Leslie simulations - again

[Mark Hammer]

I was sure I posted this the other day, and again today, but I can't seem to find a trace of it here, so here goes again.

I'm about to acquire a small Leslie speaker from a local guy (actually RE-acquire; I had one just like it but sold it in 1980 and I really miss it), and it got me to thinking about simulating rotating speakers.

Authentic simulations require one to consider the quirks of the device being simulated and going beyond merely the first thing you can find that sounds sort of like it.  In the case of Leslie speakers, our attention is drawn most to the notches created by the Doppler effect and to the cyclical nature of it.  So, some folks found an effective simulation in LFO powered bandpass filters (the PAiA Synthespin), then phasers (P90), and finally flangers near the upper end of their delay range.

The thing of it is, though, that although a spinning spekaer or speaker baffle produces swirly notches, that's not ALL it does.  As it points away from you, it also produces a slight modulation of both timbre (less treble when not facing you) and amplitude (louder when pointed at you).  So, what I'm thinking of is whether any of the existing rotating speaker sims factor in the "throb" of a real rotating speaker.

In principle, one could easily whip up a "throb simulator" module that could tack on to a basic design the way that RG Keen's LERA (at www.geofex.com) mimics the ramp-up/down time of rotor motors.  A couple of optoisolators could easily be used to gently modulate level and treble cut, and the LED half of the optoisolator can be yoked to the on-board LFO.  The nice thing is that since the notches themselves are generated internal to the flanger, the throb generator can be tacked onto the output without having to monkey around too much on the internals.

The thing I am having a difficult time wrapping my head around is the relationship of LFO polarity to throb. What *should* happen is that at the same time:
- the volume takes a little dip
- the treble is rolled back a bit
- the delay time is at maximum in its sweep (not maximum delay possible from the device)

Bear in mind that both treble trimming and notches will already create the impression of slightly reduced amplitude, so whatever amplitude modulation is imposed on top of that will have to be subtle.  Still, my guess is that you'd need to top off what the treble trim and notches do with a modest volume reduction.

Make sense?

Incidentally, it is the moving notch and treble trim that make the phasefilter mod on phasers so interesting.  I've only ever heard it on 4-stagers, and since I know from Mike Irwin's demonstrations that 12-stage phasers sound a lot more flanger-like, I'm wondering whether a 12-stager with the last 2 stages converted to lowpass (5 notches and a 2-pole lowpass filter) might do a passable Leslie simulation.

[bwanasonic]

My low-rent approach to this is to use both phase and tremelo, *synced* by ear (or eye). I use a DOD 201 and Fulltone Supa-Trem. The Fulltone has a flashing rate LED and I added this feature to my DOD 201. The throb added by the trem makes for a nice effect. The other thing to keep in mind is that, as far as I know, Leslie cabs don't have very much high frequency response. I usually roll off a bit of treble.

Kerry M

[analogguru]

Hi Mark,

I think that i must not tell you something about the Rolls RFX-147

Munky has the schematics for Dynacord CLS-222

http://www.geocities.com/munkydiy/schematics.html

and here you can find the Korg CX-3 Leslie part

http://home.germany.net/100-235778/richy2001/circuit/s3.gif

If you have problems with specific values I have an original copy and i can tell you from that. Maybe this gives you some inspiration.
I think its not necessary to explain to you the function of the Korg CX-3, but if you have any questions feel free to ask

analogguru@hotmail.com

[gez]

Funny you should post this Mark as I was working on something similar a few weeks ago (still am, but I’m taking a breather - read I’m sick of the sight of it!).  From what I’ve read the Leslie's throb is frequency dependant, i.e. lower notes receive the lion’s share of amplitude modulation.  Conversely, the doppler  effect is more prominent at the higher end of things.  

So far I’ve got pretty reasonable results following a phase-splitter vibrato input stage with a frequency selective tremolo.  I used a p-channel MOSFET inserted between the input resistor and -ve input of a inverting op-amp set up for just above unity gain.  The MOSFET’s source connects to the inverting input and the drain to the input resistor.  There’s DC bias on the FET so that the channel is fully conducting and the LFO pulls down on the gate thus attenuating the signal.  By placing a cap from drain to source, amplitude modulation is frequency selective with the lower notes getting the full throb.  I use a 5V regulator for the op-amp and MOSFET bias so that the circuit remains unaffected by supply variations.

The whole effect is quite subtle and definitely gives a more Leslie type vibe (pun shamefully intended) to things.

[Mark Hammer]

Thanks for the feedback and links, guys.

I guess it is worth noting that mimicking a Leslie is not one single thing.  Though the sound of a rotating speaker is clearly distinctive, there is more than one rotating speaker design.  More specifically, there are single and multi-rotor versions.  The one I'm about to acquire is a single rotor version with just one measly 8" speaker and a styrofoam "cheese-wheel" rotating baffle.

Of course, whether you have a single full bandwidth rotor or band-splitting  or multiple rotors, the fact of the matter is that low frequencies, by their nature are less directional, and unless one had some means of using a flawlessly sealed cabinet that restricted audible speaker output to ONLY what came out of the rotor/baffle, chances are that it IS, as noted, mostly the mids and highs being thrown around the room while the lows are more or less omnipresent and omnidirectional, like a subwoofer signal.  So, because the lows aren't efficiently being thrown around by a rotor, there is less doppler-related cancellation in that part of the spectrum.  Indeed it may be the shifting phase/time relationship between fundamentals and harmonics that makes rotating speakers sound as interesting as they do to our ears.  It's "semi-patterns" that draw our attention (read Bernie Hutchins article on "fat sounds" in JAES, 1981 for a further articulation).

Of course, what this suggests is that some sort of gentle frequency-shaping of the "straight" signal might make flangers or chorusses more interesting.  One idea that springs to mind is a simple shallow lowpass/highpass network, not unlike the famous BMP tone control.  The highpass gets mixed in with the time-delayed signal at one mixing stage, and then all of that gets mixed in with the lowpass signal at a second mixing stage.

I will have to look at the Rolls circuit again, and spend a couple of hours with the Dynacord one as well.

[Ed Rembold]

Mark,
When you study the Rolls circut,  I think it's a lesson in "How to Not do it".
As that box was a "miss" not a "hit".
Which bothers me a little,  because it's one of the few Leslie Sim  circuts I can understand.
Ed R.

[Mark Hammer]

Well, you're right about the Rolls unit being easy to follow.  I took a brief peek at it and was just about to post some comments, when I saw your note.

Unlike the Korg and Dynacord units linked to at Munky's site, the Rolls does not employ separate time-delay for the low end.  Rather, it uses what amounts to a 2-stage Small Stone circuit (2-stage OTA-based allpass) to modulate the bass.  Of course time and phase are related but not the same thing, and the same absolute time difference results in a much greater phase difference the higher up you go in frequency.

Other than simply not "nailing" it, what is it about the Rolls you don't like?  I'd be curious for opinions on analog simulators that DO nail it well.

[Doug H]

I have heard good reports about the H&K tube unit. I think Steve Aloha has some sound clips of it. But I'm not real familiar with these things.

Doug

[Ed Rembold]

Boy I hate the "Invalid session" thing!!!!
Type for 20 minutes and it's gone!
What a waste of time- I gotta go cool off.

Aron- If you see this message- see if you can do something about that problem........

Ed R.

[Gringo]

I copy the text i just wrote before submitting anything. Keeps my blood preasure at normal levels

[Peter Snowberg]

Boy I hate the "Invalid session" thing!!!!
Type for 20 minutes and it's gone!
What a waste of time- I gotta go cool off.

Aron- If you see this message- see if you can do something about that problem........

Ed R.

Unfortunately there isn't much Aron can do about that one. It's an inherent problem with the phpBB design. If you run your own server, you can adjust a value that helps out greatly, but running on a commercial host leaves you out in the cold.

There is a work-around , OK actually two of them.

(1) The easiest way is to make your reply in a word processor, and then paste it into the post box. Don't open the posting form until you are really ready to paste.

(2) Every 5 minutes or so, press the preview button. That will refresh the session.

I know it's a pain, but eliminating it from the system would take some major work unfortunately.

Take care,
-Peter

[Paul Perry (Frostwave)]

I think a major problem with trying to recreate a 'rotating speaker' sound is that when one is actually in front of a rotating speaer, the sound reaching each ear is coing from a slightly different path. So I would be thining that you really need to make TWO circuits. As a test, see whether you are happy listening to a recording of a leslie in mono, as opposed to stereo.

[Nasse]

I wonder what methods do those programmers of soft synths/virtual instruments and VST plugins use in some "Leslie" or whatever rotary simulations...some of them sound very convinsing...maybe you could turn them back to analog...you could call it analog copy of digital copy of analog.

I just fixed my self-made mixer in working order (with duct tape), I have left-right difference amp in it, maybe I should listen what the stereo part of the rotary sim sounds have in it, when the juice is extracted...

[Davefx]

I agree Paul.  The compression from that OC20 rectifier, and the boxy acoustical properties of that wretched old cabinet, and listening right in front of it, gave you that sweet quality.  When you get all of those things down, then you have a leslie sim!!! I miss my old 145. I was the only guitar player in NW Ohio that would haul one around to gigs!  The first gig I used it, my bandmates were scratching their heads.  They thought putting a plant on it for decoration was a good idea, until I played "It Don't Come Easy", then they changed their tune.  

Ahh..  the memories. :cry:

[gez]

There's plenty of stuff out there on the Leslie, but here's a nice overview - should get some of you scratching your heads!

http://theatreorgans.com/hammond/faq/mystery/mystery.html

[Davefx]

Oh, and then there's that oil hole!!! Gotta be apart of the sound!!!  :lol:  :lol:  :lol:  sorry... :mrgreen:

[gez]

Oh, and then there's that oil hole!!! Gotta be apart of the sound!!!  :lol:  :lol:  :lol:  sorry... :mrgreen:

No, the oil hole doesn't affect the sound.  The oil you put in it mind...well, that's another story!

[analogguru]

Maybe you want to build the real thing:

http://mad.topnet.com.au/~hairbear/page.2e.htm

a friend of mine has an Allsound-Leslie (German) with independent Bass- and Treble-Rotor for sale, i don´t know the price yet, if there is interest, i can ask him.

analogguru@hotmail.com

[analogguru]

another one:

http://mad.topnet.com.au/~hairbear/page.2f.htm

analogguru

[Bluesgeetar]

Well I have an H&K Tube rotosphere and I think it sounds almost dead on for Leslie.  There are lots of stressfull things to think of with a real one.  Micing that muther ain't fun.  Read an article once where Gilmour said they 7 mics on a Leslie and dialed in the perfect sound on the mixer.  Just get the H&K!  No it is designed to try and sound like a leslie not a univibe or a glorified chorus phaser pedal.  Hint though, I tried 7 of them and picked the best from the 7.  That is truly the best way to shope for new pedals.  Just hit your local GC or other large retailer and line up all of their stock and give them a go until you find that magic one.  It sounds dead on for that "Badge" sound and damn damn close for "Something in the Way"  I will say it seems to like single coils for tone.