• The Power of USB

    The Universal Serial Bus (USB) has been a prevalent standard for connecting accessories to computing devices since its initial release in 1996. The USB interface is increasingly being used in environments we would not traditionally consider directly related to computing, especially audio. The ability to carry power and data, including digital audio, along the same small, low cost and easily available cables makes USB an attractive interconnect for musicians. We are starting to see devices such as MIDI controllers, guitar wireless systems, and even effects pedals with USB interfaces.

    I carry a basic USB toolkit with me in my backpack pretty much everywhere I go. It has all sorts of uses. Here’s what you need to make your own.

    USB Battery Pack

    A portable USB power pack can be used to recharge devices such as phones and tablets and power other devices such as some wireless guitar systems. They recharge from the wall or another USB power source. There are numerous different sizes and types available, many with additional features such as an internal flash light. Lithium Ion types provide the most storage capacity relative to size and weight. It’s wise to pay a little more and be sure to get a good quality battery. Respected manufacturers will ensure that there is plenty of protection against short circuits and over voltage, current and temperature; all of which can result in serious failure such as fire or explosion.

    The battery capacity is normally rated in milliamp hours (mAh) which describes how much current the battery can deliver for how long. Watch out for low cost sellers on eBay, Alibaba etc. that are notorious for overstating battery capacity. Here are some great battery packs to consider:

    • APC M10BK : 10000mAh. Very slim for a 10Ah unit. Looks professional. Lots of safety features. On/off switch. This is the one that’s always in my backpack.
    • Griffin Survivor : 6000-10000mAh. Ruggedized with silicone surround and weatherproofing. Internal flashlight. Very easy to carry around. On/off switch.
    • Anker Powercore+ : 26800mAh. USB C and Power Delivery support. 30W capability for large items like laptop computers.
    • Naztech 13000mAh : Slim and low profile, but still powerful. It even has a USB-C output. This one is verified by Mission for use with our 529 products, and can also charge your mobile devices. You can purchase one from us here.

    USB Adapter kit

    USB-C should eventually simplify things as there is just one type of connector, but its new and there is a huge installed base of devices with different types of USB A and B connector. For the best chance of having the right connectors, you should keep a set of adapters. I carry a JDI Tech Goldx 5 in 1 kit. It includes a 10ft cable and is very robust. I’ve been using the same kit for more than five years without issue.

    Note that some of the low-cost multifunction adapters are power only, which may be all you need, but if you expect to send data (including digital audio) make sure that the adapters you use support it.

    USB Tester

    Plug-in USB testers are easily available online for around $20. Plug one in between your source and load, and the meter will display useful information such as the voltage, current and temperature. For basic function checking I use a Drok USB 3 multi-tester. It is very simple to use and has a nice clear display. If I need to do more detailed testing such as USB C, graphing changes over time, and logging to a laptop, I use a MakerHawk USB Power Tester which can work with higher voltages from USB power delivery and log results to a Windows computer via USB or Bluetooth wireless.

    USB – 9V Pedalboard Power Converter

    The Mission 529 converts any 5V USB power source into 5 isolated 9V power outputs for effects pedals. Whenever I need to power a few pedals, I use one of these with the USB battery pack. I can run a modest size pedalboard for a day or so without having to worry about finding a reliable AC power source and running large power cables. If you travel internationally, you don’t have to mess with wall adapters which is an added bonus. Click here to check it out in our store, and click here to check out the 529i to take your portable power a step further.

    USB Wall charger

    For recharging the battery pack, or just charging my phone I usually carry both a USB A and a USB C Power Delivery wall charger. Some of the devices already on the list such as the Mission 529 and the Anker Powercore+ come with their own wall chargers that you can use. If you need to buy one, I like wall chargers from Phihong because of their small size and extensive regulatory and safety testing.

    You can easily carry all of these with you in a gig bag or backpack. If you are like me, you’ll soon find that they get you out of a tight spot so often that you’ll rarely be without them.

  • Understanding Boost Pedals

    Boost pedals are a paradox; they are the simplest of devices, most with just a single knob, yet they can also be a challenge to integrate to achieve the desired effect. Much more than say a delay, chorus, or even many distortions; the heart of a good boost lies not in the boost pedal itself, but in the complex interactions between all the parts of the signal chain from the pickups to the speaker driver. This is why the same boost pedal may provide a nice lead volume increase in one rig, creamy overdrive in another; yet make mine sound like I’m using a smoke alarm as an amp. Let’s take a look and see why this should be.

    A boost pedal is really just an amplifier with a single volume control and an on/off switch. The job of an amplifier is to take a low power signal and increase it’s power level. In the case of our boost pedal, it takes the low level output from a guitar pick up and increases it before passing on to the next part of the signal chain. The number of times the amplifier can increase the output power over the input power is referred to as the gain. A gain of two means the amp should output twice the input signal, and so on.

    Boost pedals normally list the amount of boost in dB, so how does that relate to amplifier gain? Let’s take a common boost pedal value of 15dB. To convert that to gain in voltage we use the formula:

    Vr= antilog(db/20)

    Where Vr is the voltage ratio and db is the increase in dB.

    Converting +15dB gain to voltage gives us 5.623413, or about a 5.5 times increase if we round it. So with our boost pedal, a 1V input signal would be increased up to about 5.5V.

    When we put this into our signal chain, there are a couple of things going on. First, we are going to increase the input signal level into the next device. If our next device is sensitive to input level, say a fuzz for example, then we are going to get a change in behavior. Our fuzz is now getting 5.5V on the input instead of 1V. It’s like playing five times harder into the fuzz, so your boosted signal is going to be fuzzier. If we change things around though, and put the boost after the fuzz, then the fuzz is back to getting 1V on the input so the boost is just making it louder. If you place the boost in front of a pedal that’s not that sensitive to input level, such as a digital delay for example, then again the boost is mainly just going to make it louder. Of course, the signal from your guitar is not a steady 1V, it’s varying all the time, but the rule still applies.

    The same effect applies to using a boost with a tube amp. Placed in front of an amp that’s just short of break-up, the boost can be used to take the amp over the edge and start clipping, which then increases the distortion from the amp. When used with an amp that has a lot of clean headroom though, the increase in voltage may not be enough to cause clipping and the signal will just get louder.

    So this is the first thing to be aware of with a boost. The results will depend very much on where the pedal is placed in the signal chain, and how the other pedals and amp react to the increased signal voltage.

    The second factor to be aware of is what is often called ‘clean boost.’ To understand this, we have to go back to looking at the boost as an amplifier. To amplify the signal, the amplifier is taking two inputs and creating a single output from them. The two inputs are:

    1. The input signal from the guitar pickups
    2. The power supply (wall power, battery etc)

    The important thing to note here is that the load on the output is being controlled by the power supply and not the guitar pickups. The signal from the guitar pickups is modulating the power supply to provide the varying output voltage, but the eventual output power depends on the gain of the amplifier and the limits of the input power supply.

    Let’s recall our example where 15dB of boost increased our 1V signal to 5.5V, but now let’s increase our input signal voltage to 2V. (As we said the actual input signal from the guitar pickups is varying all the time, but we’ll use this as an example). Again, we’ll multiply our input signal by our gain, which is now 5.5 x 2V, or an output voltage of 11V. The interesting thing here is that to deliver 11V at the output, the power supply will have to be capable of at least that, or in practice a little more. A 9v battery is not going to be enough, and in this scenario the amp in the boost pedal will begin clipping. It delivers as much of the 11V output as it can and then stops when there is not enough power available at the power supply. It’s called clipping because if you look at the input signal as a sine wave, the output looks like the tops have been clipped off. A clipped signal will sound distorted, so our ‘clean’ boost is only clean within certain parameters.

    Some boost pedals are designed to run with higher output external power supplies to counteract this. If we could run our example boost pedal with an 18VDC supply for example, there would be enough power to provide 11V at the output to avoid clipping in our scenario above.

    Check the specs of your boost pedal to see if it tells you what voltages it begins clipping at. See if it can run with an external power supply and if so, up to what voltage. Experiment with putting a boost pedal in different places in your signal chain to see what works best for you, and remember that something that sounds one way in one rig may sound very different in another, that’s the paradox of the boost pedal.

  • Troubleshooting Your Rig

    Guitar signal noisy, intermittent, or just plain dead? Unable to make it through a rehearsal without something fizzing, popping, or blowing up? Sooner or later something is going to fail, and every rig needs some regular maintenance to keep it running at its best. If you are not in a platinum selling band with your own guitar tech, then you are going to have to do it yourself. Here are the top 5 fails, and how to fix them.

    1 – Begin at the beginning

    The most effective way to diagnose signal problems is usually to start at the signal source and follow it through to the end. For electric guitars, our practical signal source is the guitar pickups and the end is the speakers. Connect the guitar to the amp by itself with a known good cable. Tap on the pole pieces of the pickups to make sure they are working. Wiggle the cable jack around to make sure there is not an intermittent issue with the jack on the guitar itself. Turn the volume and tone pots around a few times and listen for noises. If all is good, then connect the guitar back to the rig and follow the signal through one step at a time until you are able to isolate which device, cable, or interface is causing the problem.

    2 – Cables, Cables, Cables

    I’m sure you have heard the humorous phrase used by realtors describing the major factors influencing property prices as being ‘location, location, location’. The equivalent for guitar rig failures is ‘Cables, cables, cables’. Other things can go wrong, but it’s astounding how many apparent gear failures are just down to a bad cable. Following a logical cable testing process will usually find the cause of a problem in short order.

    To identify cable problems, test one cable at a time with a known good guitar and amp. Examine the exterior of the cable for any obvious signs of physical damage such as cuts or deformities. Damage due to excessive pulling, shutting in doors, or chewing on from household pets or drummers can usually be uncovered from a visual examination.

    Wiggling the cable around at both ends can help point to an intermittent connection. A common failure point for cables is where the conductor is terminated on the connector. If the plug is not the over-molded type, you should be able to remove the cover and perform a visual exam, checking for broken solder or screw joints. Use a cable tester or multi-meter to check for shorts or disconnects. To test a ¼” guitar cable with a multi-meter, set the meter to the continuity setting and touch the two probes together. Most meters will give an audible beep to indicate continuity. The resistance display should read something close to 0 ohms. Now touch one probe to the tip (pointy end) of one jack plug, and the second probe to the tip of the other. The meter should beep, and the resistance should read close to 0 ohms. If there is no beep, and a high resistance reading, then the conductor is likely broken or damaged at some point. Also be sure to check the sleeve connection the same way; there should be continuity between the sleeves at both ends. Now measure between the tip and the sleeve. These should not be connected and there should be no continuity between these. If the meter beeps when the probes are between tip and sleeve, you have a short circuit which will need to be repaired or the cable replaced.

    TRS and XLR cables have 3 conductors rather than two, but the checking procedure is mostly the same: Making sure that you have continuity between the matching pins at each end of the cable, and no shorts between different pins.

    Cable testers designed for the pro-audio market can save a lot of time. Available from many audio and music stores, these small metal boxes can often test around 10 different cable types providing audible and visual indications of cable status. They usually cost less than a single average quality guitar cable and are a worthwhile addition to any musician’s toolkit. Look up ‘Pro Audio Cable Tester’ for more info.

    Most plugs have the conductors soldered to the terminals, but some use screw terminals, and others still often called ‘solderless’ may be an interference fit. Broken solder joints can usually be re-soldered if you have the right tools. Screw and interference terminals can be screwed back into place. If there is a break at an unknown point inside the cable itself, then it’s usually best just to replace it.

    3 – Power to the people

    Power issues are right behind cables as a major cause of rig failures. For battery powered devices, make sure to try a fresh battery. If there is any doubt, check the battery in another device to be sure it is good. Even if you are using external power, it’s worth trying a battery if the device supports it as it can help isolate if the issue is coming from the device itself, or the external power supply.

    When using external power, make sure that the supply you are using is correct. Just because the plug fits, doesn’t mean it’s the right power source. Check to see that the polarity is correct. Although a center pin negative barrel connector is common for effects pedals, it’s unusual in general, and most other consumer devices that use a similar connector are center pin positive. Well engineered devices will have polarity reversal protection which will minimize the chance of damage if you use a reversed DC power supply, but not all do. Using a reversed polarity power supply on a device without protection will usually cause serious damage, requiring the unit to be repaired.

    Check that the voltage of the power supply is correct. 9VDC is common for effects pedals, but some require other voltages such as 12V 18V or even 24V. Over voltage is more likely to cause damage than under voltage, so if you are not sure for any reason, start with a lower voltage.

    The most common power connector for effects pedals and similar devices is a 2.1mm barrel connector, but there are other diameter connectors that are almost the same that don’t quite fit. The plug may go into the jack but not make a good connection resulting in no power or intermittent issues. Check the manual for the device to make sure you have the right size power plug.

    Some devices don’t play nicely with others when the power supply grounds are common. A good quality pedal board power supply will normally have some or all isolated grounds. If your device is not behaving as expected, or is particularly noisy, try using a power output from your pedal board power supply with an isolated ground, or use a separate power supply.

    4 – KISS

    Let’s be honest here; finding a fault in your analog/digital, piezo/magnetic, wet-dry, stereo, MIDI switched, 19” rack mounted, power conditioned, wireless stadium monster is going to be a major PITA unless you break it down into smaller elements. So Keep It Simple, and work on one section at a time.

    Start with a known good guitar and a cable that you have triple checked to be working correctly. Connect to one channel on one amp and test it. Does it work OK? Good. Now add ONE THING at a time until you find the device or cable that is causing the problem.

    5 – Interaction

    Remember that sometimes an issue may not be the result of a failure, but could just be some type of mismatch. For example, some fuzz pedals will not operate correctly after a buffer and wah pedals can often sound odd after distortion pedals. There is nothing wrong with these devices, it’s just the way they were designed to work. If adding a device to a signal chain causes a problem, the device itself may not be at fault, it could be how and where it is connected.

    In this case, try using the device on it’s own with just a guitar and amp. Does it work ok now? Check the user manual for any information on different settings or use cases, and try repositioning the pedal order. If you have a lot of pedal interaction issues, using a switching unit such as the RJM Mastermind PBC can help. This lets you organize pedals into individual loops and then connect them as and when needed using programmable switches.

  • Pedal Casing: What Are the Options?

    If you are building an effects pedal, you are likely going to need a case or enclosure to keep it in. Let’s face it; the varying names and colors on the boxes are often the only differences between certain types of pedals anyway, so this is where you can stand out. The gold standard for containment in the pedal biz is the die-cast aluminum enclosure from Canadian company Hammond Manufacturing. Drilled and painted Hammond boxes provide the exteriors for the majority of boutique pedals, but that is by no means the only way. Let’s take a look at the benefits to keeping it standard, and some fun options if you prefer to go it alone.

    The Hammond 1590 Series is the baseline for effects pedals. When someone says they are using a 1590A or BB style enclosure, those are Hammond part numbers. Even though we might actually be using a box manufactured by another company, it’s often the Hammond part numbers we reference; a sure sign that they have become standard. Other manufacturers, however, make similar enclosures. New Sensor is owned by the same people as Electro Harmonix, so that will give you an idea of the types and choice of enclosures they have available. Eddystone enclosures out of the UK have been part of Hammond Manufacturing since the late nineties. There are also various far east manufactured boxes available from specialist pedal parts suppliers such as Small Bear Electronic and Pedal Parts Plus.

    A major advantage to using genuine Hammond boxes is the quality of support. Detailed documentation with accurate measurements is provided, along with 3D files in multiple formats. This may not be a requirement for the hobby builder, but it’s important for the commercial manufacturer that uses CAD and CAM tools to speed up design and produce a consistent quality product in volume.

    Die-cast aluminum enclosures are strong and not too heavy. They can be easily drilled with low-cost tools for home projects, as well as consistently machined on a production line for volume manufacturing. They provide a good substrate for finishing with paint and fixing with adhesives. They stand up well to knocks and scrapes, and can be expected to withstand many years of heavy use. These are all good reasons to use this type of enclosure for your pedal.

    Production of die cast enclosures requires custom tooling. Molds are created for the parts into which the molten alloy is injected using special machinery. Creating these molds is very expensive, and they have a finite life. All the stars have to line up to make selling such enclosures commercially viable, and as a result, there is a fairly limited range available, and they are the same ones as everyone else uses. Differentiating the appearance of your product can be a challenge with the limited choice.

    Die-cast enclosures respond well to painting in both liquid and powder. Alternatively, they can be engraved using moderately priced tools. Decals and different types of adhesive labels can be applied as well. A friendly local trophy store can be a great resource for the pedal hobbyist; most moderate sized towns, in the US at least, normally have one or two in the neighborhood. These stores will usually have laser cutting and engraving tools, and a range of different label materials. They will be happy to make one off or small runs of custom adhesive labels that can give your project a professional finish at a reasonable cost.
    Anodizing is an electro-chemical process that creates a protective coating to non-ferrous metals, particularly aluminum. In combination with certain dyes, the process can yield a distinctive finish with excellent cosmetic qualities. Unfortunately, being a chemical process, the quality of the result is very much dependent on the makeup of the base material. Die-cast aluminum products are usually alloys that are not suitable for anodizing. Other elements are added to the aluminum to provide certain properties; in particular silicon is added to improve fluidity. Silicon does not anodize, and the result is normally a dull and patchy finish. There are potentially ways around this but they are complex, expensive, and normally reserved for industries such as aerospace and professional sports where the budgets are somewhat higher.

    For an anodized finish, you’ll need a folded aluminum enclosure. A few of the specialist pedal parts stores are offering some of these now. If you are into 3D design you can create one fairly easily, and it’s a good first project if you are interested in learning. There are several free or low-cost 3D design tools that have recently become available. I’ve been using SketchUp recently, and I’ve been meaning to try Autodesk 123D. Check to see if you can find a local metal shop that can fabricate your enclosure from your 3D drawings.

    Anodized finishes can be laser etched. It’s great for small text or intricate graphics as it provides a very high resolution with sharp edges. It’s only one color though, and the color is determined by the chemical make-up of the oxide, so you don’t really have much control over it. Screen printing over anodized parts is common for commercial products. The oxide layer is non-conductive, so if you need the enclosure to act as a screen, you’ll have to have a conductive layer applied first. You’ll need to see of your anodizing shop can support this, and it adds cost.

    This leads us nicely on to screening, and the contention that a pedal enclosure must be bonded to ground. I’ve seen plenty of times comments or complaints about pedals from various sources that do not have the chassis connected to the circuit ground, but there is really no rule that says this is required, and in some cases it may actually be necessary to isolate them. Take a look around at some of the electronics you have where the enclosures are wood, or plastic, or some other insulating material.

    If you are using a metal enclosure, connecting it to the electrical ground is often done so that the chassis functions as a shield against electromagnetic interference. In low voltage DC devices, such as most effects pedals where the – and ground are usually common, connecting them all together can often help both protect against noise induced from EMI, as well as radiating EMI causing noise in other devices. However, this may not always be the case. Think about it; guitars and speaker cabinets which are effectively enclosures made of insulating wood or plastic work perfectly fine without a ground bonded metal chassis, although it is true that the EMI performance can sometimes be improved by correctly adding some metal shielding to these.

    When using intentional radiators such as wireless devices, enclosing antennae in a grounded metal box will pretty much stop it working at all. We are already starting to see wireless features such as Bluetooth getting added to effects units and digital amps so we can expect this to become more commonplace as effects become more sophisticated.
    So it’s certainly not a requirement to make effects pedals in small metal boxes. If you are handy with a saw and hammer, making a wood case would be perfectly reasonable. If it’s a gain pedal and gives you problems with EMI, then you can use some adhesive conductive metal tape on the inside of the enclosure. The same goes for plastic, and using an off the shelf molded plastic case, or making one yourself from acrylic or polycarbonate would be feasible. Since these materials are available in clear, you can even show off your electronics handy work.

    And while using square and rectangular boxes may be the most practical from a build and pedal board layout standpoint, there’s really no reason that has to be the case. If you are looking for some inspiration, look no further than the Dr. No Effects Ford Falcon Fuzz, a fuzz pedal in a toy car.

  • The Magic of Digital Effects Pedals

    Those of us interested in guitar effects pedals are spoiled for choice in the current age. There are hundreds, possibly even thousands of small and large effects pedal makers producing all manner of different effects with a wide range of features and price points. However, one thing is notable by its absence, or rarity anyway. How many boutique digital effects pedal makers can you name?

    In this blog, we’ll look at some history and reasons why this is still a specialist business, cover some basic building blocks, and provide some pointers as to how you might go about getting started with your own digital effects.

    Firstly, to answer my own question, I came up with five manufacturers I consider known for their digital effects: Line 6, Eventide, Strymon, Boss, and Digitech.

    Line 6. Parent company Yamaha Corporation. Annual revenue over $4 Billion.
    Digitech. Parent company, Harman International Industries. Annual revenue over $7 Billion. Boss. Parent company, Roland Corporation. Management buyout in 2015 and no longer publishes revenue figures, but in their last year as a public company in 2104 posted sales of over $850 Million. Seeing a pattern here?

    Eventide is a private Corporation and there are no public financials, but they have a history of innovative digital products in audio, broadcasting, and aviation going back to the early 1970’s. That leaves Strymon as the only relatively recent arrival from my list. Certainly there are others; for example, I saw Robert Keeley has some very interesting looking new digital delay and reverb units, but with new boutique builders appearing on the scene almost every week, new digital effect pedal manufacturers are rare.

    The obvious explanation is that digital products are just much harder to design and build, and that’s true, but why? And is there still a path for the start-up builder to create their own digital effects pedals?

    It’s In the Software

    To go to the trouble of flipping an analog signal like an electric guitar pickup into the digital domain and then back again to analog to be heard requires a good reason. The reason is that sometimes, doing what we want to do with the signal in analog is just going to result in something too large, expensive, noisy, and frequently all three. Analog components are not perfect; they have tolerances and add noise. They are quite large and get costly. As you add more and more of them, these factors soon get out of hand. By comparison, I can pick up digital devices with millions of transistors smaller than a fingernail for reasonable cost, and every one of them will do the exact same calculations every time. There are trade-offs. First, is that to use digital devices, I need software.

    The Sauce is in the Source

    Many people come to working with music electronics from the role of tinkerer. Maybe they swapped out some op-amps for lower noise variants in a boost, or heard that changing some resistor values would increase the gain of a fuzz. Many aficionados love looking at ‘gut shots’, and pointing out the orange drop caps or NOS transistors and how those might impact the tone. It’s often a great way to get started.

    Looking at a photo of a microchip is not going to help you much. The magic is in the code. This is what makes the pedal a chorus or a delay, a reverb or an amp. Often the hardware is similar or even identical between models of digital pedal, it’s just the software that’s different. In most cases, the code is hidden behind the digital equivalent of goop. Most manufacturers are not going to share the software ‘source code’. This is their secret sauce into which they put much of their investment, and what makes it their sound: It’s their intellectual property.

    Even if they did share it, in most cases you’d have no clue what you were looking at, or what to do with it unless you already had expertise in that area. We’ll talk about DSP here in a minute, but DSP code is usually highly specialized and requires a fair bit of education and experience. Not to mention you’ll need the tools to program and debug it. Again we’ll cover this in more detail later, but just for some click bait; a copy of Visual DSP++ and an in circuit emulator for the Analog devices Sharc DSP costs about seven thousand dollars. Add some development boards and other tools and we are already in for around 10K, and we haven’t even started. That’s assuming we already have tools like high bandwidth scopes, signal generators etc available. If not, then you can at least double that. We are beginning to see why digital effects are not for the faint of heart, nor small of wallet.

    So remember that this is the first key difference between digital and analog effects; most of the unique audio function of a digital device comes from the software, the hardware is mainly just there to support it.


    Since we are basically dealing with computers here, we need some computer terminology. Computer folk love Three Letter Acronyms. Here we go.

    ADC – Analog to Digital Converter
    Converts the analog signal from the input to digital so it can be processed by the DSP.

    DAC – Digital to Analog Converter
    Converts the processed digital signal back to analog for sending to the output.

    DSP – Digital Signal Processor
    A specialized microprocessor optimized for signal processing. Under software control, this manipulates the digitized signal from the ADC to create the effect.

    PCM – Pulse Code Modulation
    The most common digital encoding method for audio.

    CP – Control Processor
    A general purpose processor used for control functions such as handling the buttons, knobs, and display.

    SoC – System on a Chip
    A specialized integrated circuit that includes multiple components such as an ADC, DAC, CP and DSP on one physical device.

    I2C – Inter Integrated Circuit
    A standard mechanism for chips to communicate with each other

    SPI – Serial Peripheral Interface
    Another standard mechanism for chips to communicate with each other

    Although it’s possible to use a general purpose processor to do audio processing, this job is normally performed by a DSP which is a specialty type of processor designed for real time processing of signals. DSP’s are generally more complicated to program, but are much more cost and power efficient for this specific task. Sure, I can do audio processing on the Intel Core i7 CPU on my desktop. This is what Windows and Mac based systems such as Protools and Logic do after all, but it’s not really practical to run a huge power supply, large heatsinks, fans and a $300 processor in an effects pedal.

    So to build our digital pedal we are going to need an ADC to convert the analog to digital, DSP(s) to do the processing, a DAC to change the signal back to analog. We’ll also likely need some sort of control processor for booting the system and handling things such as control knob inputs, digital display and LED’s, and maybe software updates and interfaces such as MIDI and USB. We’ll also need some flash memory to store the code and save our presets.

    Think of components of this type as scientific geniuses, or world champion athletes. They are high functioning, but require a strong support group to remind them to eat, and to stop them from driving their cars into a swimming pool. Without the right structure around them, they’ll be waking up in a Vegas Hotel with a bizarre tattoo and all their credit cards missing. They need high speed clocks and often advanced PCB routing techniques, such as impedance matched traces and differential pairs, otherwise they just won’t run. Then there are various DC to DC converters as different parts run at different voltages, and voltage regulators as they are very picky about stability. Getting all these parts into a tiny effects pedal form factor probably means you are going to end up with a multi-layer board, and, of course, many of these parts are small pin pitch surface mount designed to be machine assembled, so the boards have to be sent out to a contract manufacturer for assembly.

    The bar is set pretty high for digital pedals, and producing something like a Strymon Timeline or Eventide H9 requires a tremendous amount of work in many different disciplines. It takes a team to design, build and support something competitive for the commercial market. Add to this the fact that because of the high speed clocks, the device may need regulatory approval, it’s becoming clear why we don’t see many one-man band digital effects pedals.

    So does this mean there’s no chance for small scale digital effects? Not necessarily. Recall our acronym SoC? The system on a chip puts many of the components needed for a specific application on a single chip. The choice is limited, but a lot of the work is already done. The Spin semiconductor FV-1 is an example of this. It’s getting a little old now, but check out the block diagram and you can see that most of the key parts we talked about are there. And because they are altogether on the same chip, we don’t have to worry about different voltages and chip to chip interconnects. You can order a ready to go development board that you can start testing with right away, and they provide a bunch of free algorithms to get going with. You could conceivably build a commercial product from this.

    Building Your Own

    If you are more interested in experimenting at home, there are a few projects on the web for the Arduino. Arduino is an open source prototyping platform based around the Atmel AVR (and some other) microcontrollers. There’s no real DSP on the basic Arduino, but as we mentioned before, there is no reason you cannot use other systems for audio processing. The fact that it’s not optimized for price performance on signal processing doesn’t really matter to us for a home project. Many Arduino base boards can be expanded with plugin daughter boards known as ‘shields’. There are a few projects on the web that provide documentation and kits for Arduino based guitar effects. The Electrosmash pedalShield look especially fun, and their circuit analysis of classic effects pedals are very well done.

    For a while Line 6 had the ToneCore product that provided a pedal chassis into which to plug different modules. A development module running on the Freescale Symphony DSP was available on which you could develop your own DSP code. The concept was that you could purchase the pedal bases and modules from Line 6, load your effects code on the modules, and then resell them. I don’t think you can buy these any more but, it sure was a fun idea.

  • Working With Voltage

    Most guitar effects pedals run on a 9VDC power source, which allows for a wide choice of pedalboard power supplies, wall warts, or batteries to run them on. However, there always seems to be one or two pesky pedals in the collection that complicate the issue requiring some other voltage; say 12V, 15V or 18V. Although it may seem like these pedal designers are trying to make life difficult, the reason is usually that some components used in the device require the higher voltage. It’s possible to have a boost converter in the pedal itself that would raise 9V to the level required, but there may be reasons to leave this out; such as space, thermal, noise, and cost constraints.

    So, it’s easily fixed, right? Just use a pedalboard power supply with all the necessary voltage outputs, or use one of the myriad cheap boost converters available. Well, maybe, but there are a few things to keep in mind.

    There are several pedalboard power supplies with different voltage outputs. The MXR MC 403 is a 16-output supply that has 9V and 18V outputs as well as two adjustable outputs, and a 9V AC output. It’s a very flexible supply (I have three of them in the lab as bench test sources) but the large size, weight and cost (almost $300) limits the demand for most people using smaller boards. I believe it now has to be special ordered, or you’ll have to search around for old stock or used items.

    The Strymon Zuma is a 9-output supply with two outputs switchable between 9V, 12V, and 18V. It’s also a great unit, but again is quite large and heavy, is around $250, and requires a 24V wall wart to power it.

    So, purpose built power supplies can be an option but may be large and costly. What if you are trying to build something small and low cost? There are plenty of boost converters available online at quite low cost. Pay a few $, plug 9V in, get 18V out, problem solved. Hmm, let’s take a quick look.

    Xotic Effects make a nice and popular device called the Voltage Doubler. It sounds straightforward enough, but check the documentation and it clearly indicates that it’s designed for use with some specific Xotic pedals and the current limit is 80mA. Try to power a higher current digital device and it is not going to be able to deliver enough power. Also keep in mind that voltage boosting is not free, and some power from the input source is used by the booster itself. Depending on the design and the voltage difference, this can be quite significant. A charge pump like the Voltage Doubler can be drawing over 250mA at the 9V input to provide 80mA at the 18V output, so make sure your power supply can provide enough current to power the pedal AND the voltage booster.

    There are a substantial number of off-the-shelf switching converters available from internet sellers. Many of these are quite low cost and support a range of different features such as variable output voltages and digital voltage displays. While more efficient than charge pumps, even a good switching converter will still use power itself.

    As an example, I used a generic adjustable switching converter PCB from Amazon and measured the current at a 9V input connected to an Eventide H9 at 12V at the output. Steady state current draw was around 450mA, but it momentarily passed 800mA inrush current at start-up. I was able to power the H9 at 12V from a 9V pedalboard power supply but I had to use a parallel (daisy-chain) cable and a total of four 9V outputs from my power supply totaling 900mA to provide enough headroom for the H9 to start up.

    Switching supplies that are not designed for audio use can often be quite noisy, so you would need to check this with your specific rig to make sure noise levels are acceptable. They can also get quite hot. In my test the surface of the converter chip was approaching 50 degrees C in free air just boosting to 12V at 450mA. Higher voltages, and an enclosed unit would get hotter and would likely require a heat sink.

    On that subject, if you are using just a PCB, then you would need to install it in a suitable enclosure for pedalboard use and make sure that it was installed safely to avoid any short circuits. Which, in turn, leads to the subject of safety and what protections are designed into the booster. Most pedalboard power supplies from respectable manufacturers will have extensive safety features to reduce the likelihood of damage to the pedals or supply itself in events such as short circuit, over current, over voltage and over temperature. You would want to ask yourself, does the Ali-Baba generic booster PCB have these protections? What happens if something goes wrong and it fries your $700 pedal? I connected my H9 up to one of these for the purposes of testing for this article, but I disconnected it immediately after, and I have no plans on using it for this again. I like my H9 and would prefer to keep it working.

  • Rechargeable Batteries and You

    In previous blogs, we reviewed the pros and cons of various disposable battery technologies for effects pedals. This time around, we are going to study rechargeable batteries as an alternative.

    The Contenders

    Theoretically, rechargeable versions of the alkaline batteries would be a good choice for effects pedals. The chemistry is the same, but the battery is constructed so as not to explode when being recharged! Rechargeable alkaline batteries are inexpensive to make and only require a simple charger. They are non-toxic, and have a low self-discharge; left unused, they have a shelf life of up to 10 years. Unfortunately, few companies seem to make them these days and I couldn’t find a 9V at all. Newer technologies seem to have pushed them aside.

    One limitation to rechargeable alkaline batteries is the high internal resistance which means they are not suitable for high current devices. Although this doesn’t matter for many effects pedals, technology had to evolve to support the high drain digital products we use today. New rechargeable chemistry had to be developed, and one of the first was Nickel Cadmium (NiCd).

    The common chemistry used in the early days had drawbacks. Recharging a single battery a hundred or even a thousand times before disposal should be much more environmentally friendly, especially if you have access to domestic power from renewable sources such as solar. Unfortunately, the Cadmium used in NiCd rechargeables is highly toxic and requires special processing for disposal, undoing much of the environmental benefit of recharging. The use of Cadmium is now significantly restricted in the European Union under the RoHS and REACH programs, making these pretty much unusable in Europe.

    Early NiCd cells suffered from an issue where a particular sequence of charge discharge events could cause the battery to apparently lose capacity. The story goes that this behavior was first observed on a satellite in space, but there was also a much more down to earth use case. Imagine you regularly drain a battery to a particular level, say 50% such as when using a laptop in a normal workday. In the evening you plug in the charger and leave it to charge slowly overnight. You do this for a week or so, then one day, you go on a long trip, you try to use all the batteries capacity: Although apparently fully charged, it dies at 50%, as if it ‘remembered’ it’s usual workday. For this reason it became known as the ‘memory effect’.

    In reality what was happening was the cadmium-hydroxide crystals in the cells were growing as much as 100 times, increasing the internal resistance and causing voltage depression. The capacity was actually still there, but could no longer supply the voltage necessary to drive the device. The issue can be countered by exercising (discharge /charge) and reconditioning (slow discharge to below cut off voltage). Recent design NiCd’s have significantly reduced this behavior.

    Nickel metal hydride (NiMH) is a good choice for effects pedals. They can last up to a thousand cycles with reasonable performance. They are prone to self-discharge which means they will lose some of their charge just sitting unused. However, advances have been made recently that improve this and good quality ‘low self-discharge’ 9V batteries with capacities of around 250mAh are available for under $10 each. A charger can be had for around $20.

    The new kid on the block for 9v rechargeable batteries is lithium-ion, using the same chemistry as the batteries in smart gadgets like phones and computers, but in a 9V format. The specifications look attractive; the batteries are really light, and a 4 pack with charger can be had for less than $30. It’s early days for these. It will be interesting to see how they work out.

    Pros and Cons

    + Low cost, very low self discharge, non-toxic
    – Unavailable in 9V, high internal resistance

    + High discharge rate, good over charge discharge tolerance, long cycle life
    – Heavy, toxic, low energy density

    + Light, non-toxic, good energy density, wide availability
    – High self discharge, low over charge discharge tolerance

    + Very light, non-toxic, very high energy density
    – Limited choice, low over charge discharge tolerance, unproven in 9V form


    Charging a rechargeable battery costs pennies, and with hundreds of recharges over several years, the extra initial cost is soon recovered. When they reach the end of their useful lives, disposing of one rechargeable vs. one hundred alkaline batteries is always going to be better for the environment. Music equipment such as effects pedals, wireless microphones, headphones, and portable recorders make great candidates for rechargeable batteries.

    Apart from a few niche applications such as RC car racing, NiCd is on its way out. The low energy capacity and toxic contents are 20th Century battery technology.
    NiMH is going to get the best purchase rating here. NiMH has a reasonable energy density, and should be able to provide about 20 hours of use per charge for a typical middle of the road analog effects pedal. Most of the common battery types, including 9V, are available from a wide range of different manufacturers, including the major brands. A top of the line 9V NiMH will cost about $10, with cheap ones for around $3. I’d steer clear of the real low end ones. There’s plenty to choose from reputable manufacturers for just a little more. Get a decent quality ‘smart’ charger rather than a ‘value’ or ‘dumb’ charger. The smart charger will reduce the likelihood of overcharging or shorting. If you don’t use the batteries regularly, take them out every few months and give them a full charge.

    Li-Ion gets the Most Promising Newcomer award. The chemistry is well proven in numerous electronic gadgets, computers, power tools, medical and industrial applications, even cars and airplanes, but is somewhat new to the 9V. The choice is pretty limited, but the light weight and high energy density make them appealing.

  • Power Supply Hacks

    9VDC from a battery or an external center pin negative power connector is the de-facto standard for effects pedal power. Back in the early days of effects pedals, this was a common voltage for portable devices using the PP3 style battery. Though modern microchip-based electronics use much smaller voltages, such as 5, 3.3, or 1.5, we keep the 9V format around. Many ‘new’ effects pedals are really just copies, re-issues, or minor modifications to existing designs; and they keep the same power supplies. For new designs, especially digital pedals that may require several different internal voltages, following the 9V format just makes it easier on the user, as we can use the similar power supplies and wiring for all our pedals. Sometimes though, we can make a few tweaks to add non-standard devices to our chain, or to address other requirements such as saving space, weight, or money. Here are some pros and cons for a few power supply hacks with an explanation of what’s happening, and how you can utilize them safely.


    Probably the most common trick is to connect more than one pedal to a single pedalboard power supply output. Electrically, this is connecting multiple loads to a single source in parallel (all the DC – are tied together and all the DC + are tied together). The advantage to this is that we can install more pedals than we have power supply output jacks. It’s great for adding new pedals to an existing board where all the power supply outputs are used already, or using a smaller, lower-cost power supply with a minimal number of outputs. To use this trick, you just need a daisy-chain pedal power supply cable which is available from most pro-audio stores, or you could make one yourself.

    Since the connection is parallel, the output voltage will be the same on all connectors. So if you are connecting to a 9V power supply output, you will have 9V on all the connectors. The current draw will be the total of all the pedals connected together on the chain. You will need to add the draw of each pedal together and make sure it does not exceed the current rating for the output. Note this is the rating for the single output, and not the total rating for the power supply. For example, let’s take an imaginary power supply. It has five outputs one rated at 300mA, and four rated at 100mA each. The total current rating for the power supply is 4(100)+300 = 700mA. The maximum load you can connect to a single output is 300mA.

    Look up the current draw for all the pedals you want to daisy-chain together and add the numbers together. For example, if we have three pedals and their draw is 20mA, 30mA, and 65mA, the total is 115mA. For our imaginary power supply, we could daisy-chain these together on the 300mA output, but not on any of the 100mA outputs. If the current draw exceeds the output rating, the voltage will start to drop below the 9V. The greater the current draw over the limit, the more the voltage will drop. This may cause a change in sound and noise from the pedals, and they may behave erratically or turn off completely. The exact behavior will depend on the power supply design and what loads are connected. If the supply has over current protection it may turn off the output.

    Voltage Boosting

    If you only have 9V outputs, what happens if you want to power a pedal that requires 18V or 24V? One proposed solution is to use a voltage doubling cable. These provide a single power connector for the pedal, and need to be connected to two (18V) or three (24V) 9V outputs on the power supply. Electrically, a single load is connected to multiple sources in series. This will only work with isolated power supply outputs; if the outputs are not isolated, then the cable will not connect the outputs in series as they will have a common DC – within the power supply, and the voltage doubling will not work.

    Bear in mind that when using these cables there can now be an 18V or 24V potential between points within the power supply that may only have been designed for 9V. Some power supply manufacturers may advise against using these types of cables. If you accidentally connect 18V or 24V to a pedal only designed for 9V, you may damage the pedal and the power supply. If the power supply is one of the more sophisticated digital power supplies with over voltage protection, these may detect the presence of the higher voltage and switch off the outputs for safety, so you will not be able to use a voltage doubling cable with such power supplies.

    If a cable only solution will not work, another way to do this is to use an active voltage boosting device. Several manufacturers offer effects pedal voltage converters that utilize specific integrated circuit designs such as boost converters or charge pumps to convert the DC voltage levels. These are not as dependent on the topology of the main power supply as a cable solution. They should work with non-isolated supplies, and they won’t increase the voltage on the supply side, so should not cause any over voltage issues with the power supply.

    Of course, there is no free lunch here, so there are some things to look out for. An active booster is a power supply in its own right and will have its own power limits. One I tried is a charge pump design, and can boost the voltage to 15V or 18V. It has an indicated current rating of 80mA. This means that although you will now be able to power a device at the higher voltage, that device cannot draw more than 80mA. Even if you connect the booster to a 500mA power supply output, it will still be limited to 80mA.

    You could include the booster on a daisy chain, with other devices, but then they will not be isolated from each other. The booster is a digital device with an internal oscillator that may put noise on the line, so although it will probably work for voltage boosting on a non-isolated supply, you’ll likely have to try it with your specific rig and see if any noise issues occur.

    Boost circuits are not 100% efficient, so the booster itself will use some power. Make sure to add that into your calculations. If the booster is powering an 80mA pedal, and the booster itself requires 30mA, your source output will need to support at least 110mA. Published numbers are usually nominal, assuming certain conditions such as temperature, stability etc, so it’s normally best to give yourself some headroom. If you add everything up and it comes to say 300mA, try to make sure the source output has at least around 400mA.

    Charge pump designs may not be regulated and may sag. As you get close to the current limit, the voltage may drop off. This shouldn’t be an issue for analog devices, but could cause problems with digital pedals if it drops a lot. The one I tried didn’t even get to 18V with no load, measuring 16.7V unloaded. If everything looks good on paper, then test it with your own rig to make sure it works the way you expect. There are plenty of variables and your mileage may vary.

    Power Boosting (current draw)

    If the current draw for a pedal is greater than the rating for a single output, you can reverse daisy-chain. Instead of connecting multiple devices to a single output, you connect a single device to multiple outputs. You even use the same cable. Electrically this is connecting a single load to multiple sources in parallel. In most cases this should work without too much trouble, and really the only downside is you are using up two or more power supply outputs for a single pedal.

    Here’s how to use it. If you have a device that the manufacturer says draws 150mA, but your power supply only has 100mA outputs, use the daisy-chain cable to connect two power supply outputs together, and then connect another jack on the daisy-chain to the pedal. This will spread the load across 2 x 100mA outputs, supporting up to 200mA, which will be enough for the 150mA pedal.

  • Batteries: Which One is the Best?

    I’m not sure of the reason the de-facto standard for effects pedal power became the 9V battery. Many low current pedals such as buffers, boosts, and distortions could easily be designed to run equally well on the more common AA battery type if we so desired.

    I’ll hazard a guess that history has a lot to do with it. If I recall correctly, the Boss, Electro-Harmonix, and other pedals I used in the 80’s pretty much all used the 9V battery. I imagine this same history has a lot to do with why we are also stuck with the evil center pin negative DC power connector on most pedals. I’m sure Roland must have had a good reason for using this back in the day on the iconic Boss effects, but from a product design standpoint, it’s a pain in the ass.

    Anyway, let’s get started. We are going to try to figure out the best choice of battery for effects pedals and how long they will last in each of our devices. To do this, we will need a few bits of information. Roughly in order of significance, these are:

    1. Battery chemistry
    2. Device current draw
    3. Device cut off voltage

    Battery chemistry defines the chemical makeup of the battery. The most common chemistry types for consumer primary cells are Zinc-Carbon (or Carbon-Zinc, or just Zinc; it’s the same thing), Alkaline, and Lithium. Let’s get a bit of terminology out of the way first. A primary cell is a single-use or disposable battery. These are designed to be used once and then disposed of, preferably recycled. A secondary cell is a rechargeable battery, and we’ll get to those later.

    The naming of the chemistry is all rather confusing. Zinc-Carbon cells do contain carbon, but it’s the reaction between zinc and manganese dioxide that forms the basis of the battery. We really should call them Zinc-Manganese batteries, but nobody ever does. Alkaline batteries also use Zinc and Manganese so they could be called the same thing. However, we call them Alkaline because they use a base electrolyte rather than the acid electrolyte used in Zinc batteries. Lithium batteries use a Lithium anode but are not the same as Lithium-Ion, which are secondary batteries. Got it yet? No?

    Zinc-Carbon batteries are the cheap ones you can buy in boxes of 50 for $19.99 on eBay. They are often called Heavy Duty, or Super Heavy Duty, neither of which means anything. They have a lower capacity than alkaline batteries, resulting in a shorter usable life. The body of the battery is made of zinc and forms the anode. The acid electrolyte eats into the zinc over a fairly short time giving these types of batteries a much shorter shelf life, and they are more prone to leaking. This type of battery is OK for something like, say a TV remote, but best avoided for effects pedals. You can use them in a pinch, but don’t leave them in the pedal unused for long periods.

    Alkaline batteries are the most commonly used in effects pedals. These are the Duracell and Energizer batteries that most of us use day to day.

    Lithium batteries are relatively new and quite expensive. We’ll do some calculations later and see if they make sense to use in effects pedals.

    Current draw is a nominal figure that defines how much current will flow through the device during operation. Depending on the pedal design, this can change during use, but most pedal manufacturers will publish a figure for current draw, and we can use this to calculate our battery life.

    If the pedal uses DC-DC converters, which digital devices usually do, it will have a Cutoff Voltage. This is the point at which the voltage from the battery gets low enough that the pedal stops working. These pedals will work the same all the way down to the cut off voltage, and then just stop. Other types of design may not have a hard cutoff voltage as such, they can continue working but the performance will change. It’s unusual to see a cutoff voltage published in the specs. Fortunately, there are some common industry practices around this, so we can get an estimate for our calculations.

    Head over to your favorite search engine and search for a datasheet on your battery; reputable manufacturers will publish these. If you can’t find your exact make, an equivalent will do. I use the Duracell 6LR61. Find the specs on your pedal from the User Guide or manufacturers web site and look for the current draw. For ease of demonstration I picked a few from the Roland Boss product line, and looked up the current draw on the spec sheets.

    • DS-1 Distortion – 4mA
    • OD-3 Overdrive – 9mA
    • DD-7 Digital Delay – 55mA

    There’s no exact cutoff voltage listed for these so we’ll have to estimate. It’s common industry practice for 9v battery operated products to work at least down to about 7v, so we’ll use that for our calculations. On the 6LR61 datasheet we are going to look for the constant current discharge graphs. Let’s start with the OD-3, which has a draw of 9mA. The red line on the graph is close at 10mA so we’ll use that. Draw a line across from the 7v cutoff voltage until it intersects the 10mA line. Then draw a line down to read off the service hours.

    Life of a 9v Alkaline battery in the OD-3 Overdrive

    From this we can see the approximate life of a 9v Alkaline battery in the OD-3 Overdrive is about 50 hours. Pretty neat. Let’s try some more. The DD-7 has a higher current draw of 55mA. We’ll need to go to the second chart from the datasheet for that. The closest line is 50mA, so again we’ll start at 7v cutoff, draw across to the 50mA graph and then read off the service hours.

    Life of a 9v Alkaline battery in the DD-7

    From this we can estimate about 7 hours life from the same battery in our DD-7.

    What if there is no chart for the current draw of our device? Well we can approximate it by drawing our own line. The DS-1 has a pretty low current draw for an effects pedal at 4mA. Let’s draw our own estimated graph based on the information we have.

    Life of a 9v Alkaline battery in the DS-1

    Here we can see a rough estimate of the battery life in the DS-1 would be about 200 hours.
    Modern programmable digital pedals and multi-effects with DSP’s, micro-controllers and digital displays consume quite a bit more power. A Strymon Timeline for example has a recommended minimum power supply current rating of 300mA, which would give us a battery life of less than 30 minutes. That explains why these types of pedals don’t run on primary batteries!
    A few Lithium Alkaline batteries are available billed as offering twice the capacity. Let’s take a quick look and see if they would be a good choice for effects pedals.

    Comparing the life of 9v Lithium battery vs Zinc-Carbon and Alkaline equivalent

    Here’s a chart comparing the life of 9v Lithium battery vs Zinc-Carbon and Alkaline equivalents at 50mA continuous discharge. If you recall, we used 50mA as our number for the Boss DD-7, so let’s do a quick comparison. The graph for the alkaline battery is probably an average, rather than the specific chart we looked at for the 6LR61 so the numbers are a little different, but they are in the same ball-park.
    This chart shows a service life of around 6 hours for 50mA at 6.6v cut out voltage. We got around 7 hours at 7v on the specific battery model, so it’s close enough.

    The Lithium battery is showing around 15 hours vs 6 hours for the alkaline. That’s 2.5 times the service life, which sounds pretty good. So we should start using Lithium batteries in all our effects pedals and get double or more the life, right? Well here’s the problem: Battery Junction has 9v Duralocks (essentially the 6LR61 in our tests) at $1.15, whereas the lowest cost Lithium 9v are $6. So in our DD-7 we’d get 2.5 times the life for 5.75 times the cost.

    There may be some cases where a Lithium battery makes sense. If you needed to run our example DD-7 on battery for a day at a festival with minimal opportunity to change batteries and it was worth the cost to avoid the possibility of failure? Maybe. You can also see that the discharge curve is much flatter; which means if you have a pedal with a very high cutoff voltage, above 7v for example, the Lithium might make sense, but such products are unusual.

    So there we have it, the old favorite alkaline 9v remains the best choice in most cases. The range of service life is quite interesting. Just with the three pedals in our example, we have almost 30 times difference in battery life. If you need to run your effects on batteries, it’s definitely worth making the calculations to figure out how long you should expect in each device.
    Note that we have a margin of error in our examples. If you need to be more precise, factors such as operating temperature and the battery’s internal resistance need to be taken into account. Devices with voltage regulators will draw more current as the voltage in the battery decreases and this will also impact the figures. A manufacturer’s current draw figure is a nominal value that should be used as a guideline. Even so, these details are only going to make a few percentage points difference. Unless you are designing a pedal for sale and are concerned with optimizing it for energy efficiency, using the techniques here should be quite adequate for most.

  • Inside EJ’s Box of Dead Batteries

    I’m sure you’ve all heard the story about how renowned tone magician Eric Johnson keeps around a box of partially discharged 9-volt batteries and can tell the state of charge from the sound of his fuzz pedal. There are plenty of people who are convinced their gear sounds better with different levels of battery charge. Some pedal board power supplies even come with controls that allow you to adjust the voltage range on some outputs so you can simulate a low battery. But does this really work, and if so, how?

    First things first. Many effects pedals, in particular digital effects, include voltage regulators for many parts of the circuit. Digital devices such as micro-controllers, digital signal processors, and others only sometimes run on 9v, and are very sensitive to the voltage variations. 5v or 3.3v are typical supply voltages for micros, so electronics elements such as buck and boost converters are utilized to ensure they receive a stable voltage regardless of fluctuations in the supply. If the supply drops too low for them to function, they simply shut down. If the main audio elements of the circuit in your effects pedal are powered by a regulated voltage, then using a partially discharged battery is going to have no effect whatsoever other than to reduce the run time of the device.

    Some analog devices can also be sensitive to voltage changes, and the designer may choose to regulate their voltage supply. The case here is much the same as for digital pedals; if the voltage is regulated, then using a half-dead battery or reduced voltage power supply is going to have no perceivable effect on the audio. That being said, there are some devices where varying the input voltage might influence the audio. Let’s look at those and see how it might work.

    As a battery discharges, it’s output voltage gradually reduces. If the powered device is unregulated, it will be running with the reduced voltage. This particularly impacts amplifiers such as the op-amp, diode, and transistor-based circuits in effects such as boost, overdrive, and fuzz pedals. These pedals are basically amplifiers, and the load on the output is being controlled by the power supply. The signal from the guitar pickups is modulating the power supply to provide the varying output current, but the eventual output power depends on the gain of the amplifier and the limits of the input power supply.

    As an example, let’s take an amplifier with a gain of 2 and a 3V power supply. If we provide a 1V input signal, the amplifier will try to increase this at the output to 2V. The output is 2V and our power supply can deliver 3V, so all should be well. Now let’s increase our input signal voltage to 2V. Again, we’ll multiply our input signal by our gain which is now 2 x 2, or an output voltage of 4V. Now the amplifier is trying to increase the output voltage to 4V, but the input power supply is only 3V. In this scenario, the amp will begin clipping. So, in these types of circuits, reducing the input voltage can make the effect clip earlier. It’s worth trying your boost or overdrive pedal to see if a lower input voltage has this effect.

    Distortion and fuzz pedals are more likely to be always clipping to some extent, so reducing the voltage will have a different effect. On the traditional transistor-based fuzz pedal, changing the battery voltage causes a response very similar to that of the volume control. Reducing the battery voltage reduces the signal level at the output. In combination with the existing controls and a tube amp on the edge of breakup, it gives you an extra knob to twiddle, but it does not provide a dramatic change in behavior.

    Testing with a Dunlop Fuzz Face shows a proportional reduction in output level as the voltage is reduced. The effect continues to operate down to about 5V at which point the signal from a single coil passive pickup begins dropping out.

    Dunlop Insides
    Inside the Dunlop Eric Johnson Fuzz Face. It’s a simple circuit utilizing a pair of BC 183 NPN transistors. Here, the battery input is connected to an external variable power supply for testing.

    Variable power supply
    A variable power supply allows precise control over the input voltage to the Fuzz Face, simulating a discharging battery. As the input voltage reduces, the signal level at the output reduces. Here, we are setup for 9V. The signal begins to drop out at about 5V.

    1KHz test signal
    Here’s a nice clean 1KHz test signal with the Fuzz Face bypassed.

    9V Output
    Here’s the output from the Fuzz Face at 9V with the volume and fuzz controls turned up around full.

    6V Output
    Here’s the output from the Fuzz Face with the input power reduced to 6V. The output level has reduced by about 50mV.

    As with so many things, the story of the discharged battery improving tone does have elements of truth, but it helps to understand a bit more about how it works to see what benefits may be had. In some effects pedals, this will have no impact at all since the effect regulates its voltage. In others, there is some change to the behavior either in output level, headroom, or both. Try it out with some of your pedals and see if it works for you.