A little Vibe History

Mechanical rotary tremolo systems, including Leslie's, are based on the Doppler-effect. Discovered by Austrian mathematician and physicist Christian Doppler (1803-1853) in the early nineteenth century, the Doppler-effect is the apparent variation in pitch that a stationary listener hears from a moving sound source. In practice, the loudness of the sound also appears to vary and it is this combination of frequency (vibrato) and amplitude (tremolo) modulation that give Leslie Speakers and other Doppler-effect systems their characteristic sound.

The Leslie Hi-Lo system consists of a 40-watt monophonic tube amplifier; an 800 Hz 16-ohm passive crossover; a rotating treble horn and a rotating bass speaker. The Leslie's treble rotor is largely responsible for the "Leslie Sound." The bass rotor works just as an AM device, and only for the upper two octaves or so of the bass section (200 to 800 Hz). Click on brouchure images for more information.

Basic Doppler characteristics are easily created electronically. However, most simulators don't reflect sound or produce the unique audio characteristics of mechanical systems. Rotating speaker elements also direct sound out the sides and back of the cabinet which is reflected off nearby walls and surfaces. The listener hears this combination of primary and reflected sound as a moving audio field. Adding a second cabinet further enhances the effect. Even when the audio source (organ) is mono, the rotating speaker elements and reflected sound create a realistic stereo effect.

Electronic devices that produce a "Leslie-like" sound have been available since the sixties. Cheap solid state components and the need for more portable equipment led to the development of numerous "electronic Leslies" and tremolo-vibrato add-on devices. Few early Leslie simulators were authentic enough to fool even causal listeners and rarely sounded like a real Leslie Speaker. Most relied on simple phase-shift circuitry and their primary advantages were compactness and low cost.

The Uni-Vibe was one such device. Circuit analysis of the phase shifter section suggests, that the designer(s) were attempting to simulate a Low-Hi Leslie Effect. Unlike phasers, in which capacitor values are selected to produce deep notches, the Univibe capacitors are staggered tuned so the phase-shifting is spread around in the frequency spectrum. For the boffins, here's a frequency analysis (134KB) of the original Uni-Vibe phase shifter section.