Fix your tone with a dead battery

Have you ever 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 so you can simulate a low battery. But does this really work, and if so, how? Let’s find out.

 

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 rarely operate on 9v, and are very sensitive to the voltage variations. 5v or 3.3v are typical supply voltages for micros, so DC-DC 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 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 power 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. This said, there are some devices where varying the supply voltage might have an effect on the audio. Let’s take a look at those and see how it might work.

As a battery discharges, it’s output voltage gradually reduces. Check out these previous articles for more information on how this process works. 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, lets 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, although does not provide a dramatic change in behavior.

Testing with a 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.

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 up to an external variable power supply for testing.
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 up to an external variable power supply for testing.

 

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.
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.

 

Here’s a nice clean 1KHz test signal with the Fuzz Face bypassed.
Here’s a nice clean 1KHz test signal with the Fuzz Face bypassed.
Here’s the output from the Fuzz Face at 9V with the volume and fuzz controls turned up around full.
Here’s the output from the Fuzz Face at 9V with the volume and fuzz controls turned up around full.
Here’s the output from the Fuzz Face with the input power reduced down to 6V. The output level has reduced by about 50mV.
Here’s the output from the Fuzz Face with the input power reduced down to 6V. The output level has reduced by about 50mV.

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.

A version of this article first appeared in Gearphoria.

What’s the big deal with Certification?

Have you ever looked at the underside of a pedal or the rear of a guitar amplifier and wondered about all those little symbols such as FCC, CE, CSA, TUV or UL? Why is it that we typically see those on products from the big players, but not on the boutique devices? What  does ‘This equipment has been tested and found to comply with the FCC Part 15 limits for a class B digital device’ mean exactly? In this weeks’ episode of ‘What’s The Big Deal’, we ask ‘What’s the big deal with …….. Certification?’

 

The short explanation is that these are marks indicating that the products comply with various safety and performance standards around the world. The standards vary quite significantly between different nations, which is why we often see many different marks on one product. If the manufacturer is expecting so sell their product around the world, they will often indicate compliance with multiple standards with labels on the device. The European Community has a set of harmonized standards for different types of devices. The CE mark that you see on many products indicates the manufacturer is confirming that their product complies with these standards.

An interesting thing about the United States is that the majority of the standards bodies are independent groups. There are often few laws requiring compliance to these standards to be able to sell a product. So this largely answers our question about why we typically don’t see these marks on things such as boutique effects pedals: There is no law that says they have to comply. And complying is a significant undertaking. The standards are often complex and difficult to follow. Testing requires hugely expensive specialty facilities with vast arrays of costly equipment and expert test engineers.

If certification is not compulsory, it begs the question why do the  manufacturers even bother? This usually comes down to a couple of things. In some countries, certain levels of compliance are compulsory, so a manufacturer will at least need to test to those if they expect to sell into those regions. Some standards bodies will recognize tests of other groups, so if you have to do compliance for one location, then some others may come almost for free. Some distributors or resellers may require compliance in order to resell a product, so that might have an influence. Lastly it just makes sense for larger manufacturers to develop and test to standards. It can help with design decisions and quality control, minimize support and legal issues, and most importantly, give the company and their customers reassurance that the product they have made is safe and functions reliably.

If you are manufacturing a digital device though, one set of rules in the US that you WILL have to comply with is the FCC Part 15. This recognizes that certain digital devices emit radio signals, even though this is not part of their intended operation. We refer to these devices as ‘unintentional radiators’. The radio frequencies that these devices emit have the potential to cause interference with intentional radiators, and its part of the FCC’s job makes sure that your whizz bang digital delay does not trash your neighbors WiFi or cause an international aviation incident over your back yard.

Recently I was involved in testing some Mission products for FCC compliance so I thought I would share some of the photos from the day.

Early in April 2015, myself and Missions CTO and designer of the Mission Gemini amplifier, took some Gemini units down to EMT Labs in Mountain View, CA.

Here’s a Gemini 1 inside the anechoic chamber. The amp is a 50 pound two feet high 1×12 combo. It looks tiny in the photo, which gives you some idea of the size of the chamber. The device under test sits on a metal turntable and is rotated through 360 degrees during the test so they can measure the emissions all the way around. The scary looking red thing is the receiving antenna. This is motorized and raised several meters in height, during the test, again to measure the emissions at different points.

Anechoic Chamber

Outside the chamber, here are the results being shown on a monitor. The goal is to stay below the red line, so far we are looking good as you can see.

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Here’s one of the many racks full of test equipment. A decent spectrum analyzer alone can cost $20K. EMT has many millions of dollars worth of measurement gear.

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This is a state of the art 360 degree anechoic chamber used for testing devices such as wireless routers and smart phones. The engineer told us it can take weeks or even months to complete testing on some smart phones.

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Here’s some power emissions testing being done in a room completely lined with metal.

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The door of the chamber is several inches thick with thousands of copper fingers around the frame. It’s lined with hundreds of little space shuttle like tiles for absorbing reflections.

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We had a very informative and productive day at the lab. Thanks to everyone at EMT Labs for helping us out and completing our testing within a day. I’m happy to say we passed all our FCC emissions tests.

A version of this article was first published in Gearphoria. You can read the latest articles in Workbench Confidential at Gearphoria.com

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