Distortion pedal circuit features low power and soft clipping

-November 03, 2017

Read the first article in this series, Pedal effect circuit design: Getting started

Low-power, tube-like pedal effects can now be yours with a simple JFET circuit. The LSK489, in a common-source configuration, produces soft clipping with a supply voltage of just 5 V and about 2 mA of current. With such low power, and the prospect of sub-2 V circuits, you can take your distortion pedal effects devices off the grid with just an everyday battery. 

The soft-clipping pedal circuit, (Figure 1) uses a 5 V supply and features a high input impedance that will not load down the input signal (the LSK489 has an input impedance in the 1000 Gigaohms range). The high input impedance of the circuit makes it a good match for the output of an electrical guitar (which has an output impedance in the order of 6000 to 15,000 ohms)1

Figure 1 The LSK489, when operated at 5 V in a common source configuration, will produce a soft clipped output waveform.

Figure 2 below shows the audio waveform generated (Vout1) when a 0.5 Vp-p sine wave is applied (@300 Hz). The output waveform has rounded peaks and is symmetrical. This contrasts to the asymmetrical nature of the overdriven pedal design with clipped peaks (see Pedal effect circuit design: Getting started). With the components selected, the circuit has a gain of about 3 (1.5/.05). Listen to that tone here.

Figure 2 A softer tone (Vout1) is produced with a 0.5 V peak-to-peak input signal and 5 V supply voltage.

The harmonics produced with the soft-clipping circuit are quite a bit different than that obtained with a hard-clipping pedal effect circuit. For the 5 V, soft-clipped circuit, the 600 Hz harmonic is around −40 dB lower than the hard-clipped circuit. Listen to that tone here.

Figure 3 Lowering the supply voltage from 15 to 5 V reduces the level of the 2nd harmonic of the output signal (Vout1).

The soft clipping sound file generated from the 5 V circuit can be accessed from the Linear Integrated Systems’ demo circuit library here

Sub 3V implementations offer even more 

The soft-clipping distortion pedal circuit can be directly hooked up to an amplifier. Or you can 
add your own amplifier stages to obtain more gain or to layer distortion effects. It is also possible to use even lower voltage supplies and different types of transistors. To add more gain to the design, a bipolar gain booster can feed the JFET2. In that circuit design from muzique, a 1.5 V supply is used. The LSK489 circuit above can also be run at 1.5 Volts. However, the gain of the circuit is significantly reduced and the waveform shape has a reduced duty cycle. On the other hand, the peaks of the waveform are still well rounded. 

To hook up your pedal effect circuit to the microphone jack of your PC, you can use the 2.7 V supply on the headphone jack. The high input impedance of the soft-clipping circuit allows for a direct interface between your electric guitar and your PC. The circuit is small enough to fit into a jack that plugs into your guitar3

JFETs such as the LSK489 circuits provide soft-clipping and reduce unwanted distortion due to their high input impedance, low-equivalent noise voltage and low effective input capacitance. Because of these features, pedals based on the LSK489 do not introduce unwanted or unintended distortion. 

Because the LSK489 can operate at low voltages, pedals designed with LSK489 JFETs allow for  designers to directly hook up their musical instruments to their PC’s headphone jacks and obtain 
high-quality, amplified effects wherever a PC can be found.

Mark Stansberry participates in the technology and education industry as a writer, publisher, engineer, educator, and technical research and market analyst.

  1. How to Measure the Output Impedance of Your Guitar, Dieter Hoeller, Songbirdfx, publication date: December 4th, 2016
  2. Low Voltage Boosters, Alternatives to the 9v Battery, Jack Orman, publication date: 2005
  3. Building an iDevice Guitar Interface, Jack Cooper, publication date: September, 2011
  4. A Musical Distortion Primer, R.G. Keen, Geo-Fex Cybernetic Music, publication date: 1993-2000 
  5. The LSK489 Data Sheet, Low Noise Low Capacitance Monolithic Dual N-Channel JFET Amplifier, Linear Integrated Systems 

Related articles:


Loading comments...

Write a Comment

To comment please Log In