Leslie 302C Information Manual

The captain's model guide

About this guide

This is a quick guide to Leslie speaker models suited for Hammond organs. This document is meant as a
reference for when you are browsing the classifieds or eBay and is not meant to list every little detail
about the speakers. Some models are covered elsewhere on this website, often in detail and with
pictures.

About the background

The background is Leslie finish 'Oak No 20'.

About the roto-sonics

Since most players and technicians, including myself, find the roto-sonic Leslie models less valuable,
these are listed in a separate section. Some of these speaker models may be used as is, or they can be
converted into more Hammond friendly models or simply serve as parts donors for other more valuable
Leslie models. Electro Music tended to use the same parts in as many applications as possible, so a
good many parts from, say, a model 710 may be used to restore a model 122. If you happen to prefer
the more theatre organ-like sound of the roto-sonic Leslie speaker, by all means - you are just lucky to
like the more inexpensive alternative.

About the speakers still in production by Hammond-Suzuki

Models 122, 147 and 770 are still in production as model 122A/122XB, 147A and 771/971 respectively.
They are updated in the technical design, but the basic functionality is the same as the vintage
speakers. For a while model 145 and 142 were being produced as 145A and 142A. A vintage speaker in
immaculate condition may fetch a higher price than a new Leslie, especially for the tube models. If you
are a church or a professional business, consider investing in a new Leslie from a local dealer that will
support you. Old cabinets often don't come with a warranty or a service organisation, and Mr. <insert
name here> from three towns over will most likely not help you with a replacement Leslie for Sunday
service, because the 1971 Leslie he sold you broke down at band practice Saturday afternoon. If you
are looking for a dealer of new Leslie speakers, a few of them are listed in the links section of this
website.

Leslie model guide

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Hammond Model Model 302C

Model 302C

Amplifier Type 2 channel Solid-State
Output Power 100 watts
Drum 0 (Simulated)
Horn 1
Speeds 3

The 302C is a Hammond Suzuki Leslie utilizing
a real rotating horn and simulated bass rotor.
It also is a 2 channel cabinet with both a
rotary and stationary channel.

It features both an 11 pin interface (for XB
series instruments) and a 1/4" input jacks for
use with other instruments.

It has an internal preamp so only a "standard"
foot switch is needed to switch speeds and
there are no cumbersome Leslie cables.

This site and it's content, unless otherwise noted, are the property of Speakeasy Vintage Music, Inc. All Rights
Reserved. Use of this site is governed by our general disclaimer.

Maryland Organ Service is a Service Mark of Speakeasy Vintage Music, Inc.

Hammond™; and Leslie™; are registered trademarks of Hammond Suzuki USA, Inc.

Model 302C

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UNEARTHING THE MYSTERIES OF THE LESLIE

CABINET

by Clifford A. Henricksen

Community Light & Sound

First published in Recording Engineer/Producer magazine, April 1981.

The Leslie Rotating Tremolo Speaker System has produced the same unmistakable sound for so

many musicians that it has become an international institution for the recording and performing musical arts.
Everything from the venerable Hammond B-3 organ, to human voices and electric guitars, have been processed
by the Leslie Sound, and have ended up on countless hit records. This article is an attempt to unearth the
mysteries of its operating principles, and includes a discussion of the Leslie Cabinet's practical use in both stage
and studio situations.

My love-hate relationship with Leslie Speaker began with the purchase of a Hammond organ during the
mid-Sixties. The salesman told me I would probably end up buying a Leslie for it, a statement that I dismissed as
the usual sales banter. Besides, at the time I had no idea what a Leslie was anyway. The Hammond was
purchased because that's what the Animals' organist, Alan Price, used on all the group's early hits, especially
"The House of the Rising Sun." I saw the band live in Schenectady, New Youk, where they put a Hammond
M-102 direct into a Fender Bandmaster Amp, and I had to have this sound. I soon discovered, however, that my
new musical heroes had better sound. It wasn't long before I knew that Booker T. Jones, Felix Cavaliere, Gary
Brooker and Billy Preston (and many others) were all using Leslies. This was The Sound!

My first Leslie, a single-speed Model 45 with a 40-watt tube amplifier sounded great, but it literally got lost in the

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roar of the Boston-based rock and roll band in which I was then playing. Two big Leslie Speakers were
cumbersome to move, they weren't a big improvement in level (plus 3 dB), and the high-frequency drivers broke.
From then on, it was guerilla speed shop tactics (mill the heads, oversized pistons, etc.), to the point where I
could finally be heard over any guitar player. As a result, I became an expert at Leslie repair, modification and
special use, occasionally even letting a few guitar notes into my beloved machines. I've spent the past 15 years
in and out of the Music Business, both in studios and on stage. The significant part of this time was spent in
some way dealing with Leslie Speaker Systems. As you might guess, this is a rock and roll story. I might as well
show my bias here and now. With this in mind, here's what I have learned over the years.

PRINCIPLES OF OPERATION

First and foremost, the Leslie Rotating Speaker is designed as a sound modification device. It is not a "Hi-fi"
speaker, but rather a part of a musical instrument. You buy a Leslie to change the sound of an instrument, not to
reproduce it.

The Leslie Speaker System, thoughtfully named after its inventor, Don Leslie, operates on a simple principle: a
directional sound source is rotated at constant (or variable) speed around a fixed pivot point.

At a listening point some distance from this whirling affair, three things happen. First, because the source is
directional, the intensity of the sound will be at a maximum when it points at the listener (or microphone), The
sound intensity will increase as the rotating source approaches dead center, and decrease as it rotates past this
point. The resultant effect is called amplitude modulation (AM), which is no big thing, and is a feature on any
guitar amp with a "vibrato" or "tremolo" feature. By moving closer to the rotating speaker, the inverse square law
will increase the modulation effect.

However, the big deal is the Leslie's ability to create frequency modulation (FM). As the source rotates toward
the listener, its relative velocity will increase the pitch of any tone it produces; as it rotates away, the pitch well be
lowered. This is exactly the same Doppler effect that causes a train whistle (or any other sound on the train,
such as grunting pigs, or shrieking passengers), to rise and then fall in pitch as the train approaches and then
passes.

Lastly, if you are listening in a room with any significant reverberation, a complete spatial modulation of the
sound will happen, as sound is "shot" all around and goes through multiple reflections.

If you want a real treat of all kinds of bizarre whirling horns and things, order copies of Don Leslie's original U.S.
Patents: RE#23,323 and 2,622,693, available from The United States Patent Office, Washington, D.C.20231, for
a fee of 50 cents per copy. These patent specifications are incredibly entertaining documents, and a must for
any Leslie fan.

The Real-Life Leslie

A practical and commercial manifestation of the Leslie principle may take on many forms. Indeed, the
manufacturer of Leslie Speakers, Electro Music, Inc. (now a division of Hammond Organ), produces a wide
variety. These include models with reverberation, triple channels, and rotating-cone speakers. However, any
Leslie afficionado will tell you that the Leslie Models 145, 147, and 122 are the ones with "The Sound". All three
Models share the same basic innards: a 40-watt monophonic tube amplifier; an 800 Hz 16-ohm passive
crossover; a rotating treble horn and a rotating bass speaker. Both rotating speakers are available with slow and
fast A.C. induction motors. (Older Models 45, 47, and 22 are identical, except for having single-speed rotors.)

Furthermore, these Leslie models are similar in that they all have components mounted in a three-compartment
cabinet. The top compartment houses the rotating high-frequency horn; the middle "box" the high- and
low-frequency drivers and crossover (and which also acts as a vented box for the low-frequency driver); and the
bottom compartment the low-frequency rotor and amplifier. Louvres located in the top and bottom compartments
let out treble and bass sound respectively. All the above speaker systems are virtually identical in terms of their
use and sound quality. (The larger Models 147 and 122 allegedly have a better low-end.) Both high- and
low-frequency speakers operate on the same principle: a stationary driver (loudspeaker) and a rotating acoustic

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"projector".

The "Treble Rotor"

The Leslie's high-frequency unit is largely responsible for the "Leslie Sound". Some organists actually find that
the bass rotor's slower response to speed changes is distracting, and will actually disconnect the bass rotor drive

- especially when playing a bass line. The treble unit consists of a stationary 3/4-inch throat Jensen compression
driver, connected to a vertical tube that acts as a thrust bearing (Figure 1). A twin-bell, molded black bakelite
horn, which starts vertically but flares horizontally, sits on this bearing/tube, and rotates via a two-speed A.C.
induction motor fitted with three (selectable) drive pulleys. This motor drives the treble horn at fast or slow
speeds via drive belt and belt tension spring. Direct current is sometimes applied to the "fast" motor so that it will
slow down more quickly; this is called a "D.C. brake". The treble horn actually looks like two horn assemblies; in
fact, only one is operable as a horn. The other side, a "dummy", acts as a counterweight or dynamic balancer,
providing symmetric air drag at high speed. The resulting structure rotates smoothly and without eccentric
"wobble" forces.

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At the mouth of the horn is diffuser cone, which is supposed to widen the dispersion of the horn and make a
"more musically pleasing tone". Actually, it does work. Figures 2 and 3 show typical polar response charts of a
Leslie Horn with and without the deflector; the results are dramatic. The "without" curve of Figure 2 is typical of a
"beamy" straight horn, with a lot of sound concentrated on-axis and very little sound off-axis. The "with" polar
response curve of Figure 3 is almost omnidirectional; note, however, the "lobing". As the horn revolves, the
sound will actually rise and fall a number of times, giving it an even more characteristic sound.

The horn's so-called coverage angle is taken from polar response charts such as these; it is the included angle
between -6dB points from the horn's on-axis level. Figure 4 and 5 show such coverage angles, compiled from a
series of third-octave polar response curves. Note how the beamwidth gets smaller and smaller without the
deflector. This means that if the deflectors are removed the sound will be much hotter on-axis, but much softer
off-axis. Therefore, the AM portion of the Leslie Sound will be much more severe and "choppy", as many a rock
and roller will tell you. Many Leslies played in clubs are faced backwards, with the top and bottom rear panels
and deflectors removed, because the sound "cuts" better; an effect that translates as, "You can hear it better
over the guitar player".

This beaming process will actually raise the
on-axis sensitivity of any driver, as can be
seen from the frequency-response curves of a
University ID-40 driver (a respectable, but
typical 3/4-inch throat PA unit) mounted on a
Leslie horn with and without deflectors
(Figures 6 and 7). It is interesting to note the
extra sensitivity above 1.5 kHz available
without the reflectors in place. (Note also that
the low-end - 800 Hz to 1.5 kHz - response
isn't affected by removing the reflector, and
that the response is ± 5 dB from 400 Hz to 10
kHz. This latter range could be made even
flatter with a real-time analyzer and an
third-octave equalizer in the studio.)

Now, this entire discourse is based on the
treble horn measured in an anechoic or
non-reflecting, highly absorbent room. In real
life, the horn mounted in a louvred wooden
box, which means that the picture changes
somewhat with resulting internal reflections.
The diffuser cone does another important
thing: it shifts the apparent sound source
position on the horn. With the cone in, more
of the entire range of the horn will appear to
come from the mouth of the horn; with it out,
however, while lower frequencies still appear
to come from the mouth, higher and higher
frequencies will appear to come from
progressively further down the throat. They
therefore appear to be rotating at a smaller
radius, which results in less frequency

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