Rotating Speaker - DC Motor Control

Thread Starter

SparkySam

Joined Jan 26, 2021
8
Hi all,

First post here, so hopefully I don't make any stupid mistakes...

I'm looking to build a rotating speaker for my guitar rig. Below is an image of the unit I'd like to replicate:
Capture.PNG

It consists of 2 x 100w 6" speakers being rotated between 0-400rpm. I have done some experimenting with the motor control side of things and am struggling to drive the motors as quietly as I'd like. I have opted for a 18kHz PWM motor driver, in hope to reduce the audible PWM, which has worked quite effectively, however the the motor I'm using is quite noisy, which has lead me to wonder if I am utilising the best method of driving the rotation of these speakers? I was debating using a very small VFD with a small 3 phase motor, however, I suspected I would have issues with noise, being picked up by the guitar.

I play gigs in quite large theatres (when there aren't deadly virus's around) so whichever route I take, it needs to be the most reliable and quiet.

Hoping you can help me out on this?

Thanks very much,
Sam
 

Thread Starter

SparkySam

Joined Jan 26, 2021
8
Hello,

How do you power the speaker?
In a leslie, the speaker is fixed, but a drum will reflect the sound.
This page will give you an idea:
https://www.premierguitar.com/articles/25852-diy-build-your-own-rotary-speaker
More images in google:
leslie speaker

Bertus
I'm going to use a Mercotac slip ring. I was going to use a conventional slip ring, but wasn't so sure on the reliability.

Some of the Leslie type speakers also had a rotating speaker, such as the Yamaha RA rotating speakers.

That's a really good article, I hadn't seen that before. It appears they have used the same method as me. I have just ordered a different motor, without the gearbox on, I'm thinking that is where the majority of the noise may be coming from...
 

schmitt trigger

Joined Jul 12, 2010
438
The quietest (mechanical noise) motor type will be a three phase induction. Next will be the three phase BLDC.

Both of course, require a VFD drive unit.
 

MisterBill2

Joined Jan 23, 2018
8,740
Rotating the speakers certainly has been done, but a few years ago I came up with a much simpler scheme. I used a stereo amplifier with the two speakers a foot apart and at right angles to each other. Then the signal went to the rotor coil of a surplus sine/cosine resolver. So as I rotated the resolver the signal was bothamplitude and phase shifted between the two speakers. A veryinteresying effect and very easy to produce. And I could turn the resolver as fast or slowly as I wanted, or give it a spin and let it slow down over some time. The beauty is the rotating part is already made to rotate quietly and in perfect balance, and weighed much less than a pound. And at the time surplus resolvers cost less than a dollar. New resolvers cost a lot, but surplus ones intended for 400Hz power can be cheap. And they come in many different sizes.
 

MisterBill2

Joined Jan 23, 2018
8,740
If you can get a chance to look at a real "Leslie" system that will show one way to do it. BUT I think that rotating a reflector will be a lot simpler to keep quiet. And really, you do need a bearing above and below the rotating mass. And a flat rubber belt scheme like used on turntables will be the mechanically quietest reduction method. And use plastic sleeve bearings, they are much quieter than all bearings.
 

Thread Starter

SparkySam

Joined Jan 26, 2021
8
If you can get a chance to look at a real "Leslie" system that will show one way to do it. BUT I think that rotating a reflector will be a lot simpler to keep quiet. And really, you do need a bearing above and below the rotating mass. And a flat rubber belt scheme like used on turntables will be the mechanically quietest reduction method. And use plastic sleeve bearings, they are much quieter than all bearings.
The issue is, the two leslies I have only have two speeds, whereas the one I am trying to make needs infinitely variable speed.

Thanks for the tips on the bearings etc... I will look into those
 

MisterBill2

Joined Jan 23, 2018
8,740
I was referencing how the Leslie units were assembled and built, not the motor type. For variable speed drive in the slower ranges, it may be that a brush type of reasonable quality would be good, but a brushless DC motor may be physicaly quieter, but some of them produce electrical noise. So you may have an interesting time in that regard.
 
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