The instrument
The MXR MX-104 is an early distortion pedal. MXR launched it in 1974 in a first series together with Phase 90, Blue Box and Dana Camp. Pedals with distortion effects are divided into fuzz, distortion and overdrive, depending on their circuit design and how they influence the waveform. Since overdrive and distortion use operational amplifiers and diodes, the line between the two types can be blurred. For example, the MXR Distortion+ and the DOD Overdrive 250 Preamp have almost identical hard clipping circuits.
The pedal has been used by Randy Rhoads with Ozzy Osbourne, Jerry Garcia of the Grateful Dead, Dave Murray of Iron Maiden and Thom Yorke of Radiohead, among others.
Details
The MX-104 uses a single operational amplifier and a pair of germanium diodes for grounding (parallel push) for clipping in a simple configuration with only output and distortion controls, without tone control. Output controls the overall volume of the effect, distortion controls the amount of distortion. Footswitch turns the effect on/off. The MX-104 has the following technical specifications: Input impedance 1 MΩ, output impedance 25 kΩ, nominal input level -22 dBV, maximum output level -14 dBV, noise floor -74 dBV, distortion gain 10 dB to 45 dB, 1 kHz, bypass hardwire, current draw 2.5 mA, power supply 9 volts DC. (Source: Wikipedia and MXR manual)