Music to the ears: Basics of pedal effect

Article By : Mark Stansberry

Fundamental to the reproduction of era-specific and musician-specific music is the technology contained in pedal effect gadgets. Here's how you can get started design a pedal circuit.

If you want to replicate the music of your favourite musicians, you will need to know the pedal effect devices they use and the circuitry within them. Furthermore, if you want to design new pedal effects that please your audience, you will need the right tools, the right design flows, the right components, some circuit know-how, and a little creativity.

The design of pedal effect circuits, i.e. circuits that modify the waveforms of musical instruments, is an art and science that has been practised since the 1920s. Fundamental to the reproduction of era-specific and musician-specific music is the technology contained in these pedal effect gadgets. Different artists prefer different component technology: tubes, germanium transistors, silicon bipolar transistors, MOSFETs, diodes, JFETs, op amps or any mix thereof.

The passive components used in pedal effect circuits, like inductors, capacitors and resistors, complicate the process of analog sound reproduction. Of specific mystical interest to many professional musicians is the “magical inductor.” Thought to be enchanted music’s key, the magical inductor, once placed inside a circuit generates harmonics that separate it apart from all other inductors. The idea of a magical inductor keeps musicians wondering. Is music’s holy grail locked inside the pedal device of a long-ago musician? Is the magical inductor made of a special material? Is the magical inductor lost in time and do the music giants of our times have access to it? One thing most of them agree on is that the magical inductor is one of the analog Muses and not one of the digital Sirens.

Audio circuit simulation, emulation

Audio engineers and musicians use a variety of tools to help them create analog pedal effect circuits. Among those are Digital Audio Workstations (DAW) and analog circuit simulators such as LTspice. Using these two tools together, designers can electrically analyse and listen to different wave modification circuits without building the circuitry.
These two tools give the audio designer different design flow options. Instead of design, simulate, build, and test, the designer has the exploratory option of design, simulate and emulate. After the pedal effect circuit schematic is entered into the LTspice circuit simulator and after it is simulated for basic electrical performance characteristics, such as bias, gain, noise, harmonics and frequency response, it is run through an audio emulator (which could be an expensive DAW software package or a free multimedia player). For an audio emulation, a music file such as a WAV or MIDI file is fed into the LTspice circuit simulator. The audio output file from LTspice is then played with a DAW or multimedia player and listened to on a pair of high-quality headphones.
Although this is an efficient way to listen to the circuit performance without building the circuit, the audio emulation will not necessarily produce an exact replica of the analog circuit. You could very well lose analog subtleties and associated music intonations that the actual analog circuit produces.
When the designer uses his or her ear, and musical and circuit analysis experience, the designer can mentally interpolate how the audio emulation will actually sound when it comes out of an analog circuit. To a large extent, audio mental interpolation takes experience to do well. To gain this experience, once a pedal LTspice circuit’s harmonics are examined and listened to on a DAW or media player, the designer should build and listen to the actual hardware prototype with, for example, a guitar as its input. Developing audio mental interpolation skills also requires that the actual hardware prototype waveforms on the oscilloscope be compared with the DAW’s output and the sound you hear. In this way, the designer can determine what is lost in the LTspice/DAW audio emulation harmonic analysis (Fast Fourier Transform).

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