LESLIE SPEAKERS

North Suburban HAMMOND ORGAN Society

So far we've looked at a typical or generic Leslie such as the 122 which is designed primarily for traditional Hammonds. The signal from the console enters the input of the power amplifier, gets amplified sufficiently to drive the speakers, and the amplified signal passes to the crossover network which separates it into two signals, one comprising frequencies below 800 Hz, and the other comprising frequencies above 800 Hz. The signals < 800 Hz go to the 15" woofer, and those > 800 Hz go to the treble driver.

After the sound leaves either speaker, it encounters the rotating elements which by their rotation introduce Doppler phase and pitch changes and also loudness variation which results in the characteristic "Leslie Tremolo." Don Leslie referred to the effects of his unique rotary elements as tremolo, and the term has become standard usage when associated with Lelsie speakers, but the effect is far more complex than a simple tremolo. It is so complex that I think he should have invented a new word to use when referring to the effects produced by these rotating elements. He did not, however, invent a new word and just used the word tremolo, however he referred to his early speaker cabinets as the Leslie Vibratone Speaker, so he did create at least one new word in connection with his invention. But rather than enter a complicated semantic debate, we'll stay with the term he used, inasmuch as it has become just as standard and well known as the use of the word organ in conjunction with the name Hammond, even though we all know that a Hammond organ is really NOT an organ but a new and different instrument in its own right.

I do not know why the Leslie effect sounds so much better with sine wave tones or pitches derived through harmonic synthesis as opposed to the tones from complex waves with their own harmonics, but the difference is profound. In fact the difference is so great that many electronic organs which generate both sine-wave (flute or tibia) tones and also complex tones (reed and string) are designed with separate audio channels for each type of tone, and in many of these instruments, the Leslie elements affect only the sine wave tones and not the others.

When the Leslie effect is applied to these more complex tones, the result doesn't really sound all that great, and obviously this is not just my opinion because the makers of those instruments which provided both types of tones all eventually came to develop instruments with separate audio channels for each. I would assume that they started doing this after they first applied Leslie speakers to their earlier instruments where there was no such separation.

When two or more complex tones with natural harmonics are sounded together, individual harmonics of each will be slightly to somewhat out of tune with those of the others which results in subtle interference beats in the overall result. For whatever reason, when such tones are sent through the rotary elements of a Leslie, these interference beats become very noticeable and create an unpleasant distraction. In the case of sine wave tones, and tones synthesized from combinations of sine wave pitches that are derived from the equally tempered scale, these interference beats between harmonics are not present, and perhaps that might be part of the reason why the Leslie effect is so good on that type of tone.

At any rate, when Leslie speakers were applied to other early electronic organs besides Hammonds, the makers of these other instruments soon began to separate the two types of tones. By this time, Leslie was also expanding his product line and developing speakers to work with instruments other than Hammond, and we soon began to see Leslie models with both rotary elements and straight speakers in the same general cabinet. One of the first such models was the 151 and the later 251 which included a separate 6" X 9" oval speaker on each side of the cabinet just below the top compartment. With the exception of a pair of two additional louvers, these cabinets looked exactly like regular Leslie cabinets. The power amplifiers, however included a separate channel to drive these two stationary speakers, and the input cable connections were different from those for traditional Hammonds, such as the 122.

One brand of commercial electronic organs appeared, however, that used the Leslie effect in place of any other type of vibrato at all in many of their models and that was Gulbransen. Like Hammond, the early Gulbransen instruments generated sine waves and created other sounds through harmonic synthesis of sine wave tones just as Hammond did, and thus the Leslie gave these instruments a very good, big and pleasing sound. Later on, Gulbransen developed several other instruments which included separate tone generators for complex waves and then they modulated the frequencies of these other tone generators to create a true vibrato for the complex tones, but depended exclusively on the Leslie rotors to provide vibrato and tremolo for their sine wave and synthesized voices. The smaller and simpler Gulbransen models used no electronic vibrato at all. Each spinet or console contained a simplified Leslie speaker as an integral part of the instrument and if a purchaser wished to add a separate speaker cabinet, it had to be a Leslie in order to get vibrato and tremolo.

Having worked as a dealer tech at several times for several Gulbransen dealers, I can honestly state that these instruments sounded really nice; it was always a joy to demo one of these Gulbransen instruments for the owner after completing the service call.

Somewhere during this same period, Leslie realized that running the rotary elements very slowly provided a different but equally useful effect. This was done by piggybacking a separate small motor on each of the main rotor drive motors. It was arranged so that when the small motor was not running, its shaft was held away from a rubber tired wheel on the main motor. If the user wanted the slow speed, the entire rotor (and shaft) in the small motors would move lengthwise (pulled in by the small motor's magnetic field) and would then engage with the surface of the rubber tired wheel, thus driving the main motor at a much slower speed which resulted in the rotors turning around or perhaps a little under 60 RPM or one revolution per second.

This effect they called chorale, and it provided a slow moving effect very similar to the random effect that occurs in real pipe organs and other instruments where absolute tuning perfection is not possible and where each generated tone has natural harmonics. On the next page, we have several pictures of an actual Leslie tone cabinet where you can see what it looks like inside, and also see the unique dual motor arrangement.

 

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