3.3 Leslie Speaker / Rotary Effect

Rotary Speaker

  • The Leslie speaker is a combined amplifier and loudspeaker that modifies the sound of an instrument as well as amplifying it, by rotating sound waves. It is most commonly associated with the Hammond organ, though it was later used for the guitar and other instruments.
  • A typical Leslie speaker contains an amplifier, treble and bass speaker, though specific components depend on the model. Control is achieved either by an external half-moon switch, or by a foot pedal, that alternates between two settings known as “chorale” and “tremolo”.
  • The speaker is named after its inventor, Donald Leslie. Leslie began working in the late 1930s to get a speaker for a Hammond organ that had a closer emulation of a pipe or theatre organ, and discovered that rotating sound gave the best effect. Hammond was not interested in marketing or selling the speakers, so Leslie sold them himself as an add-on, targeting other organs as well as Hammond. The first speaker was made in 1941. The sound of the organ being played through his speakers received national radio exposure across the US, and it became a commercial and critical success as an essential part of any jazz organist.
  • Because the Leslie is a sound modification device in its own right, various attempts have been made to emulate the effect using electronics. The Univox Uni-Vibe was used by a number of notable musicians, while the Neo Ventilator has received critical praise, and Hammond-Suzuki now manufacture their own simulator in a box.
  • A Leslie speaker consists of a number of individual components. The audio signal enters the amplifier from the instrument. Once amplified, the signal travels to an audio crossover, where it is split into separate frequency bands that can be individually routed to each loudspeaker.
  • Different models have different combinations of speakers, but the most common model, the 122, consists of a single woofer for bass and tweeter for treble. The audio emitted by the speakers is isolated inside an enclosure, aside from a number of outlets which lead towards either a rotating horn or drum. Both the horn and the drum are rotated at a constant speed by an electric motor.
  • The only control common to all Leslie speakers is a dial controlling the master volume. This is normally set up once and then left, since the volume is designed to be controlled by the organ’s expression pedal. Leslie recommended playing the organ at full volume with all stops or drawbars pulled out and adjusting the volume just before distortion occurs. However, the distorted sound of an overdriven vacuum tube amplifier can be a desirable sound, to the extent that modern Leslie simulators have an explicit “overdrive” setting. The half-moon switch on a Hammond organ that changes setting on the Leslie speaker between “chorale” and “tremolo”
  • Unlike most popular music amplifiers, that use jack plugs to connect to instruments, Leslie speakers use an amphenol connector to interface directly to an organ via a console connector. Older models that used tube power amplifiers used a variety of 6-pin connectors, while later models used a 9-pin connector. In all cases, for a single organ – Leslie configuration, the mains power, audio and control signals are all carried on the connector, and the design of the pin layouts varies between organs and speakers. It is also possible to connect multiple Leslie speakers to a single organ, by using a power relay that provides the necessary AC current. A separate device known as the combo preamp is necessary to connect a vintage Leslie to another instrument such as a guitar. This combines a separate AC input and line level input onto a single amphenol connector, and provide a footswitch to select between the speeds of the Leslie.
  • The Leslie is specifically designed, via reproduction of the Doppler effect, to alter or modify sound. As the sound source is rotated around a specific pivot point, it produces tremolo (the modulation of amplitude) and a variation in pitch. This produces a sequence of frequency modulated sidebands. To stop a Leslie’s rotor, a special brake circuit was added to the Leslie motor controls, that incorporated an electronic relay by producing a half-wave of direct current.
  • Much of the Leslie’s unique tone is due to the fact that the system is at least partially enclosed, whereby linear louvres along the sides and front of the unit can vent the sound from within the box after the sound has bounced around inside, mellowing it. The crossover is deliberately set to 800 Hz to give the optimum balance between the horn and the drum, and is considered an integral part of the speaker. The tone is also affected by the wood used. Tone differences, due to cost cutting using particle board for speaker and rotor shelves instead of the previous plywood, are evident in the Leslie’s sound. The thinner ply of the top of the cabinet adds a certain resonance as well. Like an acoustic instrument, a Leslie’s tone is uniquely defined by its cabinet design and construction, the amplifier, crossover and speakers used, and the motors — not merely by the spinning of rotors.
  • Because a Leslie speaker modifies as well as amplifies the sound, the output cannot simply be connected to a larger PA system if the volume onstage from the built-in amplifier is too quiet. This is particularly problematic for an older Leslie like the 122 or 147, which only has a 40 watt RMS power amplifier. Instead, microphones are placed around the Leslie, and the output from these is connected to the PA.
  • The Beatles first recorded using a Leslie during the sessions for Revolver in 1966. After John Lennon had asked for his voice to sound “as though I’m the Dalai Lama singing from the highest mountain top”, Abbey Road engineer Geoff Emerick rewired the input of the studio’s Leslie so a vocal microphone could be attached to it. Emerick used this setup to record Lennon’s vocal on the track “Tomorrow Never Knows” and claims the Beatles subsequently wanted to record everything through a Leslie. George Harrison played his guitar through a Leslie on “Lucy in the Sky with Diamonds” and “You Never Give Me Your Money”. The Beatles subsequently inspired other guitarists to use the speaker. Eric Clapton played through one on Cream’s song “Badge”, and David Gilmour used a similar setup when recording with Pink Floyd.
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