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Conversion to Single-Channel Leslie

The following write-up was posted by MikeCasino to TheHammondForum. It describes modifications made to a Leslie710 to convert it for use with a (single-channel) Hammond.

Adding a Cross-Over

It might be a bit trying to find a electronic Leslie crossover, but there is an electronic crossover available from Trek that fits the bill with a little modification. This is the Trek II LC0-1 electronic crossover modeled around portions of BobSchleicher of ElectronicInstrumentService "UniversalLeslieAdapter". I put it this way only because the ULA has older type op-amps.

The original concept of the ULA and Trek II are to adapt two channel Leslies to single channel organs using that Leslies original configuration. Not a bad sound but for me since I like to use the Leslie to really accent my playing, the spin up and down time of the heavy RotosonicDrum took too long.

The basic differences between them are frequency cutoff points, amount of cutoff or (roll off), power supply requirements and master volume control placement. First lets take a look at frequency cutoff points. Keep in mind that the popular Leslie tube amps have a cutoff point at 800 Hz or there abouts.

The original 760 active Leslie crossovers actually have a roll-off of 12dB/oct - they are 2nd order Butterworth filters. The nominal crossover frequency gives approx. 859Hz. ((L.M.V.)(M.T.C.)corrected 10/30/07)

The drawing for the ULA electronic crossover cuts off at 150 HZ with a 12 dB per octave roll off and uses a double power source. since you build this your self, you can mount the master volume where you want, but you will also need to supply a plus and minus power supply for this one.

The Trek II electronic crossover also cut off at 150 Hz with a 12 dB per octave roll off and uses a single power source right from the Leslie. The master volume is located on the circuit board, but can be modified to mount elsewhere.

For a simple clarification the more dB per roll off, the faster those frequencies are not transferred to the place it should no t go.

I used the Trek II and modified it not only because of the good design but the ease of the modification. By replacing and adding only five components (resistors and capacitors) now I have a 800 Hz electronic crossover with 12 dB per octave roll-off. It uses the Leslie's power supply which is a less of a hassle. The instruction that are supplied with it are clear and easy.

The parts to alter the Trek II LC0-1 are:

1) 28K ohms 1/4 watt. mouser pt# 271-28K min order of 10 @ $.09 each

3) 14K ohms 1/4 watt. mouser pt# 271-14K same min order & price

2) .1 micro farad caps. mouser pt# 146-250V 1K about $.27 each

Replace R2, R3 & R6 with 14K ohm resistors.

Replace R7 with 28K ohm resistor.

Replace C3 with two .1uF caps in parallel.

This will give you a crossover of 803 Hz (close enough for government work).

The Lower Rotor

Now for the lower Rotor assembly: I found one on eBay and mounted it under the 15" speaker and then inverted the lower motor assembly. There are a couple of rotor assemblies that can be used for the 15" speaker. The preferred one is ten inches in height and the second choice is one inch shorter. The shorter rotor can be blocked up from underneath. Since the Leslie 710's lower motor spins in the same direction as the upper motor, and I wanted the same effects as the big boys, I inverted the motors, changed the hardware around on the slow motor and cut a channel in the sounding board for the belt. You may be thinking to yourself that the 710 already has a belt to drive the rotor. It does but it is shorter, so I replaced it along with the spindle so the lower rotor spins at the same speed as the big boys. Since I was a little lazy, I did not invert the sounding board to place the 15" speaker on the right hand side of the Leslie, so I had to mount the power supply on the right hand side and shim out the amplifier plate 1/4 of an inch for safer clearance.

The results are awesome. I would take this unit over any 760 / 770 any day!

Hook up information can be viewed here: http://www.dairiki.org/hammond/pics/trek_LCO-1_hookup.png


TopicModification

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