outputs. For example, the Powertrain 1250 has 3 isolated 9V outputs rated at 210mA each, and 4 linked 9V outputs rated at 500mA in total. Check the current draw of each of your pedals, it’s normally in the user guide or can be obtained by contacting the manufacturer. Simply add up the current draw for each pedal that you will connect to an output, and make sure it does not exceed the rating.

Let’s use the 4 x 9V outputs on the Powertrain as an example. The total rating across all of these outputs is 500mA. If I connect 4 x 9V pedals that have current draws of 25mA, 50mA, 100mA and 200mA, the total current draw is 25+50+100+200=375mA. Since we don’t always know if the specifications are maximum, nominal, or typical, it’s best to leave some headroom, 10% seems reasonable. 10% of 500mA=50mA. Add that to our 375mA total draw = 425mA. This is less than the 500mA maximum, so this configuration is acceptable.

Current draw will be higher if you increase the voltage on pedals that support it. For example, the Mission VM-PRO will run on any DC voltage from 9V-18V. It’s nominal current draw is 3mA at 9V but it’s 4mA at 18V. Check the documentation or with the manufacturer to get this information if possible, or just make sure you have left enough headroom for the increased current if you are using higher voltages.

7. Additional Information

It is not recommended to exceed a power supplies rating or to continuously run a power supply at or very near to it’s maximum rating. It won’t be working at it’s best efficiency, and a lot of energy will be lost to heat. You may feel the power supply get quite hot if this happens, and it may even start to hum if you exceed the rated current. Some power supplies have over-current protection such as a fuse or similar safety device that will blow if the current is exceeded and have to be replaced or reset. Continuously running a supply near it’s maximum rating may also shorten it’s life.

Power supplies generate electro-magnetic interference (EMI) that can cause unwanted noise in the audio signal, especially in high gain applications  where the signal is amplified many times over. A good quality power supply will be properly designed and tested to minimize this, but it will still occur. Keep the power supply away from sensitive devices such as wah pedals. Test out the position of the power supply on your board, particularly if mounting it directly under pedals, and experiment with different positions to minimize noise.

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