Author Topic: LFO help/suggestions  (Read 145 times)

bsoncini

  • Stompbox Star
  • ***
  • Posts: 205
    • View Profile
LFO help/suggestions
« on: November 14, 2017, 11:50:44 AM »
So a friend of a friend asked me if I could make him a tremolo that goes from about 15-30 seconds to really fast ring mod type sounds.  I showed him a tremulus lune that I have and he said he didn't need as many knobs. The tremulus lune doesnt go that slow either  So anyway.  I figured I would try with a shoot the moon.

Any time I look at lfo schematics I have no idea what I am looking at.  So I figured this is a good way to learn about it. 

Heres the schematic  http://www.forrestwhitesides.com/files/up/img/STM_V1_Schematic.png

First question, is this even possible.  Messing around with c2 (10uf) and r5 (2k2) I can get it either really slow.  Ive gotten to about 15 seconds.  or really fast but can get both at the same time.

Second question.  Anyone have any good links that explain lfo's? I feel like im just plugging components in hoping to get lucky. I feel like I need to understand what the 220k resistors feeding back to the opamp and the other parts already mentioned

thanks in advance for any help

thomasha

  • Transistor Tickler
  • **
  • Posts: 74
    • View Profile
Re: LFO help/suggestions
« Reply #1 on: November 14, 2017, 12:02:05 PM »
I'm testing some LFO's in LTspice, really cool to see the thing working and check every node.

I found some information regarding transistor based LFO, or phase shift oscillator. And it's possible to calculate some values based on the transistor data and desired frequency.
Until now I tried changing the pot and some resistors, but sometimes it just won't oscillate at lower velocities.
But this is more like a tube amp tremolo, with several capacitor stages, and I guess you are talking something in the line of a schmitt trigger, with opamps.

Here https://www.electrosmash.com/mxr-phase90
you find a good explanation of how this LFO works. The one you're using has another opamp stage to change the curve form.
« Last Edit: November 14, 2017, 12:11:38 PM by thomasha »

bsoncini

  • Stompbox Star
  • ***
  • Posts: 205
    • View Profile
Re: LFO help/suggestions
« Reply #2 on: November 14, 2017, 12:09:48 PM »
That is a good idea. I have ltspice on my computer but have never touched it and don't know how to use it. Guess it is a good time to learn.

thomasha

  • Transistor Tickler
  • **
  • Posts: 74
    • View Profile
Re: LFO help/suggestions
« Reply #3 on: November 14, 2017, 12:12:56 PM »
Here:
https://www.davidhaillant.com/simple-lfo-ltspice-simulation/
it's already modelled, you can test it with this doc.

midwayfair

  • Electron Doctor
  • *****
  • Posts: 6613
  • Jon P
    • View Profile
    • Midway Fair
Re: LFO help/suggestions
« Reply #4 on: November 14, 2017, 01:16:44 PM »
15 seconds is 0.0666 hertz.

ring mod sounds will require >50 hertz at a minimum. Higher is better. Much higher is much better.

A potentiometer will give you 270 degrees of rotation, I think.

That means you need to fit about 3 hertz in every degree of rotation.

Tell him he needs to put up with more knobs if he wants to be able to dial in any sort of reasonable approximation of the tempo he's looking for in tremolo or the frequency he needs for the ring mod.

If you can find multiturn antilog pots, though, you can do what Moog's ring modulator does.

Alternatively, you can put in a switch to change the LPF cap in the lune. It's usually a big cap in the LFO, like 10uF or sometimes 100uF. Bigger values is slower time. Smaller values is faster. Of course, that's more controls.

I'm always mystified why people look at a solution and say, "Nah, I don't want the extra knob." Like, seriously ya derp, don't use the other knob if you don't want it.
Myself's music & things I make: http://jonpattonmusic.com. My band: http://midwayfair.org. PCBs of my designs from: http://www.1776Effects.com (Bearhug Comp & Cardinal Harmonic Trem); http://www.jmkpcbs.com (Hamlet+ delay & Blue Warbler envelope vibe); Snow Day OD/Flabulanche: www.madbeanpedals.com

thomasha

  • Transistor Tickler
  • **
  • Posts: 74
    • View Profile
Re: LFO help/suggestions
« Reply #5 on: November 14, 2017, 02:04:15 PM »
I think it is just important to have some control in the lower range, like a slow tremolo.
It could just jump a decade without a big change in the higher frequencies.
The thing is to define until which frequency you want a finer control.

Since whe are speaking about a LFO, I'm trying to add some offset to a LFO to use it with the loophole as the vibrato control. I'm not going to use the schmitt trigger gate, instead I'm going for the simple transistor oscillator.
The thing is, ideally the LFO would oscillate from 4.5v to 4.0v so the chip would still work. I already added a transistor to introduce the offset.
The current required, around 25mA according to the datasheet, is a little too high for my LFO. I simulated the circuit and i can mantain the tension, but the peak-to-peak voltage falls to less than 0.1. On the other hand, if I keep the oscillation peak the voltage drops to 3.0v, where the chip stops to work.


« Last Edit: November 14, 2017, 02:20:48 PM by thomasha »

bsoncini

  • Stompbox Star
  • ***
  • Posts: 205
    • View Profile
Re: LFO help/suggestions
« Reply #6 on: November 15, 2017, 01:46:43 AM »
Thanks Thomasha.

Been playing around a bit with ltspice.  What a pain in the ass at first.

I found this link on the tremulus lune lfo which is pretty informative.  http://mimmotronics.com/tttm4/

bsoncini

  • Stompbox Star
  • ***
  • Posts: 205
    • View Profile
Re: LFO help/suggestions
« Reply #7 on: November 15, 2017, 06:23:26 AM »
Thanks for the advice Jon.  I think I will put a switch with a slow and fast mode.

I played around with the fast mode today.  Put the speed pot at 200k mostly because he wants me to house this in an old volume pedale he has to use the rocker as speed control which is around 200k.  Put r5 at about 5k6 with c2 1uf.  Can go between about 40 hz which sounds close enough to me for a ring moddish sound and slowest goes about 3 hz.

I hooked the lfo to an old scope i have and just played around with values until I got to those ones.
« Last Edit: November 16, 2017, 11:25:36 AM by bsoncini »