Saturday, January 10, 2015

Organ Donor

Everybody used to have an organ in their house.  It was kind of ridiculous.  I've written about this before.  As those things started to land in garage sales and thrift shops, though, people figured out some fun things to do with them.  They've got keyboards, tube amps, foot pedals, and bunches of other goodies just waiting to be re-purposed for cooler musical projects.

About ten years ago one such organ fell into my lap.  A move out of state meant I had to get rid of it, but not before I removed a rotary speaker cabinet out of it.  Back before the reign of the transistor, it was easier to add effects to musical instruments with motors and baffles than with extra gain stages, and practical real time digital signal processing was beyond everyone's wildest dreams.

This Leslie speaker has a 6-8" speaker behind a large styrofoam baffle that spins in front of it, changing the direction of reflection.  It adds a tremolo (amplitude modulation) and a bit of vibrato (pitch modulation).  I use a light switch to turn on the single phase motor which spins the baffle.  I'm a bit embarrassed, but I was done with engineering school before I learned about using diodes or RC filters to suppress (snub) switching transients on motors and solenoid coils.  I've added a little board with an RC snubber to the speaker to eliminate that loud electromagnetic interference pops that occur when I switch off the motor otherwise.  Here's the little board that I made up for the snubber:

It's a series resistor (240 ohm) and capacitor (0.05 microfarad) in parallel with the motor to give the transients a path to ground after the switch is opened.  A schematic may be overkill, but here it is:

Here's a little video to demonstrate how the speaker sounds:

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