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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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