GGG Leslie Sim oscillator smoothing?

[JimRayden]

http://www.generalguitargadgets.com/diagrams/les_sim2.jpg

The oscillator part (Q1) is chopping too hard. It's ok if I wanted quick frequencies but it sounds bad when slow. I want more sinusoidal leslie. I haven't dug into oscillator circuit design much, so I need some help here. Actually I would like to make it even slower. Cap changes perhaps?

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Jimbo

[Mark Hammer]

The first issue Small Stone placed a 10uf cap to ground on the output of the LFO such that - given the nature of the components and speeds involved - it "de-peaked" and rounded off the LFO waveform for speeds greater than around 2-3hz.  Following this example, I did this to my Ross/Ropez phaser and was extremely pleased  at the more relaxed vocal (rather than choppy mechanical) feel it delivered at faster speeds.

You could try the same thing here by placing a cap to ground from the junction of R7 and R8.  A 2.2uf cap, in conjunction with R7 (100k) will form a lowpass filter rolling off above 0.72hz.  A value of 1uf moves that up to 1.6hz.  Either of those might do something to "tame the chop" at either higher OR lower LFO speeds.  If you look at the schem for the original E-H Pulsar Tremolo, you'll see that the "chop" setting simply reduces the amount of capacitance-filtering to ground for a sharper rise/fall in the LFO waveform.  No reason you couldn't do that here. 

Note that there WILL be some sort of audible pop when bringing that capacitor in circuit by connecting it to ground.  A suitable solution might be connecting the cap to ground through something like a 470k-1meg bleed resistor.  Use a SPST toggle to shunt the resistor when you want the capacitor to be "in-circuit".  This way, the cap can bleed off when not in use.  There will be *some* impact of the cap at all times, but the  cap/bleed-resistor combo will be acting a bit like your guitar's tone control turned up full.  Yes, it loses a smattering of highs, but not enough so you'd notice.

[jrc4558]

That, or do the trick John Hollis did with his EZVibe. Hist task was to mimick the sine-wave from the triangular one, so how did he do it? Two pairs of series-connected 1N914s across the output of LFO to ground, and consecutive LPF.

[Mark Hammer]

Smart.

What that suggests, then, is perhaps replacing R7 with a pair of series resistors, lets say 18k and 82k.  Diodes go to ground at the 18k/82k junction, and cap goes to ground at the 82k / R8 junction.  I'm not all that familiar with this circuit and what the voltage level might be at the R7/R8 junction so the actual number and type of diodes that need to be used would be to be determined.  It might be more than 2.

[JimRayden]

At a second glance, it doesn't sound much like a different wave, it sounds more like half-wave rectification.

_/\_/\_/\_/\_

The tops are smoother though. It sound like a barking chihuaua. boing-boing-boing. Another way to put it - it's two time units at the bottom, and one at top.

So I don't know if frequency filtering helps too much. Maybe there's something gone funky in the generation - overheated caps or?

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Jimbo

[Mark Hammer]

Did you already do this?  Not clear if you're commenting on what you think might happen or how it actually sounds with the mods.

Take a look at the hypertriangular sweep thing at my site: http://ampage.org/hammer/files/hypertriangleclock.gif
Perhaps that little FET-based subcircuit for altering the LFO waveform peaks will be what you want.

[JimRayden]

Yes, I did try it. I try to avoid hasty conclusions. Didn't help much.

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Jimbo

[Mark Hammer]

Bad news, but at least rapid feedback.

For my edification, what kind of LFO output are we actually looking at here?  Is it supposed to be triangular, square-ish, or what?  What sort of voltage swing is involved?  I'm asking because I don't know this circuit.

[JimRayden]

I don't know the circuit either but I'm after a sloooww sine output. Perhaps just replace the whole oscillator circuit?

Hmm, the article says leslie sound happens in the lower frequencies but it actually seems to mess with the highs. What does a man have to do to get a decent Leslie sound these days? I've been looking at Easyvibe circuit too but the goddamn LDR's are expensive as hell.

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Jimbo

[JimRayden]

Hmm, I'm not too familiar with the easyvibe. What if I reduced the number of stages to three or perhaps two? What parameter would that change? Depth?

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Jimbo