Rediscover a 2015 guitar overdrive that uses OTAs and germanium diodes for distinctive analog distortion. See why this clever circuit is still worth exploring today.
Traditional guitar overdrive pedals stick to rigid, fixed-diode setups. In 2015, Elektor presented a more versatile approach, utilizing OTAs to directly drive the current across silicon and germanium diodes. The result was an analog effects unit that let guitarists explore different clipping characteristics from a single circuit.
The Circuit
The design is built around two main OTA stages and several diodes. The input signal first passes through a buffer based on a TL271 low-power op-amp before reaching OTA IC3.A, where the amount of overdrive is controlled. A front-panel potentiometer adjusts the OTA’s control current and therefore the signal level driving the distortion section.
The schematic for the guitar distorter. (Click for PDF to zoom in.)
“The circuit … comprises roughly three parts,” noted A. J. Kohler. “The first is the input section with a buffer and a pre-amplifier, the second is the overdrive stage with the two OTAs and the diodes, and the third is the output section with a tone control stage.”
The heart of the project is its diode network. Silicon diodes and germanium diodes can be selected with a switch, allowing the player to compare their different clipping behavior. The circuit can also operate the diodes symmetrically or asymmetrically. Rather than simply placing the clipping diodes in a conventional feedback network, the design uses OTAs to drive them with controlled currents.
The Guitar Overdrive project.
After clipping, the signal passes through another OTA-based section that provides adjustable gain. A tone-control circuit follows, using a treble control derived from a Baxandall-style arrangement. Finally, another low-power op-amp buffers the output. The pedal runs from a standard 9-V battery and includes practical touches such as an LED power indicator and adjustable internal trimmers.
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Why Revisit It Today?
A decade later, this project remains interesting because analog distortion is still interesting for experimentation. Modern guitarists can buy countless digital amp models and sophisticated DSP effects, yet simple nonlinear components continue to define the character of many sought-after pedals.
The silicon-versus-germanium comparison is especially relevant. Germanium diodes have a lower forward voltage and different electrical characteristics than common silicon types, making them popular among builders chasing vintage-style clipping. This circuit goes further by giving the experimenter control over how those diodes are driven.
The Guitar Overdrive Project
The original article, “Guitar Overdrive With Genuine ‘Germanium-Sound’,” appeared in Elektor March/April 2015. Check out the article.
Editor's Note: The article first appeared in a 2015 edition of Elektor. Some of the parts and components might not be available. Still, we think you’ll find the article informative.
Elektor Magazine has been one of the leading sources of information on electronics for engineers, designers, start-ups and companies for 65 years. Our magazine is powered by an active community of electronics engineers – from students to professionals – who are passionate about designing and sharing innovative ideas.
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