From obxwindsurf@yahoo.com Mon Jan 05 14:43:51 2004
Subject:Re: Leslie tone better than MS 145 - why?

While I can't speak for the MS 145, I have built a few of my own
Leslies in various designs including 2 based on 145 dimensions with
real Leslie parts (horns & bearings, motors, rotors)

The cabinet dimensions, slot openings, centerline displacement
between bass and treble rotors, crossover components, type of wood,
how the cabinet is constructed (i.e. how it is screwed and glued) all
make a huge difference in the sound once you get past hearing the
simple AM/FM component of the sound.

Looking at the internal as well as external design of the MS 145, the
rotors and cabinet openings are DRASTICALLY different than a stock or
home-built (after stock design) Leslie. I'm also guessing that the
construction is based on something other than multi-ply furniture-
grade plywood based on the rounded off edges, and smooth-lines of the
MS 145. In addition, from the looks of the Horn and Rotor design
from the brochure these are drastically different from a stock 145.

My first forays into Leslie building involved simple cardboard, balsa
wood, and foam rotors fired into with full range speakers. While
these faked a great Leslie sound, I didn't realize a "true" Leslie
sound until I added a simple single-pole (6 db/octave) passive
crossover. I also learned that styrofoam rotors don't sound
as "lively" as wood, and balsa wood, while very light doesn't have
the same resonance characteristics as 1/4" Luan plywood (similar to
what Leslies use) in the range of tones that the lower rotor must
reproduce. You also need the lower rotor to be similar in mass so
that the rotational inertia of accel/decel is different than the horn
which has much smaller mass and thus rotational inertia. The MS 145
rotor with its funky shape may be foam or some molded non-wood
material which will come into play both in resonance and rotational
inertia.

Next the crossover: I'm sure that MS uses a 2nd order passive
crossover or even a 2nd order active crossover and bi-amping. The
key in the crossover is that a 2nd order and not a 1st order is
used. The order refers to how steep the rolloff of frequencies occur
at the crossover point. I've used a 1st order (simple non-polarized
cap as a high-pass filter to the horn). While it is passable it does
not produce the same tonal characteristics of the horn and rotor as a
stock 145 does.

In my recent foray into Leslie building (documented at
http://www.vintagemusicprojects.com) I tried to reproduce as much of
the engineering in recreating the "stock 145" sound as possible with
a few exceptions. Rotor, horn, and motors are stock 145 components.
The crossover is an Eminence 800 Hz crossover which has a 2nd order
low-pass for the woofer and a 3rd order high-pass to the horn (which
is very steep, but the division of frequencies to upper and lower are
a near perfect *acoustical* creation of a stock 145 tone. I use a
modern vintage compression driver with a PHENOLIC dome which has a
similar frequency response curve to the stock Jensen driver. My
driver can handle a higher wattage and my crossover has dynamic
tweeter protection built into it which tends to compress the
amplitude to the horn with increasing amounts of power.

I'm also using a JBL 12" M-series woofer(I couldn't beat the price on
eBay) for the bass. This has a very similar tonal characteristic to
the stock Leslie woofer, although it lacks the bass response for the
deepest pedal tones. In past home-built Leslie projects I've also
used 15" high-wattage (ElectroVoice) woofers and the only perceptible
difference to my ears is the difference in pedal bass response.

I don't use a tube amp, but rather a 120 W RMS (bridged-mono) MOSFET
car stereo amp. Since the frequency response of this is a solid flat
20-20KHz and the B4 through this Leslie only uses at most half of
this bandwidth, the amp is very adequate at not introducing any wierd
nuances. It may lack the tube warmth, but the B4 emulation of this
is more than sufficient since it is digitally modeled, and I'm
confident that my amp chassis is reproducing every nuance of the B4
modeled organ tone.

The biggest differentiator between a stock Leslie and the MS however,
is IMHO, the design of the rotary horn. A stock Leslie has a very
simple but effective horn design - a simple conical flared-bell horn
(with matching horn for counterbalance) coupled with a conical
diffuser to reduce the amplitude modulated "beaming" associated with
non-diffused rotating horns. The Leslie horn also has a bit
of "fluff" which sits in the throat to smooth out the sound, I'm
guessing. I've built Leslies with and without the diffuser and
fluff, with the logic that my horn "would be louder" without the
diffuser. Au contre're!!. Volume wise there is no perceptible
difference, however the frequency-modulated component of the rotary
horn sound has an inverse relationship to frequency WITHOUT the
diffuser. This has the appearance of making the apparent origin of
the sound to come from different depths of the horn at different
frequencies (moving down the throat as frequency increases). This
results in high frequencies being more directional or "beamy" and
less FM than the frequencies nearer the 800 Hz crossover point.

The diffuser tends to even out the FM characteristic over the entire
range from the crossover point to the acoustical cutoff point of the
compression driver.

The Motion Sound horn (see diagram at
http://www.goffprof.com/images/pro3b.jpg) has an oval-
shaped "aerodynamic for silent operation" design with a "vacuum
port", and doesn't appear to have the diffuser that a stock Leslie
has. I also notice from this diagram that it is using a TITANIUM
driver rather than phenolic which tends to be "shrieky". I
experienced this with my first horn-based Leslie which used an EV
1823M midrange driver which I believe had a titanium dome as opposed
to phenolic. Titanium, due to its lighter weight has a lower mass
and thus can support higher frequency reproduction before acoustic
rolloff characteristic of phenolic domes. The horn is also asymmetric
vertically, I assume to possess a lower profile and fit into a
smaller space. Conical horns have a narrower dispersion pattern than
oval or horizontally elliptical horns do.

Since the most audible component of a rotary speaker design comes
from the horns the differences between stock Leslie and MS may be
significant enough to "color over" all the other considerations I
pointed out above.

I'm sorry this has been such a long-winded explanation but I've got a
lot of experience in this area to share and I figured it would shed
significant light on your question.

Best regards,
Kevin

--- In CloneWheel@yahoogroups.com, "Karl M" wrote:
>
> thanks - Karl