LFO help needed

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Hello...

I am building a box with a few effects in it. Two of them are Tim Escobedo's PWM and gargletron pedals.

I have LFOs on each of those pedals (with a home made vactrol on the gargletron), but I wanted to build one additional LFO to share between these two, to be able to have more modulation options, and also to have more waveforms than the 40106 based LFOs that I currently have on each of those pedals (which only does squarewave).

So I built this LFO that I found here that someone had made based on another LFO. Here is the schem:


However, it doesn't seem to sweep as deeply, with as much variation as my 40106 LFOs do. The only mods I did were that I used 4001 diodes and use a tl072 instead of the 1458 or 4558 because they are what I had around.

Here is the PWM schem, which shows the 40106 LFO.


What could I do with the opamp based LFO to get it to put out a higher voltage and sweep more deeply? I am not sure if the problem is that it does not go high enough, or if it is that the low point isn't low enough. Would cutting out or cutting down that 1k resistor (R6) do the trick?

[sean k]

Umm, I think your problem may be that the 40106 Lfo controls the base of the transistor so when it turns on the collector meets the earth of the emitter so the signal is completely grounded... shut off, whereas with a vactrol you'll only ever get the least resistance, created by the greatest brightness, to ground the LDR can offer... so it may be 2k, 10k or even a hundred and more k over ground. Measure the resistance of the vactrol when the max voltage is applied to the LDR and what do you have?

It's not so much the swing of the LFO as the on resistance of the LDR that governs the depth. Aternately you could go with the LFO on the echobase, or at least its methodology of driving a transistor base. Use the output from the sensitivity pot through a 10-50k resistor to the base of a 3904, as in Tim's 40106, to turn it off and on and use the value of the cap to control the ramp somewhat.

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Umm, I think your problem may be that the 40106 Lfo controls the base of the transistor so when it turns on the collector meets the earth of the emitter so the signal is completely grounded... shut off, whereas with a vactrol you'll only ever get the least resistance, created by the greatest brightness, to ground the LDR can offer... so it may be 2k, 10k or even a hundred and more k over ground. Measure the resistance of the vactrol when the max voltage is applied to the LDR and what do you have?

It's not so much the swing of the LFO as the on resistance of the LDR that governs the depth. Aternately you could go with the LFO on the echobase, or at least its methodology of driving a transistor base. Use the output from the sensitivity pot through a 10-50k resistor to the base of a 3904, as in Tim's 40106, to turn it off and on and use the value of the cap to control the ramp somewhat.

I am using the same home made vactrol with both lfos. I disconnected the 40106 based LFO from it and hooked up this op amp LFO to the "vactrol" to test the LFO. I guess my point is that it doesn't seem to be generating as much brightness in the led to generate as much resistance in the LDR as the 40106 LFO seems to be able to.

Bear with me as I am sort of a noob when it comes to actually designing and combining new things... however, the 40106 lfo I used was totally done on a whim as an experiment and it worked well, so I thought I might get away with it again this time.

[sean k]

Okay, I'm with you. What about the pots? The uglyface one is down at 1k whereas the synthstick one is down at 10k so the 1k is much closer to ground so is for something low impedance driving it. Put a resistor between 1 and 2 (50k) and a resistor to V bias from pin 2 (10-20k) on the driver of the LFO to make it an amp and not a buffer which should raise the output voltage and lower the current and so raise the impedance and replace the pot with 10-50k.

I may be wrong though and that might require decoupling on the bias, which I'm still unsure of when and where myself, but I think your answer lies in this direction. You want a little more voltage and a little less current and I'm thinking a little amplification is what you want so the buffer has got to go.

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Okay, I'm with you. What about the pots? The uglyface one is down at 1k whereas the synthstick one is down at 10k so the 1k is much closer to ground so is for something low impedance driving it. Put a resistor between 1 and 2 (50k) and a resistor to V bias from pin 2 (10-20k) on the driver of the LFO to make it an amp and not a buffer which should raise the output voltage and lower the current and so raise the impedance and replace the pot with 10-50k.

I may be wrong though and that might require decoupling on the bias, which I'm still unsure of when and where myself, but I think your answer lies in this direction. You want a little more voltage and a little less current and I'm thinking a little amplification is what you want so the buffer has got to go.

Ok... I am going to need to dig into this a little bit, as honestly, I don't fully understand how the LFO works.

So I understand the bit about putting in a resistor between 1 and 2 - I assume you mean pins 1 and 2 on the opamp (which currently are just tied together).

However, what is this about the resistor from V bias to pin 2. What is V bias?

Also, you are saying to replace the 1k "sensitivity" pot (as it is labeled in the schem above) with a 10-50K, right?

[jonfoote]

hi, ive used the simple lfo found on the EA site

http://experimentalistsanonymous.com/diy/Schematics/Oscillators%20LFOs%20and%20Signal%20Generators/Simple%20Dual%20LFO.gif

if memory serves me it gives between 0 - 6V, its very simple to build and modify for the right speed range. the output has a pot which blends between square and sawtooth for a range of shapes.

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hi, ive used the simple lfo found on the EA site

http://experimentalistsanonymous.com/diy/Schematics/Oscillators%20LFOs%20and%20Signal%20Generators/Simple%20Dual%20LFO.gif

if memory serves me it gives between 0 - 6V, its very simple to build and modify for the right speed range. the output has a pot which blends between square and sawtooth for a range of shapes.

Thanks for the link.

You mention that it goes square to sawtooth. The diagram says square to triangle. Do you know which is correct? I ask because the triangle waveform is what I am really looking for (my current 40106 LFO has square and a switch for a sort of ramp/sawtooth (used a cap to ramp it off). I am looking for the smoother sweep of a triangle.

[jonfoote]

its a nice smooth triangle, i cant claim its spot on cos i havent got a scope but it sounds pretty good to my ears.