A while ago, I wrote an article about Stompbox Power Internals, mostly to force myself to learn all the intricacies of how power is wired into the typical pedal. As I've worked on building out a pedal board, I've realized that there is a lot more to power than just the battery, wiring, and DC jack inside the pedal. Lot's more. So here is a collection of topics related toPowering Pedals--I hope you find it useful in getting the most out of your pedals and pedalboard.
I get weary and grumpy when I have to read technical stuff that has lots of arcane terms, formulas, graphs n'other assorted stuff. So the last thing I want to do is bore you with the same. On the other hand, there are a few key terms we'll be playing with in this article, so bear with me if you already know them, or don't know them and don't really care to know them.
Voltage is the difference of electrical potential between two points of an electrical or electronic circuit, expressed in volts. It measures the potential energy of an electric field to cause an electric current in an electrical conductor.
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Electric current is the flow (movement) of electric charge. The unit of electric current is the ampere (A), which is equal to a flow of one coulomb of charge per second.
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Alternating Current (AC) is what comes out of the wall outlet. It alternates between a positive phase and a negative phase. Direct Current (DC) is what comes out of a battery or AC-DC adaptor.
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The majority of stompboxes run on 9 volts, direct current. 9 volts, like most things in life, is a compromise. By that I mean that it is a balance between a convenient battery size (the 9 volt PP3 type battery) and voltage requirements of a typical guitar audio circuit. Is 9 volts ideal? Well, it is fine for typical transistor and opamp-based boosters, fuzzes, overdrives and distortions. But it imposes design limitations on stompbox design, limitations that are sometimes problematic for clean boost, equalization and digital designs.
The first limitation is the amount of headroom available in a stompbox. Headroom is the maximum signallevel a given circuit can reach before compression and distortion set it in. Here's how voltage affects headroom:
A typical circuit uses a voltage divider to set a reference point. This reference point, in very simplified terms, tells the transistor or opampamplifier where the middle of the signal is, and is generally set to half the availble voltage. So half of 9volts is 4.5 volts and that value determines how muchheadroom the circuit will have.
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Now if you are talking about an overdrive or distortion pedal, then headroom is generally notimportant--you actually want to have the circuit compress and distort, so all is well. But what if youwant a clean boost? You want to amplify the guitar signal without any distortion or compression, so 4.5 volts is not ideal. And this is where we see the compromise between convenience and performance come into play.
For boost, equalization and modulation effects where lots of headroom is important, the stompbox is going to require more than 9 volts. In this case, designers will rely on several voltage augmentation techniques:
Run two 9-volt batteries and you instantly have 18 volts--double the voltage and double the headroom. This is a quick fix, but often impractical due to the increased enclosure size and the pain of having to double your battery purchasing budget. Double-battery configurations are pretty rare nowadays, but back in the early days of stompbox design when components weren't incredibly efficient, they were the norm. Look at some of Craig Anderton's designs, or R.A. Penfold's books and you'll see lots of double-battery designs.
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Another approach to increasing voltage is to dispense with battery power altogether and supply an higher-voltage (typically around 12 volts - 18 volts) adaptor. This solves the headroom issue rather nicely, but forces the stompbox to require its own dedicated power source.
A charge pump is a device that uses capacitors to boost voltage. A common charge pump devices used in stompboxes is the Maxim MAX1044 device--it is a chip that can boost voltage range to allow more headroom. There are also add-on devices like the Godlyke Voltage Converter which use charge pumps to increase voltage range.
But what about battery types? Dry-cell, alkaline, rechargeable? There are lots to choose from, but only a few options that make sense in the context of stompboxes. First, let's look at the capacity ratings of different types of 9 volt batteries:
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Capacity is measured in milliampere-hour (mAH) which basically tells you the amount of steady current passed by the battery on one hour. You can see from the above chart that alkaline batteries provide more juice than standard carbon-zinc, but way less that Lithium. Unfortunately, lithium batteries are quite expensive and are not practical for stompboxes from a cost perspective.
Additionally, rechargeable batteries deliver abysmal mAH ratings compared to disposable ones, and are also inconvenient (who wants to keep popping the battery cover and yanking batteries? Not me.)
In general, alkaline batteries are the best types for stompboxes in terms of longevity. You can certainly get away with carbon-zinc, but they'll croak faster and at a more non-linear rate.
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The markup on batteries at retail locations is unbelievable. You will typically see around 4 dollars for a name-brand alkaline 9 volt, and that is absolute insanity. Retail stores have a huge profit margin on batteries--don't play their game.
Finally, if you have to byy retail, get your batteries at a place like Costco or Price Club. Great bulk savings to be had there.
If you are serious about using effects and have, or are building, a pedalboard, ditch the battery equation altogether and go with AC-DC adaptors. We'll talk a bit more about this later, but for now, just add up the hundreds of dollars you are paying to retail stores for overpriced batteries that die and then get dumped in a landfill leading to toxic waste that will eventually lead to the zombie walking dead mutation apocalyptic scenario.
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Wait! you may say. Those are made-up terms! Well you got me there. But they are convenient monikers to use when talking about two interesting aspects of voltage modification for pedal use.
Sometimes, folks will run 9 volt pedals at higher voltages using an AC-DC adaptor. For example, you may take an overdrive pedal that is nice, but when you boost it up to 12 volts you find that it sounds a lot better. Why is that? Because of headroom, as we discussed earlier. Increased voltage gives the pedal more usable headroom. It is interesting to experiment with over-volting pedals, but only within the following constraints:
Next, on to under-volting which is also known as voltage sag. Some folks feel that certain pedals sound better when the battery is dying. This is common for fuzz, overdrive and distortion circuits. Why would a dying battery sound better? If you remember our discussion about headroom, it will be pretty bvious. Asthe battery becomes depleted, its available voltage and current decrease. This in turn decreases headroom and adds more compression and distortion.
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Of course, waiting for you battery to achieve thegolden state of mostly dead-ness is hardly practical. So you can build a dying battery simulator, or use the Sag feature of a commercial power unit to achieve the same affect.
As with over-volting, under-volting does not make sense in the context of digital pedals. They are binary beasts, either the voltage is right or it isn't.
Each pedal is going to draw a specific amount of current out of the power source, whether that source is a battery or and AC-DC adaptor. A high current draw is going to deplete your battery faster. A higher current draw is also going to require a heftier AC-DC adaptor.
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As we start out at the top, we can see that analog designs based on transistors and opamps have rather miserly requirements in the range of 3mA to 15mA. These types of pedals can last quite long with a battery. But as we move up the food chain, we can see the pedals become increasingly greedy.
Indeed, for most pedals that draw 100mA or more, batteries are very impractical. In the user guide for the Boss DD-20 GigaDelay, it explains how to install 6 AA batteries, but then goes on to tell you these are for testing purposes only.
In today's world of increasingly complex stompboxes, and with the growing ubiquity of digital/DSP designs, battery usage is often not advisable. However, if even if you use AC adaptors, you still need to understand current draw, as we'll see when talk about AC-DC adaptors and power supplies.
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As you work through the powering issues associated with stompboxes and your pedalboard, you'll eventually need to know the current draw for your pedals because you'll want to ensure that your total draw is not exceeding your available power. You can search
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