How to Modify a Leslie Speaker for the B3

A Leslie speaker is a specially constructed powered loudspeaker used to create audio effects through use of the Doppler effect. The Doppler effect produces a phase shifting and vibrato sound that is identified with Hammond organs.The Leslie speaker history is particularly associated with Hammond B3 and C3 organs, and the sound of the Hammond/Leslie combination is an element of many of the more famous records from jazz to rock. Modifications to the Leslie speaker include those to the treble and bass speakers, the motors, the passive crossover and the amplifier.

Things You'll Need

• Screwdriver
• Special Leslie adapter
• Replacement drivers
• 1/4-inch patch cables

Instructions

Use an External Amplifier

1. Disable the built-in amplifier. Pull out all four tubes and the fuse. Use the motors only, attaching them to a combo preamp or an adapter. The Leslie speaker system can then be connected to any amplifier you want to use.

2. Disconnect the crossover from the Leslie amplifier. Be sure the amplifier and tubes are not hot. Patch the crossover to a convenient input socket. Use a one-quarter-inch phone jack, or a standard banana or GR plug.

3. Attach a quarter-inch jack that disconnects the crossover input from the Leslie amp, and connect it instead to the jack.

Replace the Leslie High-Frequency Driver

1. Obtain a moderate price driver. This is important because the most expensive drivers have too much usable sound reproduction range and too large a throat to connect to the entrance of the treble horn.

2. Remove the backs from the upper and middle cabinets to provide access to the treble driver and horn. Use a screwdriver to extract the screws.

3. Attach an Atlas, Electro-Voice or University high-frequency driver to the three-quarter-inch opening to the Leslie treble horn. These drivers can handle more power than the standard, and likely aged, Leslie driver. Use a special Leslie adapter to fit the screw throat PA driver to the Leslie horn.

Upgrade the Leslie Bass Driver

1. Remove the stock Leslie bass driver. Remove the middle cabinet sides to get access to the bass driver. Access to this is already available if you have replaced the treble driver above.

2. Install a new 15- to 18-inch driver that's capable of handling higher power levels.
   The stock Leslie bass driver is a 15-inch OEM speaker capable of handling about 50 watts.

3. Add a spacer ring between the new speaker and the Leslie baffle, so greater cone excursion can be accommodated. With an 18-inch driver, part of the back panel may have to be whittled out for certain larger 18-inch speaker frames.

Vary the Rotor Speed

1. Reach around the treble motors.

2. Move the belt from one of the three drive pulleys to another. The middle pulley creates a nice effect. Avoid using speed change controls. Leslie motors are AC-induction, and their speed is determined by the 60 Hz frequency input.

3. Reattach the outside sections of the Leslie cabinet, and give your modified Leslie speaker a go.

References

• Theatre Organs.com: Unearthing the Mysteries of the Leslie Cabinet; Clifford A. Henricksen
• Hammond-Organ.com: Rumors, Myths, Facts and Lore

Resources

• Sound on Sound: How Do You Record a Leslie Speaker?