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Pedal Power

Switching power supplies? Regulated outputs? Isolated grounds? Arrgh! All I want to do is to power my pedals without sounding like a can of bees or bursting into flames. What do all these terms mean? Do they make any difference for effects pedals? Let’s find out.

The job of the power supply is to convert the voltage from your wall supply, usually around 110V or 220V depending on your geographical location, to something more appropriate for effects pedals, typically somewhere in the range of 9 – 24V. In many cases the power supply will also convert the current from alternating (AC) to direct (DC) in a process called rectification. This is because most, although not all, effects pedals require a DC supply.

Why don’t we see effects pedals just plug into the wall? Well there are some, but they are quite unusual.

The first reason is that effects pedals just don’t need high voltages like say a tube amplifier does. Most components in effects pedals are designed to work with quite small voltages. Although you could do the conversion in the pedal itself, the components are large and expensive, and who wants to make their pedals bigger and costlier? OK, don’t answer that.

Secondly, if you work with mains voltage, you’ll have to bother yourself with all sorts of regulatory issues designed to make sure you don’t give your customers the entirely wrong kind of hair raising experience. So in most cases, it’s just much more efficient for effects pedals to use an external power supply.

OK, so now we know why we need a power supply, so what about the terminology. Let’s look at some:

Switching Power Supply

A switching, or switched mode, power supply controls the output by rapidly switching between full on and full off states. The ratio of on time to off time regulates the output voltage. A switched mode power supply dissipates very little power as heat, and is consequently much more efficient than a linear power supply that is continuously dissipating power. Switching power supplies are also typically lighter and smaller than equivalent linear power supplies. One trade off is that the repeated switching generates noise that can be a problem in audio applications. A good quality switching power supply will have well designed filtering that will not cause any problems for effect pedal use. However, in very low cost power supplies, this is often where the corners are cut, so it’s probably best to avoid very cheap switch mode power supplies for effects pedals, especially in high gain applications. A good quality one will be fine.

Regulated Power Supply

An unregulated power supply delivers an output voltage as a direct ratio of input voltage. If the input voltage fluctuates then so can the output. An unregulated power supply is also designed to provide the published voltage with a particular current draw. As the current through the load increases, the voltage decreases, and vice-versa. If you have a decent multi-meter you can check this. Plug in your power supply to the wall and measure the output voltage with nothing other than the meter connected to the output. An unregulated power supply can measure several volts higher than it’s nominal rating with no load. A regulated power supply should read close to exactly its rated output voltage. Does this matter? Maybe. Digital components that need stable power supplies often have their own internal voltage regulators, so it may not matter that much. However, if they don’t then it will. A digital device receiving power outside it’s required range can shutdown in worst cases. Analog pedals often will continue to work with varying voltages, but at the extremes they may start to sound slightly different, particularly where it results in the changing of headroom on opamp derived effects.

Isolated outputs

Isolation in power supplies can refer to all sorts of things such as physical isolation, magnetic or electrical isolation between stages, transformers , optocouplers, blah, blah, it really depends on exactly what you are talking about. Power Supply isolation is a whole subject in it’s own right. However, when effects pedal power supply manufacturers refer to isolated outputs, they are almost always referring to the separation of the output grounds. That is to say, each output has it’s own return, and they are not bonded together at any point. The reason for this is to reduce the possibility of ground loops and the hum that can be generated as a result. Hum from ground loops is a common problem for musicians, and it can vary with the conditions in different locations. Digital devices can also introduce noise into the signal chain through non-isolated power supplies. The video goes into this in more detail and explains why isolated power supplies are often needed when using digital effects pedals.

There are lots of great pedal board power supplies available, and hopefully these explanations will help you decide which are suitable to you. Of course, the various features come at a price, so you will need to decide if they make sense depending on your use. If you play at home with a small number of analog pedals, and don’t typically have any ground loop hum, then a decent low cost power supply should do the job, and upgrading to a fancy one is not going to make your pedals sound any better.

If you travel with your rig, a power supply with at least some isolated outputs is usually a good investment. If you have both AC and DC pedals, look for something with AC outputs as well as DC and isolated outputs if you have digital effects. Support for variable input ranges will be useful if you travel overseas.

I’m not going to recommend any particular power supply as there are so many great ones to choose from. For what it’s worth, for small to medium pedal boards I use the Mission 529i. For large rigs I use the  MXR CAE MC403 Power System. I’ve had great results with both, but check out what’s available, because now you know what those features they brag about really mean.

1 thought on “Pedal Power”

  1. 9v was a standardized setting. All electronics are designed by constraints. For instance, there isn’t just one kind of ic. Low power op amps consume very tiny current to function but have poorer capabilities compared to low noise, higher power op amps, which have poorer capabilities in different areas compared to high gain opamps and so on.

    9v was deemed the optimum for travel, ironically being supplanted later by double and triple A platforms as either 1.5v high current or 3v doubled by stacking batteries. For walkmans and such. Anyway, a typical 9v battery is basically 6 time batteries like double a, all providing 1.5v, stacked to give 9v.

    Using a design constraint of 9v actually has large implications for other factors, like, as you said, headroom.

    To explain why higher emf matters, voltage’s pressure compared to current’s flow, a vacuum tube is a great example. Few but the smallest can run on low voltage, the rest will sound terrible. Because of lack of pressure/emf.

    Why? There’s a capacitance in all p-n junctions and vacuums, because any conductor separated by an insulator (or semiconductor) is a capacitor. Emf, high voltage difference between two points helps decrease the effective capacitance in vacuum and junction alike. One reason a higher voltage pedal can sound better.

    Hen there is sensitivity. A circuit of 9v has little room to grow if the circuit is designed to try to gain as much as possible, which is typical of 9v devices to prevent multiple low gain stages from injecting more and more noise, do it with just one or two amplifying transistos, etc. but this also means the overdrive of a 9v pedal is already at the edge so you can’t slowly dial in your crunch as with a higher voltage circuit with similar biasing specs to a 9v.

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