LFO with dual op amp (tremulous lune style), how does it work? (+trem lune mods)

[add4]

So i'm studying the tremulous lune, trying to understand how it works and to figure out how i could add the second lfo modulating the speed thing to it.
I understand that there are two paths: audio and lfo.
audio path is an amplifier using a dual op amp, with fixed gain on the first stage. The gain of the second stage is controlled by the resistor in the feedback loop and the resistor between the two stages, which happens to be a ldr, driven by a LED controlled by the lfo path. This LED blinks in a sinusoidal way, which changes the gain in the audio path, which tremolotes (new verb here) the signal.

This opens a whole new world of possibilities: any resistor or pot could be replaced by a ldr/vactrol, life is beautiful!

I'm now trying to understand how the LFO part is generated. At first i tought the opamp was some weird kind of timer-based-thing that i didn't understood, but i just realized that the IC is a 4558, dual op amp, like the TL072 used in the audio path? why not two TL072 or two 4558 then?  (<- FIRST QUESTION HERE).
Second question is: how is the sinusoidal/square voltage achieved in the lfo section? I don't quite understand what makes it happen and how it works. Would anyone care to explain that to me? i think it's essential to start being creative with these materials.

I would also be glad to take more informations about the mods/evolutions of this build:
i'm starting from the possibilities offered here : http://www.commonsound.com/kits/doku.php?id=commonsound:tremulous
but i don't find it clear at all in most/every case.

- dual LFO :
what are the differences between
- Aux LFO (to add) "superboard" - complete board with 2 4558 added ...i understand i'm supposed to get this board after mod, but if we're adding one LFO section, why 2 4558, one of them being almost 'naked' (only 2 resistors around it). what's it's role?, why not add only one LFO?
- Aux LFO (to modulate main) - 1 LFO added, to what, where? how does it interact with the trem LFO section? From what i understood: a LFO generates an oscillating voltage, you want to convert that to a resistance so it can control any pot. here we want (apparently) to control the speed pot with the lfo, so why can't i find a second vactrol/ldr in place of the speed pot to control that? or are we using the voltage to control it? if yes, how?
- Aux LFO (to add) mod board -> TWO Lfo added ??!? that would with the main board above, same questions then?
- Two speed mod: is here a diagram somewhere?
- combination mod: same question : is there a diagram to that somewhere? what i should do is not clear to me..

Sorry about the long post.
I searched the forum for this topic and couldn't find the answer, sorry if it has been posted already.
Thanks for your help and insight!
Peace,
Arnaud

[PRR]

> why not two TL072 or two 4558 then?

1) TL072 used to cost more than 4558.

2) TL072 can flip-out when inputs go to extremes.

> how is the sinusoidal/square voltage achieved in the lfo section? I

There's no true sine in there. Triangle and square.

"triangle wave generator"

http://www.google.com/search?q=triangle+wave+generator

http://www.play-hookey.com/analog/triangle_waveform_generator.html

http://www.freecircuits.net/circuit-63.html

The Lune uses a degenerate form of the 2-opamp integrator-Schmitt, where the integrator is replaced with a passive R-C network. This gives roundy triangle (roundy the wrong way to be sine-like). While this seems to save one opamp, in fact it needs the buffer so it is still a 2-opamp affair. It also upsets the Schmitt with a Spacing control. It's pretty funky.

> any resistor or pot could be replaced by a ldr/vactrol

Resistors (including 2-terminal reostats) only over a limited non-linear range.

True 3-terminal potentiometers are very difficult to replace with LDRs.

[chemosis]

always wanted a trem lune with dual lfo. but I don't know enough yet. the aux lfo out sound sweet to

[garcho]

Do you want two identical, simultaneous LFOs that each have their own independent controls? That's pretty simple; you duplicate the LFO section and stick another LED in front of the LDR. It's far from perfect but works enough to get some interesting interactions and cool textures, especially messing with the depth controls.

Do you want one LFO controlling the other's functions? As Paul aka PRR mentioned, creating a "true" potentiometer out of LDRs is way more involved than it might sound. On the Tremulous Lune the controls that are configured as variable resistors (as opposed to a true potentiometer), and those you may be able to modulate with an LDR instead of the pot are "spacing" and "speed". Speed would probably be a more interesting effect, but it would mess with your time. Modulating the spacing might be subtle but something you would actually use.

You could replace the LFO with something more sophisticated, like these LFOs on a chip from Electric Druid. Or something with an OTA that you can modulate with CV (control voltage) a la synthesizer.

[ElectricDruid]

You could replace the LFO with something more sophisticated, like these LFOs on a chip from Electric Druid. Or something with an OTA that you can modulate with CV (control voltage) a la synthesizer.

Thanks for the mention! The control inputs on the StompLFO and the other Druid chips are just 0-5V inputs, so you can modulate them with a control voltage very easily too. There's a ton of Eurorack modular synth modules based on various Druid chips that do just that...

IMHO, the best thing about messing with the LFO in circuits like this is that you can add a lot of features and flexibility without risking the actual "sound" - you're not screwing around with the analog audio path at all. For me, that's the best of both worlds.