pedals

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Intro to Effectsblog

March 16, 2014
  • 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.

  • 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.

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