captaincoconut
Member
Intro
I'm mainly a Hammond organ player these days and recently bought a pristine looking and working Leslie 122 amp for my setup. For those not in the know, the Leslie 122 has a built in power amp using two 6550 power tubes. It's really only a power amp stage, because there usually was a tube preamp already built into any Hammond organ.
The Leslie 122 uses a 6 pin plug to connect to the organ using the following pinout:
1. Audio (balanced)
2. GND
3. Line voltage
4. Line voltage
5. B+
6. Audio (balanced)
Back in the day that was a very neat solution, because there was only one cable necessary to run both audio and line voltage (230V or 117V AC) from the organ to the Leslie. Switching between fast and slow speeds (tremolo and chorale) was done by superimposing a control voltage of more than 35V DC onto the balanced audio signal. That control voltage is picked up by one of the two 12AU7 tubes you can see above. If that tube starts conducting, a relay switches from one motor stack to the other motor stack. Due to the balanced nature of the audio signal, that control voltage cannot be heard through the speakers. Pretty neat actually!
Unfortunately, that plug makes it a pain in the ass to connect anything to the Leslie that isn't a vintage Hammond organ. As I'm playing a Crumar Mojo Classic that only has a standard 1/4" output I had to come up with my own solution.
Requirements
I spent some time at the drawing board and came up with the following requirements:
1. The box had to be pedalboard friendly.
2. I wanted a transformer-isolated audio input in order to allow balanced as well as unbalanced input (pretty much a built-in D.I. box). No preamp in the box itself, because I wanted to use an external preamp.
3. Create the control voltage for speed switching from line voltage.
4. Allow speed switching via a standard TRS-footswitch or MIDI.
Here's the switching and audio input circuit I came up with (there's a resistor missing in series with the zener diode that I later added). It's pretty simple, actually. The control voltage is superimposed onto the balanced audio lines using two closely matched 10k resistors. The two large 10uF capacitors protect the transformer secondary from any dc voltage. If relay K1 is closed, the control voltage of 48V is pulled to GND (Slow -> Fast) and vice versa. The second relay K2 switches a 12V zener diode in parallel to that control voltage, effectively limiting it to 12V. This is in preparation for the use of a Trek II EIS relay that you can build into the Leslie. A stock Leslie 122 only allows slow and fast speed. There is no stop mode on any Leslie 122 ever made. By using that Trek II aftermarket module, one can add a stop mode to a Leslie 122. The Trek II module senses the input control voltage (0V, 12V, >25V) to change between the three speeds (Fast, Stop, Slow). I haven't bought that unit yet, but wanted my Leslie interface to be prepared to send that voltage for the stop mode. That's why there's a second relay on the pcb.
Here's what the construction looked like in Fusion:
Results
I chose to use a 1590D size enclosure that I had drilled and painted by Tayda. The audio input is isolated by a high quality Lehle transformer.
The whole switching circuit is microcontroller based. A simple Arduino Nano workalike is more than enough for this. It handles the input from a 1/4" TRS input or a standard 5 pin MIDI DIN connector. The optocoupler for the MIDI circuit is hiding under the microcontroller. There's also a hex-switch allowing to change the MIDI channel, if necessary:
Here's the 6 pin Amphenol plug used to connect to the Leslie. I had to come up with a solution to mount this inside the enclosure because while it's easy to get the plug itself here in Europe, the metal flange used for enclosure mounting is only available from parts dealers in the United States. So what I did is to come up with an adapter using FR4 that I added to my latest PCB order from JLC. Works flawlessly and seems super sturdy, but only time will tell!
...and that's pretty much it! I just had to write some code for the Arduino and it fired right up! For the preamp itself I'm using the fantastic sounding Behringer MIC300. It adds a really nice growl (think Jimmy Smith, not Jon Lord) to the sound, is very well built and silent.
Cheers!
I'm mainly a Hammond organ player these days and recently bought a pristine looking and working Leslie 122 amp for my setup. For those not in the know, the Leslie 122 has a built in power amp using two 6550 power tubes. It's really only a power amp stage, because there usually was a tube preamp already built into any Hammond organ.
The Leslie 122 uses a 6 pin plug to connect to the organ using the following pinout:
1. Audio (balanced)
2. GND
3. Line voltage
4. Line voltage
5. B+
6. Audio (balanced)
Back in the day that was a very neat solution, because there was only one cable necessary to run both audio and line voltage (230V or 117V AC) from the organ to the Leslie. Switching between fast and slow speeds (tremolo and chorale) was done by superimposing a control voltage of more than 35V DC onto the balanced audio signal. That control voltage is picked up by one of the two 12AU7 tubes you can see above. If that tube starts conducting, a relay switches from one motor stack to the other motor stack. Due to the balanced nature of the audio signal, that control voltage cannot be heard through the speakers. Pretty neat actually!
Unfortunately, that plug makes it a pain in the ass to connect anything to the Leslie that isn't a vintage Hammond organ. As I'm playing a Crumar Mojo Classic that only has a standard 1/4" output I had to come up with my own solution.
Requirements
I spent some time at the drawing board and came up with the following requirements:
1. The box had to be pedalboard friendly.
2. I wanted a transformer-isolated audio input in order to allow balanced as well as unbalanced input (pretty much a built-in D.I. box). No preamp in the box itself, because I wanted to use an external preamp.
3. Create the control voltage for speed switching from line voltage.
4. Allow speed switching via a standard TRS-footswitch or MIDI.
Here's the switching and audio input circuit I came up with (there's a resistor missing in series with the zener diode that I later added). It's pretty simple, actually. The control voltage is superimposed onto the balanced audio lines using two closely matched 10k resistors. The two large 10uF capacitors protect the transformer secondary from any dc voltage. If relay K1 is closed, the control voltage of 48V is pulled to GND (Slow -> Fast) and vice versa. The second relay K2 switches a 12V zener diode in parallel to that control voltage, effectively limiting it to 12V. This is in preparation for the use of a Trek II EIS relay that you can build into the Leslie. A stock Leslie 122 only allows slow and fast speed. There is no stop mode on any Leslie 122 ever made. By using that Trek II aftermarket module, one can add a stop mode to a Leslie 122. The Trek II module senses the input control voltage (0V, 12V, >25V) to change between the three speeds (Fast, Stop, Slow). I haven't bought that unit yet, but wanted my Leslie interface to be prepared to send that voltage for the stop mode. That's why there's a second relay on the pcb.
Here's what the construction looked like in Fusion:
Results
I chose to use a 1590D size enclosure that I had drilled and painted by Tayda. The audio input is isolated by a high quality Lehle transformer.
The whole switching circuit is microcontroller based. A simple Arduino Nano workalike is more than enough for this. It handles the input from a 1/4" TRS input or a standard 5 pin MIDI DIN connector. The optocoupler for the MIDI circuit is hiding under the microcontroller. There's also a hex-switch allowing to change the MIDI channel, if necessary:
Here's the 6 pin Amphenol plug used to connect to the Leslie. I had to come up with a solution to mount this inside the enclosure because while it's easy to get the plug itself here in Europe, the metal flange used for enclosure mounting is only available from parts dealers in the United States. So what I did is to come up with an adapter using FR4 that I added to my latest PCB order from JLC. Works flawlessly and seems super sturdy, but only time will tell!
...and that's pretty much it! I just had to write some code for the Arduino and it fired right up! For the preamp itself I'm using the fantastic sounding Behringer MIC300. It adds a really nice growl (think Jimmy Smith, not Jon Lord) to the sound, is very well built and silent.
Cheers!
