MXR MX-101 Phase 90 Effects Pedal

The Phase 90 phaser effect pedal was the first pedal sold by MXR. It became one of the best-known phasers. The original model had a simple orange-coloured housing with a logo in cursive script, a 1977 version in block letters. The variants also differed in effect due to different components. MXR went bankrupt in 1984 and was taken over by Dunlop in 1987, which still produces the early pedals. 
The phasing effect, a delay time-based effect for distorting guitar, keyboard, drums and voice, was discovered by American composer Steve Reich in the mid-1960s. Status Quo, Pink Floyd, Genesis, Led Zeppelin, Queen, etc. used it in the 1970s.

The phasing effect is created by passing the audio signal through two different signal paths. One part of the signal remains unchanged, while the other is sent through a series of all-pass filters that shift the sine waves in time. The shifted signal is mixed back into the original signal, with the waves whose phases are shifted by 180° cancelling each other out. By modulating the phase shift with a Low Frequency Oscillator, a rotation effect is created. The MXR Phase 90 has the following values: input impedance 1 MΩ, output impedance 10 kΩ, nominal input level -20 dBV, nominal output level -20 dBV, noise floor -96 dBV. The speed of the phasing effect can be changed using the speed knob.