Battery Powered Overdrive Pedal for Guitar Effects

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 minimal amount of batteries used, a small light weight pedal can be made if desired. All of this combined with likeable sound effects makes it a leading overdrive design.

Amplified guitars appeared in the early 1930's. However, at that time early recording artists strove for clean orchestra type sounds. By the 40's DeArmond manufactured the world's first standalone effect. But at that time amplifiers were valve based and bulky. During the 40's and through to the 50's 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 60's 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 amplifications.

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 make it more than apt. Further, the circuit will be able to work on 4.5 V (1.5 V center line +3 V) or 6 V (3 V center line +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.

AGain=1+(R2/R1)

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 arrangement with formulas producing interesting conclusions. This topology is an important crux relied upon by the final overdrive circuitry which will incorporate it as a main core several times to maintain a working model.

If we look at things a little simpler, then for a certain frequency f

AGain = 1 + (2ттfC1R2 / [2ттfC1R1 + 1])

and

AGain = 1 + (2ттfC1R2 / [2ттfC2R3 + 1])

The actual equation for AGain at a particular frequency f is thus:

which breaks down further to produce a final formula:

As evident this is analogous to the addition of the simplified equations above except for the inherent unity gain of the amplifier which is constant. In summary the frequency response gain of each high pass feedback topology leg is compounded.

The aim of such arrangements is to obtain a more uniform amplification of the input signal over the frequency range so that at higher frequencies where the OpAmp’s gain is reduced, we can introduce more gain. At low voltages the sound can be preserved through those low frequencies even though the headroom is not very high.

Circuit Diagram

Circuit Explained

The SLG88103/4V incorporates innate input protection to prevent overvoltage at its inputs. Extra protection diodes have been added at the initial stage of overdrive input for extra design robustness.

First stage amplification acts as a first stage high impedance buffer and amplifies initially to prepare for the overdrive stage. Gain is around two although it varies with frequency. At this stage care needs to be taken to ensure that the amplification stays low, as any amplification at this stage is multiplied into the overdrive amplification.

Following through to the overdrive stage, where the signal will undergo large gains, frequency selective amplification again ensures that the higher frequencies get that boost for a more consistent amplification, and consecutively we induce clipping using two diodes in forward conductive mode. A simple low pass filter forms the tone, and this leads to a simple volume potentiometer and a buffer to drive the output.

Only three of the on-board Operational Amplifiers are utilized, and the last remaining one is wired appropriately as per “proper setup for unused OpAmps”. If desired, 2 x SLG88103V’S can be used instead of the single SLG88104V.

A low power light emitting diode indicates an on-state. The importance of it being a low power version cannot be understated due to the low quiescent currents and running power of the SLG88104V. The main power consumption from the circuit will be the power indicator LED.

In fact, due to the extremely low 375 nA quiescent current, the power consideration for the GreenPAK SLG88104V is very small. The majority of power loss is through the decoupling low pass capacitors and the emitter follower resistor. If we measure the current consumption of the complete circuit’s quiescent current, it turns out to be only about 20 μA, increasing to around a maximum of 90 μA when the guitar is in action. This is very small compared to the 2 mA consumed by the LED and is the reason that the use of a low power LED is imperative. We can estimate the average life of a single AA alkaline battery to drain from full to 1 V is around 2000 mAh* at a discharge rate of 100 mA. A decent new pair of batteries producing 3 V should then be able to source over 4000 mAh. With the LED in place our circuit measures a 1.75 mA draw from which we can estimate over 2285 hours or 95 days of continuous usage. Because overdrives are active circuits our overdrive can produce “a hell of a kick” at minimal current usage. As a side note, two AAA batteries should last around half the time of the AA.

Below is the working model of this overdrive circuit. Obviously, as with any pedal, the user needs to adjust the settings to find the sound that is most apt for them. Turning the amp’s mid and bass higher than treble seemed to give really cool overdrive sounds for us (as treble was harsher). It then resembled the warmer old-fashioned type of sound.

Because of the GreenPAK SLG88104V’s tiny package and very low power consumption, we have succeeded in achieving a low power overdrive pedal that is less bulky and runs on only two pencil type batteries for a great length of time.

AA batteries are more readily available, and there is the possibility that they will not be changed for the life of any working unit, making it extremely easy maintenance and ecologically friendly. Further, it can be built with a small number of external components, so it can be low cost, easy to make, and as stated previously, lightweight.

* Source: Energizer E91 Datasheet (see bar graph), powerstream.com

Figure 8: Ch1 Red, Ch2 Yellow. Ch1 Overdrive Input, Sine Wave, 304 Vpk-pk, 513 Hz. 200 mV/Div, Timebase 2.5 mS. Ch2 Overdrive Buffer Output, Clipped Sine Wave, 1.024 Vpk-pk 513 Hz. 200 mV/Div, Timebase 2.5 mS

Figure 9: Ch1 Red, Ch2 Yellow. Ch1 Overdrive Input, Sine Wave, 304 Vpk-pk, 1 kHz. 200 mV/Div, Timebase 1 mS. Ch2 Overdrive Buffer Output, Clipped Sine Wave, 992 mVpk-pk 1 kHz. 200 mV/Div, Timebase 1 mS

Conclusions

In this project we have constructed a low voltage low power overdrive pedal.

Aside from handling the analog processing for GreenPAK’s mixed signal IC’s and other digital semiconductors, GreenPAK’s Rail to Rail Low Voltage, Low Current OpAmp’s have been shown to be useful in overdrive circuitry. They are autonomous in many other applications and especially advantageous in power sensitive applications.