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Power supplies and batteries


Typically we power most of our effect pedals by 9V DC from a battery or a 2.1mm DC socket with a negative tip. Some purists might argue that powering effects off a battery is the only way to go. Power supplies bring a small potential for problems, but are terribly convenient if you need to power a whole pedal board. Better for the environment, and better for cost too. A perfect power supply can provide performance no lesser than a battery.

The first problem is that they can introduce hum. Your guitar signal is an AC signal - that is it swings from a positive voltage to a negative one as a wave, just like the power supply from the power point. If a little of the mains (power point) AC can find its way into your signal, you pick up the power point hum frequency. This can be through capacitance or poor filtering. Australia's power supply has a potential of 230V-240V AC @ 50 Hz (50 full cycles per second). Other countries can use lower voltages (around 110-120V) and 50/60Hz, but the principles are exactly the same.

The AC voltage is usually divided by a transformer. This converts electrical energy into an electro-magnetic field, then back into a new transformed voltage, also giving an electrical isolation (the output metal no longer physically touches the input). The output voltage of a transformer will vary however based on the voltage on the input (from the power point) and the load on the output. The mains voltage from the power point isn't stable as the city load is always changing. Not only do we not know the exact number at any time, but it jumps quickly. For example a large electrical appliance turning on in yout home may cause a spike in the supply.

When you buy a simple transformer pack (AC or DC output) it might tell you the voltage output along with a current value measured in milliamps (mA) or amps (A). For example 500mA is equal to 0.5A. This will mean that the output voltage should be at the stated value when that specified current load is applied. You can't over-feed a pedal too much current. The current rating just means how much energy can sufficiently be supplied should it be required. So with so many variables we can't really know very well what the voltage will be or how stable it holds. A transformer power supply with nothing plugged in will read much higher than stated. And if it is only driving a low load then the device will receive that overly high voltage.

Transformers aren't terribly efficeint also. As with any transfer of electricty there is some loss. They will burn off energy just sitting there as the coil winding works similarly to a heater element. This is why they get warm even plugged in away from the device using it. But we probably only need a small one to run some pedals and we won't leave it plugged in all day.

The small kinds of switchmode power supplies give a more exacting voltage output as they aren't greatly effected by loading or voltage input. They can often be run from any country's voltage and frequency and still put out the same voltage accurately. These do not offer an electrically isolated output and are run without any transformer using active electronics. They also operate on their own switching frequncy, which can be a problem, but usually operates outside of the low bandwidth of the human audio range. They are usually magnitudes more efficient than a transfomer and should draw almost nothing when left plugged in but with no load. They don't take up much space nor are very heavy.

The output of a transformer is still AC, just lower voltage. This is then rectified with four diodes so that both the positive and negative voltage swings can be used as a single wave going from a positive voltage to zero. This means that the power supply is now pulsing at 100 peaks per second.

To be made into useful DC for pedals, we need to be able to supply power during the low points of the peaks where the voltage is dropping to zero. To do this we attach a capacitor on the line. The capacitor charges when the peak is high and supplies the stored voltage while the wave is dropping. This is now filtered DC. The larger the capacitor the smoother the DC output as it can hold more charge and drop less voltage.

There's still the issue of voltages not being stable. A pedal can respond differently at different voltages, or even be damaged. The filtering capacitors may never give enough smoothing to eliminate all hum. You can also introduce spikes and pops from the mains AC, for example when a nearby refrigerator kicks in. To overcome this problem you can then regulate the DC with a voltage regulator integrated circuit. By supplying the regulator with a few more volts than the needed output, the voltage can be stabilized very cleanly.

There's also a potential issue with ground loops if you have multiple pedals sharing the same supply. Noise can also be transferred from one pedal to the next if you are sharing the same supply, especially in the case of high current digital pedals or pedal with high frequency switching circuits. Pedals that work with positive grounds can not share the same supply as pedals with negative ground without a convertor, as they will short out the power supply. Generally only old PNP transistor circuits have positive grounds though, and are were only run off battery.

Here's a diagram visually representing every stage mentioned above.

 

Power supply diagram. AC transformed AC. Rectified DC. Filtered DC. Regulated DC.

 

Having a transformer electrically isolates your pedal power supply Earth from your amp Earth, which diminishes Earth loop hum problems. Every pedal does not always needs its own individual isolated power supply and regulator. In most cases you can just have one high quality power supply daisy chained to every pedal without causing noise problems. This might assume however that every pedal has its own adequate filtering, which is not always guaranteed to be the case.

 

 

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Copyright © Darron Thornbury