Rotosphere Circuit Analysis

[liquids]

So, I'm looking at the H&K - Hughes & Kettner - Rotosphere schematic(s). 

In the end I'm looking to mostly simplify the circuit a bit, or streamline it for my own uses.   Hopefully these links work, but these are the two sections I understand the least.  The schematics came from 'the other' website, but I uploaded them to the gallery here;  hope that is acceptable to everyone.

http://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=48627&g2_GALLERYSID=3bef53a3853c15ba5cc071f2421f1d70

http://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=48630&g2_GALLERYSID=3bef53a3853c15ba5cc071f2421f1d70

I find the schematics hard to follow.  I find the oscillators odd.  I also don't quite get BBDs.   So, for starters, if anyone can take a stab at anything they 'get'...well, I'd sure appreciate it, since this isn't your average chorus.

One thing I think I notice - from a bit of schematic analysis (marking up the schematic), and in accordance with an online demo - the 'rotor' side of the circuit doesn't seem to get modulated...it has an LFO, but the sound of the 'low frequencies' is either simply being subtly AMPLITUDE modulated (tremolo), and/or phasered?   
Following, it seems like the HPF/horn frequencies are being chorused 2x over?  Seems like the "L out" has dry + 'high pass filtered & modulated signal 1' + just a touch of 'high pass filtered & modulated signal 2'  and "R out" has the opposite, but high/horn frequencies are effectively modulated 2x over, no? 

I'm not interested in the tube/drive stages, personally.   I could always use a fet preamp for that anyhow, personally...

One note - this is the MKI schematic, I believe...the unit is 'stereo' or, at least, dual input & output....however, it does say in the manual, while if used 'mono' you use the R input, for a more dramatic leslie effect, use the R input, but use the L output....wish I had saved the manual since I thought it would be easy to find again, but now I can only find the MKII manual.

[liquids]

Okay....

Well, two things I noticed, after I spent quite a few hours simulating a good part of the LFO section...with quite a bit of troubleshooting, not unlike a complex circuit on the breadboard....

The main LFO creates an 'even' triangle wave with softened, rounded edges, where you would 'probe' G3R, G3L, G2L, G2R.   I was a little concerned about needing to duplicate the entire LFO circuit, which I just assumed was overly complex in a cost/benefit ratio, for my own purposes and desires, which are simlification.   Note that I am looking for a simplification because, for one, I have a real leslie horn cabinet with a crossover and everything, and can power it to get the real thing (!) as I have posted elsewhere about.  And I love it...but, due to some life stuff, traveling, debating the effort needed to finagle a setup that utilizes that cabinet to the fullest or all the time, etc., and since I sometimes need to use headphones, I want to see if I can get 'close' but via a different approach than most take.  That is, more 'complex' in topology than just a chorus pedal set fast, or a vibrato pedal, but not as (over-)complicated as the rotosphere pedal.

All the circuitry involved, and the 'critical' part is right at the end, before the LFO tap(s) feed the BBD clock - where you see the LFO feed what I now believe to be a wave-shaping section primarily involving two transistors, two diodes, and a handful of caps & resistors...p3...right before the MN3102.

I will simply and vaguely put it out there, that I simmed a far simpler LFO that could NOT do a convincing 'rounded triangle wave,'  but used it to 'feed' the aforementioned network in LTspice simulation, similarly, and, voila, similar if not identical results if lfo speed is taken into account and within a general range of LFO shapes.

Hope this works out to be more or less that simple (or 'good enough') in realtime.

[fuzzo]

I've been looking for leslie simulator these days. There're some schematic floating around on the internet and the rotosphere is pretty complicated . The idea of leslie simulator, more how I understand it actually , is to split the signal into two paths , low and highs (cross frequency at 800hz). Each going into a bbd delay line (chorusing effect ? ) (the dynachord unit , SL222 or something) with a different oscillator speed , treble going faster that basses (or the other way) .

In the RFX47 (by Rolls) there're several paths. the signal goes into a 2 phase shift OTA then a LPF , but also into a HPF to go into a BBD line (MN3207) . The three signals (dry, highs ,lows) meet at the end via a op-amp mixer. Still remains the two oscillators with two differents speeds and the ramp control.

The rotosphere seems to work in the same manner but the schematics I have are really hard to read.

[liquids]

I believe you mean the Rolls RFX147.

I saw the phasing/filtering approach, and, while that may be a 'part' of the 'real' sound, and may be really the critical element after AM (amplitude modulation) of what happens to the rotor, post-crossover, for the low frequencies and all the high frequencies filtered out, in a leslie cabinet, speaker facing downward, I'm not into even duplicating that.

There is a demo of the rotosphere that comes up on youtube - first, I believe - they pan between the LPF and HPF side, and it sounds like that is the major thing you are hearing when the LPF side is being heard...in addition to some AM.

Alas, I will say that, I have my BBDs and such on the way, but, I remembered that my TC electronics rack unit has Chorus.  I used it for a faux leslie sound, but it never quite pleased.   I remembered that I could sort of concoct a way to do something with it thought - split the signal, HPF one side and 'vibrato/chorus' it, and sort of LPF the other half of the split signal, and run that into the amp as well, though somewhat awkwardly...with no effect, modulation, etc...and only a way to volume control the 'dry' signal with some high frequencies trimmed...the ability to truly and heavily LPF the signal was the weak point here but HPFing the vibrato signal was very controlable.

Anyhow, it was a significant improvement over modulating the whole signal.  Can be easily done via analog means.  One can take this as far as one wants - all the modulation control, difference in rotor transition speeds, AM, FM, Phasing, crossover notches, simulation of close miking, dual micing, LFO shape, etc....but, I do have 'the real thing' as far as the leslie horn in a box...I'm looking for simple and 'surprisingly good' for headphone use or amp use, as opposed to 'surprisingly complicated' for ideal implementation via distinctly different sounding, dual amps running simultaneously, or switching back and forth, utilizing some kind of crossovers, etc, and the like, without blowing stuff up, and with no less than 1/2 of the real thing in hand making the 'real' sound (leslie horn cab).

To each their own...

Honestly, the rotosphere schematic is daunting to look at, but I've traced it out by 'painting' over the schematics to trace difficult lines via drawing colored lines.  AFter that it isnt as hard to see.  The most complex part is the LFO, which I think takes up a whole page, and is unnecessarily complicated IMO, and definitely unecessarily complicated for my own needs.

In short, it's a booster--->dual triode tube-->HPF & LPF split (with some options)--->a lot of weird and possibly overkill additional filtering--->dual modulation paths on the HPF signal that gets mixed together with some dry, a lot of one modulated HPF signal, and a l ittle of the 2nd modulated HPF signal, with the LPF signal mostly seeing just AM, maybe some minor phasing simultaneously or instead (I'm predominantly ignoring that part), LPF and HPF signal mixed back together, and output.  The LFO section is a whole page of complexity that may be unmerrited, all for creating a rounded triangle LFO wave with uncessarily complex means to switch speeds/ramp up & down at independent speeds for HPF signal and LPF signal, plus 'breaker.' All the LFO complexity is more or less completely warped by a simplistic wave-shaping circuit snippet for modulating the HPF signal, the circuitry of which is similar to what the Arion Chorus LFO is pushed through, IMO and how I see it...

It's not simple, but it can be simplified, IMO, especially if you don't want to retain all of the rotosphere's features, and especially if you are OK and spend some time dialoging (or circuit simulating) what is happening here and there. I think the designers sort of worshiped the dynacord unit and said 'ain't broke here, won't fix it' rather than simplify intelligently...

To me it is kind of a humorous scenario - the rotosphere is the 'best regarded analog simulation' of "the real thing dual speed Leslie."  However, "the real thing Leslie" seems to have a lot of variations anyhow...so, even what is the sound of 'the real thing leslie' is debatable.  The argument that, from a science perspective, doppler/FM, AM, and phase, and 'shape' of the frequency through rotations, plus other acoustic phenomenon are all critical might be overstated.  The ears can only hear so much of that anyhow.  Cost/benefit - if phase cancelation could be isolated in an example of 'the real thing' and it were almost imperceptible to the ears in person - let alone on record, when mic'd, then mixed with other instruments playing, etc....how worthwhile is it to spend time trying to duplicate/dial that in?  If AM is present, but almost so subtle, in some cases - depending on the room the leslie is in, how close it is to a wall, how far the mic is from the cabinet, the recording, and mixdown - as to be comparable to a tremolo circuit just shy of unnoticable - how much circuitry is merrited in duplicating that element, let alone all the variants?  Etc.

Some people are after 'certain' leslie sounds. I get that.  Some people don't realize that, like captain foldback's page notes, the sound of steppenwolf's organ is NO horn and no crossover, and just sending everything including high frequencies through the huge down-firing leslie speaker with it's rotor on fast, with the amp in overdrive.   Well, that's different than what I'm more or less comparing to, 100% - the sound of driving an MTI rotophaser, which is strictly a leslie horn with a passive crossover, but no amplifier and no low rotor - just an output for the LPF(?) side of the crossover.   Gonna sound way different if I try and simulate that...which is what I'm doing.

One thing is - whether it's george harrison's guitar on certain beatles tracks, through a leslie/vibratone, steppenwolf's organist, charlie hunter, or steve winwood on "gimme some lovin," Warren Haynes and John Scofiled on some tracks, or john medeski - the 'sound' is unmistakable as "what is that?" when it's "on" no matter what tool they're using to get it - leslie 147, rotosphere, spinning horn, vibratone, full-frequency driven lower rotor, or ibanez CP-9 set fast.