Introduction

For the love of music or for the love of electronics, the aim of this project is to show how critical the SLG88104V Rail to Rail I/O 375nA Quad OpAmp with its low power and low voltage advancements can be to revolutionize overdrive circuits.

Typical overdrive designs on the market today run at 9V. However, as explained here we have been able to achieve an overdrive that is extremely economical in its power usage and running on such a low VDD that it can work using only two AA batteries at three volts for extended periods and extremely long battery life. To further preserve batteries left in the unit, a mechanical switch for disengagement is used as standard. In addition, as the footprint of the SLG88104V is small with a minimal amount of batteries used, a small lightweight pedal can be made if desired. All of this combined with likable sound effects makes it a leading overdrive design. 

Amplified guitars appeared in the early 1930s. However, at that time early recording artists strove for clean orchestra-type sounds. By the '40s DeArmond manufactured the world's first standalone effect. But at that time amplifiers were valve-based and bulky. During the '40s and through to the '50s even though clean tones were prevalent, competitive individuals and bands frequently turned their amps volume up to overdrive status and the distortion sound became increasingly popular. In the '60s transistor amplifiers started to be manufactured with the Vox T-60, in 1964 and around the same era to further preserve the distortion sound which was very sought after at that time the first distortion effect was born.

Prerequisites

Analog or digital processing of music signals can provide new effects, and active overdrive effects recreate the overdriven clipping effects of those early valve amps.

Usually unwanted and minimized in terms of amplification the opposite is true in terms of this effect. Clipping produces frequencies that are not present in the original sound and that could have been partly the reason for its appeal in the early days. Strong and almost square wave related clipping produces very hash sounds that are inharmonic to its parent tone, while soft clipping produces harmonic overtones and so generally the sound produced depends on the amount of clipping and depletion with frequency. It’s the strong belief of this author that the quality of an overdrive pedal depends on its proportion of harmonic to inharmonic tones throughout its range and its ability to preserve the harmonic tones at higher amplification.

Overview

Below is an overview of a proposed circuit, the aim of which is to preserve existing signals and produce those overdrive sounds. Using the SLG88104V allows an Overdrive pedal running on 3 V using two AA batteries which are much more widely available and less expensive to purchase than 9 V PP3 batteries. If desired, AAA batteries can be used instead, although the extra capacity of the AA makes it more than apt. Further, the circuit will be able to work on 4.5 V (1.5 V centerline +3 V) or 6 V (3 V centerline +3 V) if desired, although not necessary.

Selective frequency amplification - important modification to accomplish amplification at lower voltages.

Explanation and Theory

We choose to use the non-inverting topology of the amplifier as a base for the gain stages due to its high input impedance and easy adaption for frequency selection.

As we have seen, the gain in this set up is solely conditional on the feedback. If we convert this as a high pass topology, gain will be dependent on feedback and input frequencies as per some overdrive arrangements. Further, if the filter feedback circuitry is doubled, then the topology will apply one range of responsive gains to the input and then a further different set of responsive gains.

This setup can serve to both clarify the design and allow a more frequency directional/selective amplification. Below is the diagram of such an arrangement with...