Choosing the right power supply for your pedals
Published in PM May 2009
Technique : Stagecraft
Batteries are simple and effective as a power source, but if you have more than one or two pedals or high-current devices, they can be costly and inconvenient too. The answer may be a mains power supply, but there are still a few issues to get to grips with.
There are thousands of effects pedals on the market today, spanning the range from the inspiring to the mundane problem solver, and from mass-market to boutique, and even in the era of commercial, all-in-one multi-effects units, many players still prefer using their own combinations of individual pedals. Powering them, however, is another matter: batteries generally work just fine from a sound point of view, but they are expensive, ecologically suspect and also prone to failure partway through a gig unless you change them frequently. You could use rechargeable types but they tend not to last as long as alkaline batteries, so although arguably greener and definitely cheaper, they are more likely to let you down.
If you put a multimeter on a brand new 9V battery you will see that it is often actually providing over 9.5 Volts. This will drop to around the nominal 9V, however, as soon as it is asked to power a pedal, as the pedals circuitry will be drawing more current than the multimeter. A battery is, therefore, an example of an unregulated supply: the more current you draw from it, the lower the voltage will drop. Ohms Law (you remember that, dont you?) states that there is a fixed relationship between voltage, current and resistance within a circuit. With a battery powering a pedal, the resistance of the pedal is a fixed value (as is the internal resistance of the battery, which forms part of the power supply circuit), so the current drawn by the circuit will be determined by the applied voltage. However, the power stored within a battery is finite and as that begins to be used up, the voltage will begin to drop off, reducing the amount of current available within the circuit. Some people feel that certain designs of vintage distortion pedal actually sound at their best when the battery is beginning to die and the supply current is limited. For most devices, however, current delivery within the specified range is vital to their audio performance.
Regulated or unregulated?
The simplest unregulated power supply is made up of just a transformer with its secondary winding feeding a diode rectifier to convert the AC waveform to DC — a diode conducts in one direction only and so allows only the positive-going (top) or negative-going (bottom) half of the AC waveform to pass through. A smoothing capacitor then levels out the bumps caused by having only half of the waveform. Unless well-designed, such power supplies can allow some hum to get through (engineers call this ripple), and as more current is drawn the voltage may drop significantly while the ripple increases, especially if the transformer isnt very generously rated and can only just supply the required current. The output voltage is also dependent on the mains voltage, which in the UK can be anything from 217 Volts to 253 Volts, which translates to over a Volt of possible output variation for a 9V supply. Some pedals take an unregulated supply and simply use it instead of a battery, while others use elaborate internal circuitry to improve the quality of the supply by regulating it or even using it to drive a circuit designed to produce a higher voltage. Pedals that use an AC input supply invariably have internal power-supply regulation circuitry, often deriving both positive and negative supply rails to power audio ICs that require a split power supply.
Regulating a power supply simply means using additional circuitry to keep the output level constant, and this can be done using a switched-mode power supply similar to the ones used in mobile phone chargers, or by using a solid-state series regulator fed from an unregulated supply that supplies a little more voltage than is ultimately required. Regulator circuits compare the actual output voltage with an internal reference, often derived from a Zener diode that is designed to conduct at a specific voltage, and then reduce the supply voltage to match the reference. More often than not this is done using a dedicated regulator chip.
Switched-mode power supplies convert the incoming 50 or 60Hz mains to a very high frequency, allowing small power transformers to be used, many of which can also adapt automatically to the incoming mains voltage over a range of perhaps 80 Volts to 255 Volts, so that there is no need to switch voltages when travelling to countries with different mains supplies. They are lightweight and efficient but almost impossible to fix — if they fail, you just buy another one.
Individual pedal power supplies tend to have no ground pin connection, because they dont need one as they are transformer-isolated, so youre unlikely to suffer ground-loop hum problems when using them. Where a power supply is designed to daisy-chain to multiple pedals, ground-loop hum shouldnt be a problem as long as the power link cables are kept as short as possible.
Mix and match
If you only have one or two pedals then using a separate mains adaptor for each is a possibility, provided you use some kind of pedalboard with a mains distribution block attached, as flimsy power supply leads trailing across a stage are asking for trouble. Irritatingly though, many power supply adaptors are too wide to sit next to each other in a typical mains four-way or six-way distribution block, so you have to leave an empty socket between them. However, if you have more than two or three pedals, it makes sense to use some sort of universal powering system, though choosing the right one for your needs isnt always straightforward or even possible. It is OK if you use, for example, only modern Boss 9V pedals as they use a common powering format (see PSA and ACA box, overleaf) and, indeed, Boss sell their own power supply system capable of powering multiple pedals, but when you mix and match pedals from different sources there are some things to be aware of. The simplest power supplies merely split the output from a single transformer and rectifier pack to feed the required number of pedals. These generally comprise a simple unregulated supply (a simple rectifier with smoothing) feeding multiple output sockets or a daisy-chain cable. As long as the pedals require the same voltage and are of the same polarity, this should be OK — but only if the current rating for the power supply is greater than the current consumption of all the pedals plugged into it (see Calculating current box).
Problems arise when the pedals in your collection are designed to operate at different voltages, a different polarity, or even AC rather than DC. The polarity issue isnt always easy to clarify either, as theres little information on how the power supply is used once inside the pedal, so if you plug in two pedals from different manufacturers, even though they both need the same DC voltage, you may find that the signal ground between the units when connected up shorts out the power supply, risking damage both to the pedals and the power supply. If you measure between the negative battery terminal and the signal cable screen using an Ohmmeter, and get a short circuit, then it is pretty safe to assume the pedal has a negative ground and will play nicely with other negative-ground pedals that require the same voltage. However, there are exceptions and nobody explains this in their handbooks, as they tend to rather simplistically assume youll be using one power supply per pedal. The powering arrangement inside digital pedals, in particular, can also be quite complex.
Thats where some of the more sophisticated power systems score. They may appear to be costly but what the best ones give you is a set of outputs that are electrically isolated from each other, as each is fed from a separate, secondary winding on the mains transformer, and in most cases the individual outputs will be electronically regulated so that the voltage remains stable regardless of how much current the pedal draws or how much the local mains voltage fluctuates. The best such systems offer a choice of output voltages, and provide a series of connection cables that will work with pedals of either polarity. In this case you need to ensure that each output is rated to provide enough current for the pedal connected to it. Most analogue pedals take very little current but some digital pedals take rather a lot. The spec sheet for the pedal and the power supply should provide this information.
As an example of more sophisticated powering systems, the Voodoo Lab Pedal Power range and some of the T-Rex Fuel Tank series have fully isolated and regulated 9V DC outputs, and if one pedal should happen to short out for any reason, its power feed will shut down but the other pedals wont be affected, as they would be if all were fed from a simple power supply using a splitter or daisy-chain cable. The Pedal Power 2 Plus powers both 9V pedals or the higher-voltage Boss ACA type (again, see PSA and ACA box, left), while two further outlets work with high-current Line 6 digital modelling pedals such as the ToneCore range. They even provide a pair of user-adjustable voltage power outlets so you can lower the voltage to get that fading battery effect, as mentioned earlier. T-Rexs Fuel Tank Classic offers eight 9V DC outputs that all share a common ground, in addition to one 12V, 500mA DC output with isolated ground, and one 12-Volt, 500mA AC output with isolated ground. There are other supplies available designed to offer solely AC outputs, as needed by units with higher current requirements — especially anything with a valve in it being run at a worthwhile voltage.
Of course you may still have a weird pedal (or two) that needs an AC supply at one of the less common voltages, or one that uses both plus and minus DC feeds down a three-core cable, so none of these off-the-shelf solutions can be expected to work for everyone. Nevertheless, if you can power the majority of your pedals from a single source, then having to find room on your pedalboard for the odd awkward PSU to power the rest isnt such a big deal. One thing you will have to bear in mind, though, is the possibility of mains transformers radiating hum into your signal leads. There is really very little you can do to tackle this issue other than making sure that transformers are located as far away from the signal path as possible and that all signal leads are kept as short as possible. Dont feel tempted to try screening everything with copper foil — this is electromagnetic radiation, and not so easily stopped. In practice, a separation of about six inches is often enough for all but the most susceptible of pedals (usually distortion processors), but when you are assessing hum levels do make sure that you have a guitar connected with its volume control turned all the way down, so you can eliminate any hum being injected via the signal path. Also, be sure to get a reference hum level to compare to by connecting a turned-down guitar directly to your amp, otherwise you could find yourself chasing down pedalboard hum that isnt actually originating from your pedalboard!
And if youre really determined to have everything running from one mains inlet, there are many electronics engineers who could handle the relatively simple task of dismantling your wall-wart power supplies and fitting their transformers and components into a new housing fed from a single mains source, in effect creating a bespoke power unit for you. However, dont try to do this yourself unless you have the necessary skill in electrical and electronic engineering!
While it is impossible to cover every powering requirement, we hope that this article helps explain why a simple power supply capable of running a handful of 9V pedals that all use the same grounding system might only cost you £20 to £30, while one with separately regulated, isolated outputs can cost well in excess of £100. On the other hand, if you have that many pedals, just work out how long it would take you to get through £100 worth of batteries. It might not be as long as you think, especially if, like me, you keep forgetting to unplug the guitar lead! 0
To establish the suitability of any power supply for a given application you need to know its output voltage and its maximum current rating. The output voltage must match the requirement of the pedal and the current rating must match or exceed the pedals current demand. If you choose a supply with a higher current rating, it will work perfectly well, but if you use one that is under-rated, you risk generating hum in the audio and an over-heating and ultimately failing power supply, assuming that it works at all. If you are not sure, over-rate the current capacity, but never feed a pedal a higher voltage than that specified. You will damage it. Of course, there are pedals that can work on a range of voltages, notably the T-Rex units, which are quite happy anywhere between 9V and 12V, and many of the more boutique designs, which will often operate between 9V and 18V, with corresponding headroom advantages at the higher voltages. Just make sure you read the manufacturers literature properly before trying it.
If you are powering a whole pedalboard from a single supply you need to be able to calculate the total current requirement of all the pedals in order to determine the necessary supply rating. If you are lucky, every pedal will have its current requirement clearly printed on the bottom and all expressed in milliamps (mA) or fractions of an Amp (eg. 0.5A, which is simply another way of expressing 500mA). In that case you can just add them all together and pick a supply that has a current rating safely in excess of the total demand. If not, a little basic maths is called for: you might find a power consumption figure instead, expressed in either Watts or VA (Volt/Amps). Watts and VA are the same thing, so to derive the current figure we need for our calculations you simply divide the Watts or V/A figure by the supply voltage — for example, 1.8W divided by 9V equals 0.2. The result is in Amps, so 0.2A would equate to 200 milliamps. Job done.
Boss ACA and PSA — whats the deal?
Early Boss pedals, including classics such as the OD-1 Overdrive and CE-2 Chorus, were designed to be used with a relatively simple, unregulated power supply, known as the ACA adaptor. This was specified as a 9V adaptor, but actually provided closer to 12V at its output when off-load. This was dropped to something around the normal 9V inside the pedal with a 470Ω resistor and a diode. (The diode being there to prevent incorrectly wired PSUs, or PSUs with reversed polarity, from causing damage to the pedal.) Given the comparatively low current draw of the circuit designs — about 8mA for the (all-analogue) chorus and as little as 4mA for the overdrive — this strategy worked OK.
For example, using Ohms Law (see box, right), we can see that the 8mA load of a CE-2, passed through a 470Ω resistor, would impose a drop of 3.76V on the original 12V DC supply, reducing it to a little over 8V. An OD-1, with its 4mA load, would reduce the supply voltage reaching the circuitry to just over 10V.
Adding more pedals to the chain pulled the supply voltage down, but there was voltage to spare, anyway, as the supply was 12 Volts, with the regulation occurring inside the pedals.
As the pedal designs began to get more sophisticated, however, particularly with the addition of digital circuitry, the unregulated strategy started to unravel a bit and Boss made the switch to the current PSA 9V regulated adaptor. This, of course, necessitated also ditching the internal resistor and diode, no longer needed as the supply was both the correct voltage and regulated.
Anyone whos tried plugging a PSA adaptor into an old-style Boss pedal, designed for use with an ACA adaptor, however, will know that the LED barely lights up, and while the pedal usually works, it is quite clearly not running optimally, with increased background noise and distortion. Except when you connect a signal lead to a modern, PSA-designed pedal sharing the same supply, the thing springs into life: this is because the shared ground bypasses the dropping resistor and diode, allowing it to receive the full 9V. This is perhaps coroborrated by the fact that switching to a transformer-isolated supply, where there is no common ground between the pedals, dims the LED again.
If youve got a collection of Boss pedals and both PSA and ACA adaptors, and you are not sure which goes with which (of course, many people will have removed the rubber base where the necessary information is printed to mount the unit on a pedalboard), then the safe course of action is always to plug in the PSA adaptor first (with a jack in the signal input to turn it on). If it lights up fully, its a modern pedal that expects a regulated 9V supply (a modern PSA). If it lights up dimly, its expecting an unregulated 12V supply via an ACA, or the simulated ACA output on some power supplies. If you have neither of those, you can cheat it into working properly by connecting it to a modern, PSA-powered pedal via both signal lead and a daisy-chained power supply. The one to avoid is plugging a 12V ACA adaptor into a modern pedal that has no dropping resistor or diode, and is designed for 9V use only.
Ohms Law — worth knowing!
V = IR (V = Volts, I = current in Amps, and R = resistance in Ohms). And if V equals I multiplied by R, then V divided by I equals R, and V divided by R equals I. So, if you have any two of the parameters you can always work out the third. The resistance figure is the total resistance within the circuit you are calculating, so if something is not adding up youve probably forgotten to include the internal resistance of the battery or power supply (otherwise you could calculate that a 9V PP3 connected to a 1Ω resistor will be delivering 9A of current, which is plainly nonsense).
If this is still all a bit hazy, the usual water analogy often makes it clearer: think of voltage as the water pressure in the pipe behind a tap, the current as being the water that flows when you open the tap and the resistance as being how much you have closed the tap. If you increase the resistance, by closing the tap further, less water (current) will flow. At the same resistance (tap setting) any increase in water pressure (voltage) will cause more water (current) to flow. The three parameters have a fixed, interdependent relationship.
Published in PM May 2009
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