Exploring Pedal Effects with JFET Amplifier Circuits

Creating unique pedal effects doesn’t always require complex circuitry. In fact, many classic sounds are built by simply overdriving a JFET amplifier, tweaking input signals, or using clever circuit designs to shift filter center frequencies.

For instance, a single JFET amplifier can produce a clean, natural-sounding signal—as long as the input voltage remains within the normal operating range of the amplifier. This depends on the JFET’s gain and the supply voltage you’re using. Once you start pushing the input beyond these typical limits, that’s when the magic of overdrive and harmonic color begins to emerge.

Check out the circuit diagram and simulation data below to see how this basic building block can become the heart of your next creative pedal design!

Figure 1 - A common source amplifier, based on a JFET, is the basis for the design of distortion pedal effect circuit.

Figure 2 - The output of the JFET amplifier shows little distortion

Figure 3 - Examination of the FFT, one can see that the 2nd and 3rd harmonics are all below 20 dB of the 1st harmonic (300 Hz)

In order 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.

Figure 4 - Applying a 3 Volt peak-to-peak input signal to the JFET amplifier results in significant distortion. In this case the top of the waveform is clipped. 

The Fast Fourier Transform, below, indicates that the harmonics generated at 600 Hz, 900 Hz and 1200 Hz are significant (near 0 dB).  When the distorted and non-distorted waveforms are listened to on Microsoft’s Media Player you can discern the difference. The bass sound has been muted significantly.

Figure 5 - Overdriving the JFET amplifier generates 2nd and 3rd harmonics that overpower the 1st harmonic

Instead of overdriving the circuit, you can lower the power supply to produce a softer tone. The figure below is the waveform generated (Vout1) when the power supply was lowered to 5 V and the source resistor increased to 1000 Ohm.

Figure 6 - A softer tone (Vout1) is produced with a 0.5 Volt peak-to-peak input signal and 5 Volt supply voltage

Comparing the FFT of the 5 Volt with the 15 Volt circuit, one sees that the 600 Hz harmonic has been reduced to around -40 dB. When comparing the actual audio of the two circuits, one can hear the difference. The audio of the 5 V circuit is much more subdued than the 15 Volt overdriven circuit.

Figure 7 - Lowering the supply voltage to 5 Volts reduces the level of the 2nd harmonic of the output signal (Vout1)

For a more detailed discussion of distortion circuits, see the article by R. G. Keen, A Musical Distortion Primer