Leslie 760 to Hammond XK-3 Interface

Here is my interface to allow a Hammond XK-3 to be connected to a Leslie 760.

It is not as straightforward as I had expected. The first problem was to work out how the XK-3 Leslie output worked. Unlike the older Hammond organs, little was available on the Internet and the XK-3 manual was little help so I was obliged to work it out for myself using a multimeter. Firstly, the XK-3 relay contacts are designed to switch positive voltages whereas the 760 Leslie and its near cousins use negative voltages. Hammond fitted diodes across the relay contacts of the XK-3, had they omitted them then polarity would not have mattered but as they did, a conversion circuit is needed. I chose to use miniature 24v relays. Secondly, the XK-3 has a Leslie detection circuit, operating at 24 v. When a Leslie is detected, the audio output is not subjected to the internal Leslie simulation. If a Leslie is not detected, the simulation is applied. My initial design did not recognise this fact, consequently the real Leslie was fed with a simulated Leslie signal. The result was not ideal. Connecting the Leslie 760 28 v output to the Leslie detect input of the XK-3 solves the problem. What's more, by switching this line as shown in the diagram, further flexibility is achieved. With the switch open, the Leslie stops spinning and the simulation signal is output.



The circuit board is very simple. I used 5-pin XLR connectors with 6-core cable intended for fire alarm installation. These provide a cheap and reliable connection and the plugs don't fall out - unlike the traditional Leslie connectors.There do not appear to be any noise problems or any other noticeable changes caused by the cable swap  My Leslie has been modified with a separate power cord, something which is necessary in this application and, in my view, desirable for all applications as electrical safety is much improved.


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