Building the Dying Battery Simulator

Update 20 February 2009: New diagram showing how to install a dying battery simulator directly in a pedal.

Since a vintage fuzz circuit is a bit touchy to begin with, the balance of settings between transistor values, bias, and the volume and fuzz controls is a very interactive process. You can get things sounding really nice by playing with all these variables. But then you start changing the volume knob on your guitar or change other variables, you start to lose tone in various ways. On the fuzz circuits I’ve worked with, dropping the supply voltage to between 6v and 7v starts to clean up the tone based on other variables such as your pickup and guitar volume.

I’ve also built a clone of the Big Muff PI Fuzz pedal. The 9v drop circuit has great applicability there also—it gives me a greater deal of control over the circuit, especially when I change the guitar pickup selector or the volume. So how do you go about building the Dying Battery Simulator (DBS)? The good news is that this about the simplest project you can build, so it is easy to try out on your own pedals.

A couple of starting points:

• Voltage depletion is also called "sag" by some folks. The same general concept I'm covering in this project may be referred to as a 9v sag circuit. I learned to avoid using the word "sag" as many folks confuse it with tube amp sag which is an entirely different matter.
   
• This idea works on analog pedals. It is almost certain that it won't work on digital pedals, or pedals that require DSP or other on/off binary stuff.
   
• A 9 volt alkaline typically yields 9.5 volts fresh out of the wrapper. A dying old battery starts to get spongy at around 7 volts.
   
• Obvious question: “Will lowering the voltage damage my pedal?”. Easy answer: No, it won’t. Think about it—if a pedal could be damaged by lower voltage, what would happen when your battery started dying?
   
• The inverse of this project, i.e. a voltage "booster" is not a good thing. You can lower the voltage without hurting anything. You can also raise the voltage into your pedals, but you will hurt things. In the best case scenario, you'll burn out the limiting diode, worse case you'll get a ball of smoke, stink, and and empty slot in your pedal board. It's not like overclocking your CPU, different concept altogether.
   

Series Resistance

A key characteristic of a battery is that it presents a series resistance. As the battery dies, this series resistance increases. So in order to simulate that characteristic of a dying battery, we insert a resistor between the battery and the circuit. And to make it adjustable, we use a potentiometer instead of a fixed resistor. So to build a dying battery simulator using series resistance, it is as simple as this:

This approach works great, and is very simple to build:

To build this, you'll need two 2.11mm DC jacks and a 10K ohm linear pot. Mount the parts in a small enclosure and wire it up.

As a Voltage Divider

Another technique is to replace the series resistor with a voltage divider. In this case we wire the potentiometer differently so that it actually divides the voltage.

We also add a small resistor between the potentiometer and ground so that we can't accidentally turn the voltage sag all the way down and overload the power supply. Here's the schematic.

Note that I'm using a 10K pot, but just about any value pot will work, i.e. 100k, 1Meg, etc. The value of the resistor sets the low end of the voltage reduction. The following table shows some values to try:

You'll see that this is quite similar to the series example, except the potentiometer's wiper becomes the output and lug 1 goes to ground. Here's how you would wire it up:

Adding a Meter

The next step would be to add some type of visual indication as to the current voltage as you play with the drop control. Let’s say you dial in the perfect sound with your drop at about 90% full on (i.e. your drop knob is at about 9 out of 10). Wouldn’t it be nice to be able to actually see the actual voltage?

Not a problem, simply add an analog voltage panel meter to your circuit:

An analog panel meter in the 0-10vdc or 0-15vdc works fine in this arrangement. Notice how the meter taps into the 9v circuit after the potentiometer. If you tapped in before, your meter would always read the full voltage value, regardless of the potentiometer setting. Of course this sounds obvious, but I thought I’d point it out since I’ve made that mistake. This approach works for both the series and the voltage divider configurations.

Pristine Original Tonality

Finally, if you want to get very fancy, you can add a switch that takes the meter out of circuit, but leaves the potentiometer in circuit. Why? Well, I’m not sure, but there may be some circuits where the meter acts as an inductor, or adds additional resistance that make affect the behavior of the overall effect. I have yet to come across a circuit where there is any audible difference with the meter in circuit, but if you want to ensure the pristine original tonality of your effect, add a switch that disconnects one side of the meter.

You can wrap this all up neatly in a typical effects enclosure also. Get a smallish size enclosure, drill a hole for the potentiometer, and the meter and switch if you want to include that. Drill holes on either side for the 9v DC adaptor plug. Wire it up and you’re ready to go voltage spelunking on all your pedals.

Be sure to use a Discrete Power Source

When you use an AC adaptor with a voltage sagger, be sure to note that the voltage will sag across the power source--this means that if you use a daisy chain adaptor, all the outputs on the daisy chain will be sagged.

For example, if you use a daisy chain power supply like the OneSpot or PowerAll with a voltage sagger, all outputs will sag by roughly the same amount. Since you are generally not going to want to sag voltage to all your pedals, use a separate AC adaptor for the voltage sag.

Nope, don't want to sag all the pedals

A better way, separate power supplies,
one for the voltage sagger, one for everything else

Adding it Directly in a Pedal

So what if you want to put the dying battery simulator directly in a pedal instead of having an external unit? Very simple--just find where the 9 volts + comes out of the power jack and insert the DBS there.

Here's a simplified pedal before the operation. You can see that the middle lug of the DC jack is what supplies power to the circuit board.

To add the DBS, simply insert a 10k ohm linear pot in series with that wire, so it looks like this:

You'll see that I've chosen the series potentiometer approach here--it works great directly in the pedal. Of course you'll have to figure out where the new potentiometer will fit in the enclosure and then drill a new hole to accommodate it.

Of Voltage Regulators and Mathematics

If you want to go further and learn about how to truly regulate voltage and other power supply variables, there are a number of great resources on the web.

• A Super Pedalboard Power Supply: Great article by R.G. Keen which includes coverage of dying batteries: http://www.geofex.com/Article_Folders/pedalbdpwr/pedalbd.htm
   
• Patents on Parade: Where Jack Orman talks in detail about batteries, depletion, patents, and existing circuits. http://www.muzique.com/lab/patent2.htm

Some folks from a more technical background would claim that the simple potentiometer approach is not sophisticated enough to accurately model a dying battery. I have two thoughts for that: first, it is super simple and pretty accepted within the DIY community. Secondly, if it is good enough for Zvex, it is good enough for me:

“put a 5k linear pot in series with the positive terminal to the circuit. that's all you need to simulate a typical dying 9V battery. the resistance range is almost identical to the chemical build-up over time using a pedal that draws around 5mA. if you don't believe me. try it, and then substitute a dead/dying battery and listen for a difference with the pot shorted. batteries can't die without building up series resistance. likewise, the reason for voltage drop is a result of the increased series resistance! i put it to you... the lowering of battery voltage is a result of exactly the same mechanism as increased series resistance. a pot in series with a battery will most certainly simulate the dying of a battery.” – Zachary Vex

Parts List

These are the parts I had lying around, along with links to Mouser for ordering. You can use just about any kind of linear pot--I've listed a 100k value, analog panel meter and SPST switch.