CMOS Distortion

CMOS stand for "Complimentary Metal Oxide Semiconductor (Field Effect Transistors)," meaning that CMOS integrated circuits contain MOSFETs. At some point a rumor got started the MOSFETs "sound like" tubes. This is a subjective issue, and beyond the scope of this article, but the rumor inspired at least two CMOS IC based distortions with "Tube" in the title in the 1970s. The first was a DIY project offered by Craig Anderton called the Tube Sound Fuzz. The other was Electro Harmonix's Hot Tubes Overdrive (1978?). There may be others from the 1970s; I'll be on the look out. The IC Sound Tube Wizard appeared in 1986.

In the boutique pedal era, this circuit reappeared as the Way Huge Red Llama, the Blackstone Appliances MOSFET Overdrive, and the Schumann Lion, to name just the ones I know of.

Anderton's Tube Sound Fuzz cascades two stages of CMOS distortion. Small caps are included in the feedback path to reduce gain to unity at ultrasonic frequencies. The IC is a CD4049 hex inverter which means there are 4 more inverter stages on the chip that can be used for more distortion, or a "Ch B", or perhaps useful as bypass circuit controllers.

The CD4049 is normally used as a digital device where the output will be low if the input is high and vice versa. Placing a resistance between input and output introduces negative feedback which turns the inverter into a linear amplifier.