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.

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.

 

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.

 

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.

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).

 

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.

 

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.

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:

 

All Pictures and Text
(c) Copyright 2003 by Stephen Cyr
Last updated August 21, 2003
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