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About three months ago I decided
that I wanted the true "Leslie" sound. I had been using
the Native Instruments B4 software for the Hammond/Leslie sound
for the Church worship band, in which I play keyboard. But while
the Leslie simulator in the B4 software is pretty good, its still
not quite "there." Not being able to just go out and
buy a Leslie 122 or 147 (price tag too steep for my budget--800
to 1200 US$ depending on condition), I decided to build a Leslie
from parts that I discovered could be acquired on Ebay. First
I bought a horn and driver assembly from a Leslie 147, then an
upper motor unit.
 First I bought a horn and driver assembly
from a Leslie 147, then an upper motor unit. My first step was
to build a prototype - I wanted to make sure I could make this
work.
Shortly after I built the prototype,
I acquired a Hammond Model A organ, which fortunately had been
wired to present an unbalanced output to a 1/4" phone jack.
Although the organ's guts came from a Model A that was probably
built in 1935, it is installed in an A100 cabinet, from which
all the amps and speakers have been removed.
Not being able to afford a
genuine Leslie amp from a 122 or 147, I decided to use a component
I already had: an Alesis RA-100 stereo power amp. To make this
work, I needed an active crossover unit. I found a unit made
by Rolls--an SX21 Active Crossover, which is simple and compact--just
what the doctor ordered! I managed to pick up a used one for
about $50. To test the setup, I connected everything as shown
below: |
Note that the diagram shown
here DOES NOT include anything for the control of the horn rotation
speed. That's because at this point I hadn't come up with a solution
for that yet. Once this was connected, I started the organ, and
WOW! Even with no bass rotor, this setup blew the B4 software
away! The bizarre thing is that I thought I could hear the vibrato
/ tremolo / Doppler effect in the sound coming from the bass
cabinet as well as from the rotating horn. Interesting how the
brain puts in stuff you can't really hear. Speed switching was
accomplished by unplugging one motor and plugging in the other.
Not exactly well-suited for a performance situation, methinks.
More about speed switching later. |
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Note that the diagram shown
here DOES NOT include anything for the control of the horn rotation
speed. That's because at this point I hadn't come up with a solution
for that yet. Once this was connected, I started the organ, and
WOW! Even with no bass rotor, this setup blew the B4 software
away! The bizarre thing is that I thought I could hear the vibrato
/ tremolo / Doppler effect in the sound coming from the bass
cabinet as well as from the rotating horn. Interesting how the
brain puts in stuff you can't really hear. Speed switching was
accomplished by unplugging one motor and plugging in the other.
Not exactly well-suited for a performance situation, methinks.
More about speed switching later. |
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 For 3 or 4 weeks I pondered what sort
of cabinet I would put this stuff in. For a while the plan was
to build a cabinet myself, and I drew up a set of plans. The
cabinet was to be slightly smaller than a Leslie 122 or 147,
with an exterior finished in oak.
But after working out the cost
of this approach, and after noticing that empty Leslie cabinets
could be bought on Ebay, I started scouting around for something
like that. Requirements? It had to be adaptable to the 122/147
configuration, structurally sound, and the finish in good shape.
A cabinet with an upper shelf already in place would have been
nice, but alas, it was not to be! Eventually, I found the Leslie
55C cabinet pictured to the left in Southern California for $45
In case you're interested,
the cabinet DOES fit in the back of a 4-door Chevy S10 Blazer--in
fact you could put two of them in there if you were so inclined!
Driving 450 miles with that Leslie cabinet in the back was, shall
we say, "Interesting." Fortunately we had other reasons
for making the trip--getting the cabinet was an added bonus. |
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 This cabinet was indeed empty, but
since the Model 55C originally contained only a 15" woofer
and a rotosonic drum, there was no shelf for the upper rotor
horn, and no louvers at the top of the cabinet. For two weeks
after bringing the cabinet home, I mulled over the best way to
go about cutting the louvers for the upper horn, and did some
research online to find out how others had approached the problem.
It turns out that Leslie used some very special equipment to
cut those louvers, and that duplicating the Leslie louvers was
well beyond the capabilities of me and my equipment. I decided
that I would use my router to cut 1" wide slots with 1/2"
radius corners, spaced about 1-1/4" apart on the front and
sides of the cabinet. Next I needed to come up with a jig that
would guide my router in cutting these slots, and the result
was the routing jig shown below. This design allowed me to reconfigure
the jig so that I could cut the slots in both sides and the front
of the cabinet. This was the first action seen by my router in
several years. |
 For the side slots I was able to clamp
the jig to the cabinet to get some stability, but for the front
slots there was nothing to clamp to. When I put the jig in place
for the upper slot in front, I noticed that part of the jig rested
over where the lower slot was supposed to be. So I mounted the
jig directly to the cabinet with two wood screws, knowing that
the two screw hole would be routed away later. For the lower
slot, the jig was reconfigured to use the moulding across the
top front of the cabinet to stabilize the jig. |
 The other thing I discovered is that the 1/2"
straight router bit I started out with cuts out too much material
in a single pass. It took a long time to make the cut, and the
force required tended to bend the jig, resulting in an uneven
cut. For the front slots, which were done last, I started with
a 1/4" bit, then switched to a 1/2" bit for the second
pass. This way, the finish cut only removes 1/8" of material,
making for a much better and smoother cut. |
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 This picture shows the jig laid across
the front of the cabinet after the top slot has been cut. If
you look closely you can see the two screw holes in the cabinet
just below the slot. They are in just the right place so that
the other slot will make 'em disappear.
Also, I used slots that go
all the way across the front for consistency with the bottom
louvers, which also go all the way across the front of the cabinet. |
 The router is shown here in place to make the
second pass on the lower slot on the left side of the cabinet. |
 Once the slots had been cut, I installed
mounting strips and cut the shelf for the upper horn. The shelf
is a piece of 3/4" plywood, cut slightly smaller than the
interior dimensions of the cabinet, which was 26-3/8" by
18-7/16" The picture below shows the shelf in place, but
not permanently installed, since I still needed to make the cut-outs
for the horn and driver assembly and the two-speed motor stack,
and drill the mounting holes for these components. |
 The cutouts for the motor and horn
assembly were located using the prototype mounting board as a
template, with some modifications in the shape of the motor cutout
to make installation of the motor a little easier. Once the cut-outs
and holes had been made, it was time to permanently install the
shelf with glue and screws, and mount the components on the shelf.
The next step was to test the rotating horn & motor to make
sure everything was aligned properly. |
 In this picture the power cord for
the fast motor is plugged in, so you can barely see the rotor
as it spins. |
 This picture is a more detailed look
at the motor, belt, idler wheel and pulley that drives the upper
horn. The upper motor has a pulley with three grooves for the
belt--the speed of the upper horn depends on which of the three
grooves you putr the belt in. The idler wheel maintains tension
on the belt, and you can vary that tension by bending the arm
the idler wheel is mounted on. The amount of tension has an effect
on the rate at which the horn spped up when you switch from slow
to fast (that is, from chorale to tremolo). |
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Speed switching is done essentially
the same way it's done in any two-speed Leslie: a switch operated
by the organist applies voltage to the coil of a relay, which
switches AC line current between the fast and slow motors. I
needed the following components to make this work:
- A small single-pole, double-throw
(SPDT) with a 12-volt coil (Radio Shack, about $7)
- A 12-volt power supply (Radio
Shack, about $15)
- Two standard household AC
outlet plugs (Home Depot, 44 cents each)
- A box to mount the AC wall
sockets in (Home Depot, about $5)
- Wire (Lying around the workshop)
- A small metal box (About $7
at an electronics store)
- A Single Pole Single Throw
(SPST) switch--preferably one with quiet operation (about $3
at Radio Shack)
Here's how its wired up:
The wall sockets are wired
to the output terminals of the relay so that one of the two socket
pairs is energized, depending on whether the relay coil has a
voltage applied to it or not. The fast motors are plugged into
one pair of AC sockets, and the slow motors plugged into the
other pair. When the Fast/Slow Speed Control switch is closed,
one pair of motors is running, and when the switch is open, the
other pair of motors is running.
Unfortunately, as I mentioned
elsewhere, this setup is rather noisy. Flipping the switch generates
a *POP* in the speakers--apparently the removing of current from
the motors generates a spike in the AC line that generates the
noise in the Power Amp, even when the Power Amp gets its power
from the Power Conditioner.
After I posted this story in
the Clonewheel Group's Files section, I got a suggestion from
one of the members that some capacitors in the relay circuit
might suppress the noise. So I got hold of a schematic for the
Leslie 122 power amp, and sure enough, there were two .1 uF capacitors
connected between the common and output terminals of the relay,
like this:
With this modification, the
relay circuit generates a slight pop occasionally--the difference
is quite dramatic. Here's what the system looks like:
If you look carefully you can
see the relay hiiden behind some of the wires to the right of
the AC sockets that the motors plug into. The switch box in the
other picture has been replaced by another one with a quieter
switch in it, too. |
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 I don't have the lower rotor drum yet,
so the woofer is to be installed without a rotor for the time
being. The woofer came orignally from a Yamaha combo bass amp
of mid-80's vintage. This is an 8-ohm woofer slightly larger
than a standard 15-inch woofer--it measures 15-3/4 inches in
diameter, with a 15" diameter bolt-hole circle. This meant
I had to drill new mounting holes for it in the lower shelf.
The picture to the left shows what the cabinet looks like with
all of the components installed
After cleaning up all the sawdust
from the inside & outside of the cabinet, I wheeled it into
the living room and connected it up to the Hammond, the same
way as before. WOW again! Another pleasant surprise: this woofer
seems to go well with this cabinet, and the bass seems to be
deeper and richer than it was with the Peavey bass cabinet I
had tested with earlier.
Next, I connected up the speed-switching
circuitry discussed earlier, and now I can switch speeds from
the Hammond Console. The only problem is that the switching sometimes
(most of the time, but NOT always) generated a noticeable POP!
from the speakers--apparently switching off a motor's power causes
a transient that the power amp cannot filter out. So I connected
the power amp and crossover to a Furman power conditioner and
there was an improvement, but the POP is still there to a lesser
degree, but it was still not good enough. The last step was to
add capacitors in the relay circuit as discussed in the section
above on speed switching. |
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 At first I put the power conditioner,
power amp, and active crossover in the bottom of the cabinet
as shown at the right. But will that work once I get a lower
rotor installed?
If you look closely at this
picture you can see the felt and foam strips I added to the back
of the cabinet to seal the woofer chamber a little better. I
was planning to make a small box to go around the upper motor
as well to isolate the woofer chamber from the upper horn chamber,
but after hearing what the speaker system sounds like, I've decided
it's not something to worry about. |
Right now I have three components
left to install to complete the project:
- The bass rotor. I bought one
of these on Ebay, but it has not arrived yet.
- The two-speed motor for the
bass rotor. I have the motor, but it needs to have the low-speed
motor replaced, so I have ordered the replacement from Goff Professional.
- The speed control switch.
I have ordered a Leslie Chorale/Tremolo switch from Goff Professional.
When it arrives, I will install it on the Model A front rail
and connect it to the Leslie cabinet
Here's what the inside of the
cabinet looks like now with the bass rotor installed. The one
I bought from Ebay arrived, and it took a little more work to
get it installed. The basic problem was that the 55C cabinet
was designed with a Rotosonic Drum below the woofer instead of
a bass rotor of the type found in Leslie models 122 and 147 (and
many others). The space below the woofer shelf is 4 to 5 inches
taller that in a 122 or 147 cabinet, so I had to built a little
stand for the lower bearing for the rotor spindle.
The motor for the bass rotor
has a brand-new slow motor too--I bought that from Goff
Professional in Connecticutt. I also mounted a really nice
power strip from the Home Depot on the floor of the cabinet,
and put the power amp, crosoover, and power conditioner into
a 4-space rack case, just as I promised. Here it is:
The organ and the power conditioner
are plugged into the power strip in the bottom of the cabinet,
so now the power strip's on/off switch becomes the power switch
for everything. Nice!
Here's two more pictures of
the interior of the cabinet:
By chance I found a reference
to a company in Columbia California that makes very compact audio
amplifiers, very much like the ones that are made for car stereo
applications, but AC-powered. You can put two of these little
amps side-by-side in a single rack space, and they develop 50
watts per channel into 8 ohms. I picked up one of these on Ebay
for about $95 used, and it now powers the Leslie. The next three
pictures show the Stewart amp in place, Velcro'd to the bottom
of the cabinet, with the Rolls crossover Velcro'd on top of the
amp:
In the last picture I pulled
the amp out so you can see its front panel, which has input levels
controls, a power switch, and channel overload lights. This amp
is quite impressive--the sound is great, and its in a package
that's 1-3/4" by 8-1/2" by 10-1/2" and weighs
5.5 pounds. The inputs are 1/4" phone plug jacks, and the
speaker output is a small terminal strip.
The last step was to modify
the original back panel so that the back of the upper horn chamber
is left open. The picture below shows the back of the completed
cabinet, with the original back cut into two pieces horizontally
and installed to the back of the cabinet separately:
The next picture shows the
bottom rear panel with jacks installed to connect the Leslie
Tremolo/Chorale switch (on the left) and the audio signal from
the organ (on the right):
And finally, another shot of
the completed Leslie from the front:
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