Author Topic: LFO questions  (Read 15978 times)


LFO questions
« on: February 02, 2010, 01:39:28 PM »
I am starting to become fascinated by the possibilities of LFOs and how they can control circuits, but the truth is I don't understand them very well.  I have been searching and reading post hereand all over the internet looking for schemtics and explantionsof various LFOs.  I hope to start with some very simple designs, the waveform(s) aren't that important yet.  My first project would probably be to create a tremolo, using the LFO to drive an LDR/LED.  I already have the LDR/LED and gain control worked out, and tried a lot of experiments with a 555 but had seemingly unresolvable issues with ticking.  

In my various searches I found this LFO diagram:

I breadboarded figure 1 using a TL071 but get no oscillation.    The diagram does not specify power, so I hooked 9V+ directly to pin 7 and ground to pin 4.  Is this correct?  I measure 9V at pin 7 and a constant  4.5V at the output pin 6.  I have experimented with different values for C1 but still no change.  

Thanks for any suggestions, general links to beginner info about LFOs, and suggestions for other simple LFO designs (preferably using very standard opamps or transistors I would likely have on hand) that I can experiment with.
« Last Edit: February 02, 2010, 01:41:07 PM by oliphaunt »


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Re: LFO questions
« Reply #1 on: February 02, 2010, 02:00:10 PM »
That's my diagram, from this (unfortunate) thread: The figure 1 LFO certainly works, and similar LFOs appear in the Phase 45 and Phase 90, where it wiggles JFETs. You should be able to detect a voltage swing at the output, and even drive a LED from it (via a 1k resistor say), but to make it drive other stuff you need to add a buffer as in figure 2. Figure 4 is an extension of figure 2, the lower the value of R7, the wider the LFO will swing.

The way you have power hooked up looks correct, do you also have the upward pointing arrow at the top of R1 connected to 9 volts? Maybe try another opamp, a dual perhaps. Sometimes just breadboarding again from scratch will make things work. The value of C1 won't really affect things that much, C2 is the timing cap.

Here's another take on that same LFO topology I've been playing about with recently, this time using diodes to convert the triangle wave into an upward or downward sawtooth:

All these variations will also give a square wave at the output of the opamp.
« Last Edit: February 02, 2010, 02:01:45 PM by frequencycentral »

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Re: LFO questions
« Reply #2 on: February 02, 2010, 02:15:06 PM »
Thanks FC.  I do have R1 connected to 9V+.  I have also tried a couple of TL071 opamps, different values for C2, and largely breadboarded from scratch, and still no luck, the voltage stays constant at the output.  I have a fair bit of experience breadboarding, so I'll keep at it and look for any more errors on my part.  

The dual LFO you linked to is next on my list.

« Last Edit: February 02, 2010, 02:16:58 PM by oliphaunt »


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Re: LFO questions
« Reply #3 on: February 02, 2010, 02:22:41 PM »
Other observations re figure 1, 2 and 3:

You can reduce the value of R4, which will give a wider sweep.

You can reduce (or remove) R5 for faster maxumum speeds.

You can increase the value of C2 or the speed pot for slower speeds.

It would also be worth looking at the Tremulus Lune LFO (and varients) schematic for other variables that you can implement:

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Re: LFO questions
« Reply #4 on: February 02, 2010, 05:40:51 PM »
Hmm, I've tried a TL072, JRC4558D, 2 TL071s, carefully rebuilt the entire breadboard each time, accounting for the pinouts between single and dual opamps.  The single OAs give me a constant 4.5v at output, the duals give me a constant 8.25v at output. 

It's frustrating to be beaten by such a seemingly simple circuit when I've successfully built ones 10 times as complex, in terms of parts count.   I seem to have bad luck with oscillators though...


Re: LFO questions
« Reply #5 on: February 02, 2010, 06:10:21 PM »
I'm not overly farmiliar with those circuits but they look like schmitt triggers into intergrators to me. I've built up one that uses 2 opamps, one for the schmitt trigger and one for the intergrator, and it works perfectly. It's a very common circuit, if you put opamp schmitt triangle into google images it should come up with a bunch.


Re: LFO questions
« Reply #6 on: February 02, 2010, 06:46:04 PM »
Still no luck with the first LFO I tried but I am having some luck with this the right schematic here:

I plugged in these values using a 4558.  These are some of the values from the first LFO I tried, plus others I found by experimenting.  Does this look like it's in the ballpark?:

The square wave output works, but I'm not getting anything but constant voltage from the triangle output,which is ok for now, I'll work with the square.  I connected the output to a homemade LDR/LED and put the LDR in the path of some audio. It works as a tremolo, but there is some ticking present.  Does anyone have any thoughts on how to filter the ticking out? 

Another random thought, I have used the LDR/LED to control the volume simply by inserting it in the line and the resistance knocks the level down.  I then imaged I would follow that with a gain stage to maske up for volume loss.  Could I use the other half of the op amp for the gain stage, or would the ticking issue likely be unavoidable?


Re: LFO questions
« Reply #7 on: February 03, 2010, 04:31:50 AM »
+1 on the frustration.

Have tried TL082 dual, LM741 and LM358 (since it only needs a single supply - none of this + / - 9v business) with no success whatsoever.
Just a constant output.

The only chip I've had oscillations on is the 555 (noisy, square wave) and 7555 (quieter, still square wave).

More digging is required I think - although I'll have a try with the links kindly posted above.

I wonder if there'sa GEOFEX article... <scratches chin>...
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Re: LFO questions
« Reply #8 on: February 04, 2010, 06:46:50 AM »

I built (figure 2.) from the original post (dual op-amp).

And I thought it didn't work, as I was using an LED as an indicator.

Then I started collecting voltages for debugging - and they were rather wobbly.

Op-Amps used were TL082 and LM358 (pinouts are thankfully the same).

Voltages are as follows...

Pin    Reading (v DC)
1      1.3 to 6.3 swinging.
2      4.4
3      2.6 to 4
4      0
5      2.9 to 4.4 swinging.
6      2.15
7      3.3 to 4.4 swinging.
8      7.39.

Taking output from either pin 1 or 7 (as I have been doing) - shows a constantly lit LED - because even the lowest 'swing' on these outputs is greater than the forward voltage of the LED (square green thing I had lying around).

Introducing a 2k2 resistor between pin 7 and the LED's anode gives a very pleasing throb (please don't be smutty - I'm just happy that something finally oscillates after playing wih this for *many* hours).

Oliphaunt - apologies if it seems like I've hijacked your thread.  I think we're aiming for the same thing (quieter oscillator for Tremolos).
I hope these voltages are of some use to you in your debugging.



Taking output from pin one, through a 270R resistor gives a square on/off result.
Taking output from pin seven, through a 22R resistor gives a triangle ramp up/down result.
« Last Edit: February 04, 2010, 07:08:40 AM by newfish »
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Re: LFO questions
« Reply #9 on: February 05, 2010, 09:52:38 AM »
Newfish thanks for posting your findings.   I will expereiment with htose myself.

I have been able to get the second schematic I posted to work but when I put any other circuit on the same breadboard sharing the power supply I get tons of LFO ticking.  A 100U cap from 9V+ to the output takes away th top end but leaves a throbbing low end pulse that a high pass filter on the audio circuit still can't rid of.  I have also put a filter on the power supply ot the IC, it's helps but nowhere near enough.

Should I expect better result from a dual op amp LFO?


Re: LFO questions
« Reply #10 on: February 05, 2010, 10:46:27 AM »

I'm aiming to make / fudge circuit no.2 into a basic passive trem this weekend.

The idea is to keep the signal ground separate from the oscillator's ground. 

I *did find that using a 22uF Electro between the Anode and Cathode of the LED / LDR combo (Vishay SFH618-2 in this case - Maplin (UK) have them) helps to really smoothe things out.

This was with a 7555 (low power 555) oscialltor which was *very* noisy.

I've yet to try this set-up with a dual op-amp 'rig' (as above), but will post my findings.

As far as 2 x single op-amp vs. 1 x dual op-amp go in terms of noise, I really don't konw.

Maybe look at filtering on the output of pins one and 7 before it goes through the Buffer - so there's more 'signal' and less 'thump'?  Dunno - just thinking out loud.
E.g. skinny cap to ground for particular band of frequencies?
...or a voltage divider between supply, ground and pin 8 - so there's less supply to spike with when the wave changes up or down - though this may well need an *extra* (or higher gain) buffer afterwards to allow the output to still drive an LED.
 Again - have yet to try these ideas - am sat at work...

Best of luck with this - I reckon there's some decent mileage figuring out a quiet, reliable oscillator.
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Re: LFO questions
« Reply #11 on: February 06, 2010, 01:04:29 AM »
I’m not familier with how those circuits work but I use them as building blocks in by builds. I have, For example, used the Tremulus Lune LFO to do the LFO in the Easy Vibe and connected one LED to 4 LDR. (you have to connect a ~R150K in series with the speed pot to get the correct speed range)

If you are looking for some thing to drive a LED/LDR combo with I would go for the Tremulus Lune LFO as frequencycentral suggested. Its’ easy and really versatile.

If you are looking for some thing that will control the connection to GDN like controlling how often and how long a circuit connects to ground the EA tremolo LFO will work. Among other tings I used it to control the LFO in frequencycentral Tube tremolo.
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Re: LFO questions
« Reply #12 on: February 06, 2010, 06:17:29 PM »
Thanks Solderman,

I've had a look at the TT, and it might take me a week or so to get thet hang of what's happening (if not longer).

I really like the look of the 'smoothness' control and the 'fine tune' pot in addition to the speed control.

Now to see if I can make more hours in the day to play with these ideas...


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Re: LFO questions
« Reply #13 on: April 11, 2010, 12:38:16 PM »
The value of C1 won't really affect things that much,

I've been messing with this oscillator yesterday and:
What I see is that changing the value of C1 will change the shape, the frequency and : the wave may not reach zero volts if C1 is too high.

With 0.01uF the wave will retain it's shark fin (integrator) form.  With higher values you can have a more sinusoidal form, but it will reduce the frequency of oscillation at the same time.  With a 0.22uF the wave will be sinusoidal and will not touch the 0V absyss (and this even if you connect the V- pin of the opamp to a negative voltage (if I remember well).

Thanks Rick for that snippet, it's been fun messing with that.