A low cost DIY Battery life and sound quality extender

A simple add on for those times when a battery is better than a wall supply

I know that title sounds like one of those "snake oil" claims, but hear me out and you'll see its not a tall tale at all. Most battery powered circuits are designed to offer reasonably consistent sound quality throughout the useful life of a battery, but only to a point. From some of these battery discharge curves, you can see that a typical 9V Alkaline battery starts its life at 9 volts (or slightly higher), then drops gradually at a rate depending on load current. They drop more quickly toward the end of life. (Note that the mAh capacity on the bottom graph axis is intended to be a multiplier of the actual milliamp-hour rating of the battery.)

Obviously the voltage drop tends to be more gradual with well made guitar pedals, since they are designed to operate with minimum current drain. My old MXR Dyna-Comp "Red box" (a popular compressor pedal from back in the 1970's) only draws about 5mA ( that's only 5/1000 of an ampere). But even if a 9V pedal was designed to work down to 6V, the sound the pedal was intended to produce will tend to suffer or at least become inconsistent as the voltage drops. For example, the speed or depth setting of a chorus pedal, or the sustain of a compressor or overdrive pedal will change and require re-adjusting over time. Then the bigger problem. At some point the pedal will either stop working entirely, often suddenly sounding like unusable crap without any warning. This happened to me at a recent performance where I forgot to check batteries before hand, and I generally do Not like to let a performance problem happen again if I can avoid it. Hence, this article!

The usual solution

Since it would be ideal if the pedal(s) could operate from a steady 9.0 volts all the time, the obvious fix is to wire all pedals to a common power supply that plugs into available AC. Sometimes though this is not practical. For me, there are times I just want to go to a jam and bring one small distortion or multi effects pedal. Or what if you have active electronics inside your guitar? Just about all acoustics do these days, as do some electrics. I personally liked the sound of that MXR "Dyna-Comp" pedal so much, I built it right into my Les Paul guitar many years ago. The last thing anyone would want is another cord to supply wall power to a guitar. The point is, there are times when its still convenient to be able to rely on the battery. If you're lucky, your device incorporates a monitor which lets you know when the battery is going South, but with pedals those cases are few and far between.

My solution

Here's a couple of pictures of the boost regulator I used. This particular board is designated "MT3608" and very small, barely 1-1/2" x slightly less that 3/4". With a little ingenuity, this is small enough to fit inside most pedals or guitars. Note that It also has a nominal efficiency of 96%. This may seem like a 4% battery loss. But when you add in the extra time your dying battery will continue to provide full rated voltage, its almost like getting 200% (or higher) efficiency, in terms of time. How much more time? Of course your mileage will vary. That "Dyna Comp" pedal circuit I described only draws 5mA, and usually sounds "OK" until the battery gets down to about 6 volts. That's a good design and not all pedals will tolerate such a voltage swing. So for a test, I connected that Dyna-Comp pedal circuit to one of these boost converters using that same battery that "died" on me during a recent performance. I also included that "low battery LED" add on I described. Here's my circuit. For now ignore the optional Filter Capacitor, resistor, and Zener Diode.

The LED lit fairly brightly right away, indicating I was really running on a pretty dead battery. In fact, my DVM showed it was now down to only about 5 volts. At that voltage, it would be useless to power the pedal at all. But guess what... That boost regulator maintained the 9.0V output for nearly another hour, until the battery input finally crossed 2 volts! That's much better than I would have predicted looking at those battery discharge curves! Especially considering that in order to output the same output voltage as the input drops, the converter must draw progressively more current. But with the boost regulator, my "Dyna Comp" pedal circuit worked perfectly right up to the bitter end. Like I said, your mileage will surly vary with battery quality and the current required by your pedal. But several advantages are indisputable:

• You will definitely get full "brand new battery" voltage through the entire life of the battery.
• You will get significantly longer usable life from a battery, because the pedal (or other audio circuit) opartion won't deteriorate until the battery is really totally shot.
• If you are environmentally conscious (or even just cost conscious) this will squeeze every bit of available energy from each battery you buy.
• As a bonus, with a simple LED and resistor add on, you can be warned well in advance when the battery should be changed. With a little experience, you'll learn to predict how many hours of usable battery life is left, from the brightness of the "low battery LED "

Not just for 9V!

Remember that these converters are adjustable, which offers even more application possibilities. Maybe you already have a common 9V supply running on AC, but have one or more 12V pedals. You can use one of these boosters to directly convert the available 9V to 12V. This boost converter is probably a much cheaper solution than adding another power supply, and will almost certainly take up far space on your pedal board. Or maybe you'd like to add a portable option to your pedal board, so it can run without being plugged in to AC. There are many rechargeable "gel cells" and motor cycle batteries that typically are 6 volts. In fact you'd be hard pressed to find one in a 9 volt model. But the boost converter opens the possibility of running your 9 volt pedals from these 6 volt sources. And by the way... these particular boost regulators are capable of output currents up to 2 amps. That means you probably can power quite a few pedals from each one of these regulators.

Another possibility is to use a boost regulator along with a 3.8V (nominal) lithium cell. This may be more of an option for a designer wishing to build a battery powered device or pedal from scratch. An 18450 size Lithium rechargeable cell can have so much higher a mAH rating than a 9V, it may be advantageous to design with one of these, even though it means extra circuitry for safe charging and protection of the lithium cell.

Optional Zener Diode Explained

One issue with any usage not starting with equal input and output voltages ( like our original example of a 9V pedal with 9V battery) is that the simple two part "Battery Low" warning LED will require an extra Zener diode in a case like that. A zener diode will completely block all current until a certain threshold is reached. By choosing a zener whose reference voltage is equal to the difference between the input and output, we can ensure that the "low battery" LED won't start to glow until the battery starts to decline. For example, if a 6volt battery is used to generate 9 volts, we would want to add a 3 volt zener diode to the "low battery" LED add on. Of course in the case where the boost converter is not used with a battery (for example to drive a 12 volt pedal with a 9 volt AC wall supply), no battery warning circuit is needed.

Optional Filter parts

As for the capacitor and resistor "filter", this is a bit technical. The parts shown will work in most cases, but if you need to fine tune your circuit this will be helpful. For now feel free to skim to the last sentence of this paragraph if this gets too deep. During bench tests with an oscilloscope, and using a 1K load (to offer a load of approximately 10mA) I noted oscillations in the range of tens of Khz on the output of this boost regulator. It was a "sawtooth" waveform, which is typical of unfiltered output from a boost converter. These seldom reached an amplitude of more than 20mV peak-to-peak, depending largely on the difference between input and output voltage and the load applied. Its frequency increased and its amplitude decreased as the input voltage got lower. This makes sense and correlates with the way a boost regulator works. However, it does reveal that the "switching frequency" listed in the specifications on these regulators is completely wrong. And oscillations in frequencies anywhere in the range of audio frequencies can pose a problem for high gain pedal circuits.

Well as it turns out, my "Dyna Comp" compressor was probably the worst case situation, because a compressor increases its gain as the source signal decreases. That's how it makes guitar notes seem to sustain forever, but this is also why compressors tend to get very noisy when you stop playing completely, and usually need to be turned off immediately afterward. Fortunately, much like the output of a transformer based power supply, this oscillation is easily filtered with the addition of a few parts. The 47 ohm resistor and 50uF capacitor shown made the oscillation virtually undetectable at the output, over a wide range of loads and voltage settings, at the expense of a very small voltage drop. At the actual 5mA load of my "Dyna Comp" pedal, this drop amounted to about 0.25 volt loss at the "Output Test Point" shown in the diagram, when compared to the actual VOUT+ terminal from the boost regulator. But that is easily compensated by adjusting the boost level until the voltage at the test point was equal to the desired voltage. Bottom line, add this capacitor and resistor to eliminate the chance of any unwanted side affects or noise from the boost converter's oscillations.

As an addendum, I'd like to point out that I've worked with several boost converters over the years, and have found the need to add some kind of output filtering to be pretty much the norm, when the intent is to power an audio circuit. In fact I'm puzzelled why more manufacturers don't make provision for filtering components right on their PC boards.

Other setup Details

This may or may not be apparent, but recall we said a boost converter maintains the same output voltage, with an input voltage equal or less than the output. Well that's true but if you think about it, at the point where the input voltage is exactly equal to the output, the boost regulator must stop doing anything, and this could cause an instability problem. A brand new 9V alkaline battery will actually output almost 9.5 volts for a very short time. So If the boost converter is set to output exactly 9V, it won't be doing anything for a while, and then will begin to turn on when the battery voltage begins to dip under 9. That may sound like a convenient situation, but in reality the transition can get a little ugly and unstable when the two voltages are very close. To avoid this, Its actually best to set the boost regulator to output at least 1/4 of a volt more than the max voltage a brand new battery could ever offer. So if a new 9V battery outputs about 9.5V, a boost regulator setting of about 9.75 should keep it in a state where it is always at least operating, and is therefore stable. 9.75 volts is NOT a problematic voltage for any 9V pedal. But, remember that the filter network (the capacitor and resistor) described earlier is going to drop about 1/4volt anyway. So in the end, once properly adjusted, both the battery and pedal will start out receiving 9.5V (equal to a "brand new" battery voltage) while the boost converter is kept "happy", outputting at least 1/4" more directly at its VOUT+ terminal.

Total Cost? Virtually Peanuts!

I've purchased several of these boost converters for my own projects, so if you'd like one or more of them to experiment with, you can buy them directly from Elfin Technologies . I'll include a low current LED, 1.8K resistor, as well as the filter parts (47 ohm resistor and 50 uF capacitor) as part of the kit. This will work for the simple case where initial battery input voltage and output voltage setting are equal. If you think you may need a zener for a different battery voltage setup, I'd suggest discussing your application with me before ordering. I may or may not have the zener diodes on hand with the correct voltage for the project you have in mind.

Please be aware that this page is intended to be for a DIY project. I'm only making these boards and parts available as a convenience. I'll confide that if you look for these boost converters on a China sales website (like aliexpress.com or even some ebay stores), you'll probably find them so cheap you'd have to wonder how they could possibly make any money selling them. And if you're an electronics enthusiast, good chance you have the other parts already. I have to charge a little more than China, and I also don't get the shipping discounts these larger businesses (or China suppliers) get. But I'd advise buying at least 2 if you do order them from here, because the shipping will be about the same.

Finally it should go without saying that building a circuit like this into any pedal or other device will require experience in soldering, and basic equipment such as a DVM for taking electrical measurements. Of course you also need some experience with mechanical improvising anytime you attempt to take apart and modify a device, and basic shop tools are a big help too (for example, a drill to make a hole for the "low battery warning LED). As always, I have to remind you that such mods pretty much void the terms of your warranty. Of course if you purchase these from here, you'll also have a reasonable amount of guidance at no extra cost. But no liability can be assumed for any harm done to your equipment or devices, regardless of my extended assistance or advice. As another option, if you'd like such a mod done and don't feel confident doing it yourself, feel free to write me using the contact form. Perhaps I can make the whole mod for you if you're willing to pay for shipping, along with a fair charge for the bench time.

Thanks for reading, and I hope you find this helpful!

Prices, should you choose to order from me