In this article, we’ll talk about headroom, an argument often used by effect pedal manufacturers. We’ll look at the influence of headroom on the circuit itself, to understand how changing the supply voltage can change the characteristics of the effect. some audio electronics reminders Before getting into the theory of what the headroom is and how it will affect the circuit, let’s review the basics of signals and audio electronics. what is an audio signal? An audio signal is an alternating signal, that is to say it will vary in time in a repetitive way. It can be characterized by several values: its amplitude in Volts, and its frequency in Hertz, i.e. the number of times that the signal will oscillate or repeat itself, per second. The most common alternating signal is the sinusoid, which is often used to represent an audio signal. The sine wave is the elementary component of the audio signal. Indeed, any alternating signal can be decomposed into a sum of sinusoids of different amplitudes and frequencies. That’s why it is so often used to simplify the representation of an audio signal. bass signal at 55hz and its fourier transform. source wikipedia. The signal on the left comes from a bass playing a note at 55Hz. We can see that the pattern between 0 and 0.018s is repeated over time, this is the period of the signal. The signal doesn’t look like a sinusoid at all, but it can be decomposed into a sum of different sine waves. To visualize the decomposition of an alternating signal, we do something called the Fourier transform. The graph on the right represents the Fourier transform of the bass signal. This graph shows the sum of all sines that compose the signal on the left. Each peak indicates a sine, with its own frequency and amplitude.For example, you can notice that the signal is composed of a first sine at 55Hz, which is the fundamental frequency. Then another sine at 110Hz with a slightly lower amplitude, then another at 165Hz stronger, then at 220Hz, etc… These are the harmonics of the signal. These harmonics give the signature of the signal, they are what make the sound of an instrument, but they are also at the origin of the saturation in a signal. amplification with active components To make a boost or saturation effect pedal, we need to amplify the signal. For this we use what we call active components. The active components allow to amplify the signals by increasing their amplitude, but for that they need to be powered. Two kinds of active components are used for amplification: the transistor and the operational amplifier (Op-amp). a transistor and op-amp circuit which allow to amplify the signal. the limits of active components and saturation One thing to know about active components is that they cannot amplify a signal beyond their supply voltage. If a circuit is powered at 9V, and it amplifies a 1V sine wave by a 10 factor, the output signal cannot be 10V amplitude. Instead, the output will look like a sine wave with the edges cut off. The output signal cannot go beyond the supply voltage of the circuit. In red, a sinus that saturates at the output of a TL072 op-amp. This is what we call clipping, which is at the origin of the saturation. Since the output signal is still alternating, but no longer looks like a perfect sine wave, we can assume that the circuit has added harmonics to the original signal, and therefore saturation. This is the basis of any saturation pedal! The choice of components will directly affect the way the circuit will saturate, in a more or less violent way, and generate different harmonics. These harmonics will make the characteristic sound of the different saturation pedals. the headroom in transistor circuits We saw that the signal saturates as soon as its amplitude reaches the supply voltage. Depending on how a circuit is designed, this saturation can happen very quickly, when you hit the strings of your guitar. The limit at which the signal will start to saturate depends on the supply voltage. This is called the headroom. By increasing the supply voltage of a circuit, it is possible to increase the saturation threshold. The circuit can thus saturate only on the strongest attacks, improving dynamics. On transistor circuits, the saturation comes directly from the transistor which is at its amplification limits. The power supply will therefore have a great influence on the dynamic range of the circuit. This is what our Tape Preamp features, a JFET transistor boost, with a special power supply circuit that allows to vary the voltage from 3 to 27V. The dynamics of the pedal can be directly controlled by a potentiometer, for a great versatility. discover the tape preamp the headroom in op-amp clipping circuits reminders about clipping As much as transistors are known to provide a natural and warm saturation, it is not the case of op-amps, which saturate in a rather brutal and non-musical way. That’s why we often find on the op-amp circuits a clipping structure with diodes, in order to limit the amplitude of the signal and make it saturate way before reaching the limits of the power supply. A diode is a component with a threshold voltage, which means that the voltage at its terminals cannot exceed a specific value. The threshold voltage varies according to the kind of diode. To give an idea, the threshold voltage is about 0.3V for a germanium diode, between 0.5 and 1V for silicon diodes, and up to 2 or 3V for LEDs. Some germanium and silicon diodes. Generally, two diodes are connected in opposite directions, each one cutting the negative and positive ends of the signal. And of course, you can put several diodes in series to add their threshold voltages and increase the clipping threshold. But also put a different number of diodes in each direction, to make an asymmetrical clipping. The two most common arrangements are soft and hard clipping circuits. The first one produces a smoother saturation, while the other will produce a sharper clipping with more harmonics. an op-amp with diodes configured in soft clipping and hard clipping. the difference between od, disto and fuzz the op-amp slew rate When using clipping diodes, the signal cannot reach the supply voltage. We can then wonder what is the point of increasing the supply voltage of an op-amp. But op-amps have another limitation, which is called the slew rate. Simply explained, the output of an op-amp cannot reproduce instantaneous or too fast signal changes. When there is a fast variation at the input, for example with a square wave signal, the output will tend to react with a ramp that will gradually reach the correct value after a certain time.The greater the amplitude of the signal and the faster the changes, the greater the effect of the slew rate. in blue the input signal, in red the output of an op-amp. source texas instruments. Of course, this time is extremely short, and is not perceptible on low frequency signals. But as we have seen, an audio signal is much more complex than a simple sinusoid, and can present sharp peaks and fast variations, especially during the attack. And this is even more the case in a saturation pedal, whose purpose is to add harmonics to the signal. A bad slew rate can therefore alter the definition of the high harmonics and the attack, when the amplitude is the highest. Some pedals have used this effect to produce a characteristic sound, like the ProCo RAT and its LM308 op-amp, known for its bad slew rate. But to achieve a transparent overdrive which respects the input signal, it is better to have a good slew rate. the advantages of the headroom on op-amps During the development of the Klon Centaur, from which our Savage is inspired, the engineers realized that increasing the supply voltage of the op-amp would improve its slew rate. So they developed a specific power supply circuit allowing to convert internally the traditional 9V to 27V of headroom, to supply a portion of the circuit. The Klon Centaur schematic. Source Electrosmash. We are not going to analyze the schematic of the Klon here, it will be the subject of a future article. But you can notice that only the op-amps located after the clipping (D2 and D3) are supplied with 27V. This allows to keep a maximum of harmonics added in the clipping section. The headroom have also an impact on op-amp circuits, but this time it affects the transparency and the fidelity of the signal, rather than the dynamic range. To change the dynamic range of an op-amp circuit, you should play with the clipping diodes, by choosing different threshold voltages. This is the case with the Ego Driver, which is inspired by a well-known green overdrive using an op-amp and two clipping diodes. The Ego Driver not only offers greater fidelity and richness thanks to an internally generated 18V headroom, but also different levels of dynamics and compression thanks to multiple interchangeable clipping diodes. discover the ego driver conclusion Increasing the supply voltage to change the headroom can change the character of a pedal. The power supply has a real impact on the dynamics in the case of transistor circuits, while in the case of op-amp clipping circuits, the headroom will improve the transparency and fidelity of the audio signal. The dynamic range of an op-amp can also be modified by using different clipping diodes with different threshold voltages. It is therefore interesting to experiment with the supply voltage of a pedal when the manufacturer indicates that this is possible. But be careful, ONLY if the manufacturer indicates a voltage operating range! Otherwise, you probably lose your pedal and your warranty.