Music to the ears: Pedal effect design

Article By : Mark Stansberry

Musicians favour pedal effect circuits created with specific types of components or with a specific circuit design because of how sound changes with them. Here's a design example.

« Previously: Music to the ears: Basics of pedal effect

There are literally thousands of different pedal devices and variants on the market. Some of the most popular effects include distortion fuzz overdrive the Wah delay flanger boost phaser and tremolo. There are also hundreds of different ways to create a pedal circuit. Musicians often favour pedal effect circuits created with specific types of components or with a specific circuit design; that’s because the subtleness of the sound will often depend on the specific circuit design and the types of components used.
A distortion pedal effect circuit is relatively simple to build. A basic one-transistor amplifier can be used. However unlike the design of an amplifier where the goal is to produce an exact duplicate of the input waveform with little or no distortion the design goal of the pedal distortion circuit is to add just the right amount of distortion. There are several ways to do this. The most common is to alter the bias point of the amplifier. In the JFET amplifier design below the bias resistors and the source and drain resistors can be varied to introduce different levels of distortion. Another common way is to overdrive the amplifier. One more way is to modulate the voltage supply.
In the circuit that follows a common source amplifier configuration was used. It was designed to introduce as little distortion as possible. Specifically a bias point and a low enough input voltage level was used to avoid clipping. The JFET was selected because it of all discrete transistor devices mimics the operation of the now-ancient vacuum tube.

Figure 1: A common source drain amplifier based on the LSK489 JFET is one basis for a distortion pedal circuit.
To capture the audio with LTspice as shown in Figure 1 a wave statement was used as a SPICE directive:

.wave “Preamplifier_Common_Source_JFET_LSK489” 16 16000 V(Out1)
The statement specifies the audio output wave file that the audio output is to be stored in Preamplifier_Common_Source_JFET_LSK489.wav. The command also specifies 16-bit resolution sampling 16000 samples per second sampling rate and the node to be recorded Out1 (the output node of the amplifier). LTspice allows for sampling rates up to 4 billion samples per second (1) way more than you will need for high quality audio and video recording!
With a 300 Hertz 0.5Vpp input signal the common source amplifier outputs a 300 Hz signal (4.5 Vpp) that is 180 degrees out of phase with the input (Figure 2). If you do not want the phase shift you would have to include another common source amplifier (the LSK489 contains two JFETs per package) to introduce another 180-degree phase shift or use a common drain or gate configuration.

Figure 2: The output (Out1) of the JFET amplifier shows little distortion.

To introduce some overdrive distortion the input signal is increased to 3 volts peak-to-peak. The resultant output waveform is given below. As can be seen it is asymmetrically clipped at the top (2).

Figure 3: Applying a 3V peak-to-peak input signal to the JFET amplifier results in significant output distortion. In this case the top of the output waveform is clipped and the baseline voltage is around 3V.  

LTspice’s Fast Fourier Transform (FFT) simulation feature gives the designer a way to determine the magnitude of the harmonics generated from a circuit. This allows the designer to get an idea of the sound effect generated before hardware prototyping. Below (Figure 4) the Fast Fourier Transform of the 300 Hz output waveform is illustrated. The FFT indicates that the 1st harmonic is around 3 dB. The Fast Fourier Transform also indicates that the harmonics generated at 600 Hz and 900 Hz are significant (in the 0 dB to 7 dB range). The intermodulation distortion (harmonics that are not a multiple of the 1st Harmonic 300 Hz) are also present. But for the most part they are below 80 dB and are not audible. Low intermodulation distortion can be attributed to the LSK489’s very low effective input capacitance.
When the distorted and non-distorted waveforms are listened to and compared on Microsoft’s Media Player you can discern the difference. The clipping reduces the bass and increases the pitch (attributed to the higher level of the 2nd and 3rd harmonics).

Figure 4: Overdriving the JFET amplifier generates 2nd and 3rd harmonics that compete with the 1st harmonic.

Next: Effect simulation, emulation »

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