Low Frequency Oscillators (LFOs) and why they are hard to design

[boogietone]

Dude! How to follow that? 

[PRR]

> How to follow that?

Thanks for asking. We have a lot of frogs in my bog. Hygene is important.

[Taylor]

Thread of the year?

[jonasx26]

Thread of the year?

Just might be 

[Gurner]

Excellent....a thread that makes me think I've taken acid - bookmarked.

[earthtonesaudio]

I hope you don't mind Paul, this thread inspired a change to my avatar graphic.

[PRR]

> Excellent....a thread that makes me think I've taken acid

Come down to earth and READ what R.G. posted.

"The underlying issue is not the LFO at all. It's whatever they are driving. Each thing a LFO drives has its own different needs for whether it needs a voltage or a current drive, what the minimum and maximum levels for these voltages or currents are, and what amount of variation you get per change in the LFO quantity."

So the illustrations of car-lifts and paint-rollers have a point:

1) Determine and write-out or draw-up your VCO ("lift") output.

2) Determine and write-out or draw-up the requirements for your "thing being driven". Include variations for device tolerance, and for artistic control ("Depth").

You guys are all good with levers. If you need to bend a box, shim a sagging step, or pry a frog from under the porch, you get the situation in your mind, then go grab the right pry-bar or scrap 2x4.

What R.G. seems to be pointing out is: VCO connection problems also need a good understanding of what has to be moved how far and how hard. He offered a few examples.

It is actually a problem in Analog Computing. Multiplication and addition of voltages. Back in the 1950s there were college courses. While multiplication by less-than-One can be done with passive dividers, and addition can always be done by sticking a battery in series, the more general cases used the then-new Operational Amplifiers. This allows multiplication by larger factors and adding voltages controlled by pots instead of extra odd-voltage batteries. (Although in our case, the VCO can generally swing more than we need, so we rarely need multiply-up.)

To build your skills and tool-kit, LOOK at all the VCO-controlled things you can find. What is the VCO output? What is needed at the thing being controlled, for max and for less-depth? How did they make the necessary changes to the VCA voltage to give the desired voltage at the thing?

[boogietone]

Actually, I have an situation that pertains to the topic of the thread. I have put off posting on it because I have not finished experimentation yet. But, it does seem apropos since R.G. opened up the discussion.

Take our beloved EA Tremolo (GGG version http://tinyurl.com/3oawnst) and replace the coupling JFET, Q2, with a MOSFET, specifically a BS170 (don't forget to mangle up the legs due to pinout). Let us ignore just how I ended up with this configuration for the moment and ask the following. What will happen? Will it work?

Yes, it does, after a fashion of course. The result is more of a soft slicer/chopper effect. It is not very abrupt - quite nice IMHO. It is only active for the upper half of the rate pot and with the depth pot at least 2/3 ON. From what I can tell through measurements with my DDM, changing the rate pot actually changes the amplitude of the LFO or the DC offset of the LFO. I cannot tell at this point. I did not measure similar changes with the JFET.

This is exactly the kind of thing that R.G. described at the top of the thread, Mark Hammer succinctly summarized, and PPP adeptly put into pretty pictures.

The design goal is to keep the slicer type of effect but get the MOSFET to work over the same range as the JFET version WRT rate and depth. I have done some LTSpice sims with limited success. I am not sure that I have a good model for the MOSFET.

Any thoughts on how to get the frogs to fill the entire bog?

[Gurner]

> Excellent....a thread that makes me think I've taken acid
Come down to earth and READ what R.G. posted.

Wow, CAPS LOCK ...it's like its 1974 & I've not handed in my homework 

As it goes I had no problem with the preceding text or concepts, but when frogs came into it,
then so did flashbacks to teenage evenings involving magic mushrooms...

[asatbluesboy]

> How to follow that?

Thanks for asking. We have a lot of frogs in my bog. Hygene is important.

Any idea on how to make it always front-to-back? Diode for ramp-down?

[PRR]

> Take our beloved EA Tremolo

This "lift" has coupling-cap after the LFO. In isolation, the wobble is always centered around ground. The Depth pot runs from near-zero wobble to a several-Volt wobble around ground.

> replace the coupling JFET, Q2, with a MOSFET

The JFET is full-ON when gate is near or above ground, goes off with a couple volts negative.

The MOSFET is off for negative or ground, only comes on with several volts positive.

That coupling/depth network is "not right" for either. Apparently the fact that the JFET is on or more-on most of the time is no fault.

A more refined JFET interface might return the Depth pot to a small negative voltage, about half the JFET turn-off voltage. However that's a complication the designer avoided.

The interface for a MOSFET would resonably return the depth network to a small positive value. Perhaps +3V. Surely adjustable for MOSFET threshold and player taste.

So in mechanical terms you need a block of wood (up for MOSFET, a "down-block" for ideal JFET) and a less-short lever (reduce R7) for the higher-voltage MOSFET.

Sorry about the all-CAPS.

[PRR]

>> Take our beloved EA Tremolo

> This "lift" has coupling-cap after the LFO. In isolation, the wobble is always centered around ground. The Depth pot runs from near-zero wobble to a several-Volt wobble around ground.

> replace the coupling JFET, Q2, with a MOSFET

Or in pictures:

I got the wobbling lift suspended so the average arm height is near ground.

I find there are two types of sump-pumps. One has the shaft out the bottom and needs stroke from ground to 3" down, the other out the top 3" above ground and stroked an inch each way.



Do not forget Longworth's Power Hammer With Movable Fulcrum:



The wheel goes round. The lever works the hammer. The fulcrum can be slid on the lever for short or long strokes, depending what you are hammering.

[Morocotopo]

Soo...

I need to have an LFO in my car´s trunk, just in case I get a flat tire? Or include in my gig bag a car lift, just in case the damned chorus refuses to sweep?



Sorry, no offence, funny analogy. Would be nice, for those like me not so LFO-savvy, some actual schems examples, to fully get the ideas.