Leslie 122 amp - doubling the power?

[b3groover]

Thanks, Nick! It's always good to meet another Hammond player/nut! As far as the trio, we're working on our sophomore record as well as another record backing a tenor man. Should be fun!

Scott, I've been throwing around the idea of asking you to design a syncrotor that would be compatable with single-speed Leslies, but I realize that's a lot of work.

I've talked to the guy that runs this small company: http://www.rtclogic.com/ Really cool guy and his boards look nice. I want to try one. However, they don't incorporate stopping the Leslie at the same spot. That is absolutely crucial for recording. I don't even miss the chorus speed much on my 21H. I really just miss the rotors stopping in the same spot, since the 21H is in my studio. It's a total bummer to play a bitchin' solo and then have the rotors stop 180 degrees away from the mics!!!

 

[Scodiddly]

I'm happy enough to see somebody else going with the idea of the syncrotor in a modern design.

The one thing that's really important is that it be silent... my first experiments with magnetic reed switches were audible, so I had to figure out how to use Hall effect sensors instead, and go through the trouble of finding a supplier.

 

[PRR]

> stopping the Leslie at the same spot. That is absolutely crucial for recording. ...It's a total bummer to play a bitchin' solo and then have the rotors stop 180 degrees away from the mics!!!

That's tough. And somehow I thought the rotor was belt-drive? That means the motor can't be sure where the rotor is, because the belt slips.

Here is an 1880-style approach (no new-fangled microcontrollers).

Your main run/stop switch (not shown) applies motor power normally in Run. In Stop, it applies power through two switches S and K and a resistor.

There is a cam on the rotor with a bump under each horn mouth.

When not-Run, the motor mostly coasts. But when a horn mouth is at the desired stop-angle, switch S shorts the motor. A shorted motor slows down faster than a disconnected motor. So it tends to stop here. But we don't want instant-stop (or do we?). Inertia plus the resistor means the rotor tends to stop here, but at high speeds will just slow.

If it does not stop at the intended angle, it coasts until the cam-bump hits switch K. This applies a Kick of power to the motor, to ensure that it will always coast to the next stop point.

So it will coast, slow, kick, coast... and eventually stop on switch S, the brake switch.

The trick is to make both actions gentle (with the resistor and cam-bump length), so it will slow down fairly smoothly, yet give slow/kick forces larger than friction forces that might make it stop at a wrong angle.

If a rapid-stop is acceptable: wire the console Run/Stop to drop a stick in the rotor when the motor loses power. Said stick of course hits a horn or rotor-notch at the right angle.

 

[PRR]

Ah, even simpler.

Keep the cam-bumps. Lose the switches.

Rig a felt pad on a leaf spring so it drags on the bumps, but not between bumps.

Rotor really wants to stop on a bump.

To avoid drag in Run position, rig a solenoid to pull-off the pad so it clears the bumps. Wire it across the motor.

 

[Scodiddly]

Perhaps I should explain what the "syncrotor" does. I'm certain somebody here could do a better design than my old CMOS-logic box.

A Leslie rotor (spinning horn, spinning bass drum) has two motors, a "fast" motor that's coupled to the rotor via a belt, and a "slow" motor which is coupled (when on, sort of like an automotive starter motor) to the fast motor shaft via a rubber-edged disk. The usual Leslie control has two positions, fast and slow. The older models only had a fast motor, so the control had fast and stopped positions.

A few years ago a friend who plays jazz organ wanted to have a "stop" position on his Leslie so he could sound like some of the really early jazz organ records. So I ended up building a little solid-state relay circuit to switch the motors on and off, with a three-position control switch. But of course as a sound guy I was bothered by the fact that in the stopped position the horn might end up pointing in the opposite direction as a microphone - bear in mind that while a Leslie has two horn-shaped things on top, only one of them produces sound - the other is for mechanical balance.

So eventually I had a box which cleverly used the slow motor to control the stopped position - I'd discovered that the rotor always ended up stopped in the same place, if it was the slow motor that was running. So, I had (on each rotor) a Hall effect sensor and a magnet, and triacs to control the fast and slow motors. If the control switch went from "slow" to "stop", then as soon as the sensor was triggered the slow motor would be shut off. If the control switch went from "fast" to "stop", the fast motor was switched off, the slow motor switched on, and a timer started. The time interval was set (with a knob) to give enough time for the slow motor to drag the rotor down to the regular slow speed, at which time the "slow to stopped" logic went into effect. This was all done with the classic CMOS logic chips contain AND gates, OR gates, buffers, etc., and a lot of thought went into reducing the number of chips as much as possible.

I ended up building somewhere around five of these boxes, wire wrap circuit board and all. These days it would be easy enough to do the design with a microcontroller and add a fair amount of functionality (MIDI would be really cool) while reducing the size. But I'm not really that interested in doing it, especially if there's somebody who's worked with microcontrollers before and won't need to learn it all from scratch. It would even be possible to elimate the slow motor (the part that wears out most often) by having better control of the fast motor (which is just about indestructible).

As I mentioned in an earlier post, the whole thing would have to be very quiet to be usable. The early Leslies with no slow motor had an optional brake for the fast motor, which either shorted out the motor or sent in DC to slow it down in a reasonable amount of time. Apparently it was rather noisy to use. All the Leslies had a regular relay to switch speeds, and you can actually hear it on some recordings.

 

[b3groover]

Thanks, Scott. Your design is really useful. I showed it to Joey DeFrancesco when he played my rig and he was blown away. If someone could design one to work, like you said, just with the fast motors, providing all three "speeds", and have it made with microprocessors, I think they might have something.

I emailed the guy at RTC about the idea.