Voltage Controlled Triangle LFO w/ Common Components

[Ohioisonfire]

I have been bread boarding LFOs for the past few days trying to find a nice triangle wave with a good range that can be controlled easily with an expression pedal.

The very common integrator/comparator circuit (found in many pedals, CE-2 for example) that produces a triangle and square wave works quite well, and can be tuned to work with a specific expression pedal value. Unfortunately the rate of the oscillation changes if an expression pedal with a different pot value is used.

I stumbled across the voltage controlled option below found in the LM358 datasheet:



This can be tuned to work quite well with a wide range of frequencies and can be independent of expression pedal component value. On the breadboard it works great with either an LM358 and a single supply (+9V), or a more generic op amp (i.e. TL072/4558/etc) and a dual supply (+9V/-9V).

I have tried many configurations on the breadboard and can get it to oscillate with a single supply but run into issues I can't fully wrap my head around.

• Is it possible for this circuit to work with something like a TL072 on a single supply?
• If not, is there another voltage controlled oscillator I should be looking at that uses common components?

*Note: Before anyone asks, I will try to draw up some schematics of the single supply adaptions I tested on the breadboard.
*Note 2: Happy to try any suggestions and follow up with scope measurements of the results.
*Note 3: You may see this cross posted if you are in other DIY pedal groups.

[ElectricDruid]

It looks to me like it probably needs an op-amp whose output can get close to ground. If the miniumum op-amp output is (say) 1.2V like it might be for TL07x, then the transistor never really switches off.

Have you got a link to that version of the datasheet somewhere? The current sheet linked from TI's site doesn't have the application notes or any useful circuits...sigh.

[Ohioisonfire]

I noticed the same thing regarding the latest datasheet, you can find an older copy with some digging. Search for "LMx58 datasheet". Lots of example circuits in there, very little to no write up on them however.

https://www.ti.com/lit/ds/symlink/lm158-n.pdf

[ElectricDruid]

Thanks, that helps. comparing your single supply version with their dual supply one is quite interesting. There's quite a lot of differences, so I'm not entirely surprised TL072 only likes the dual supply version.

For example, in the original circuit that "+V/2" on the right-most op-amp would be 3/4 of +V on a single supply version. You've got it set in the centre. Don't know whether this matters.

The two "ground" connections on the tranny and the left-most op-amp you've left at ground instead of shifting them to Vref. It seems to work fine like that, but it's a significant change compared to the dual supply.

[m4268588]

It is possible to operate the TL072 with a single supply, but the oscillation frequency variable range is limited due to input limitations.
I tried it on such a circuit.
https://mrotqch.web.fc2.com/stomp/OSC/osc_test_40.png

	MIN.	MAX.
LT1013	6.88Hz	189.8Hz
TL072	76Hz	186.2Hz

[ElectricDruid]

It is possible to operate the TL072 with a single supply, but the oscillation frequency variable range is limited due to input limitations.

Ah, yes, good point. The CV needs to stay within the input range.

The divider on the comparator is crucial too. You can tweak those values to increase the amplitude of the triangle, but you have to stay within the range that the integrator can actually produce. I've played with this in the sim.

[amz-fx]

A good read about this type of vco:

https://sound-au.com/project162.htm

regards, Jack

[ElectricDruid]

Should have known Rod Elliot would have the low down!...

[R.G.]

 I was quite taken with that circuit when I first saw it some years ago. What's not to love? A voltage controlled oscillator on a single supply. I found that it was fairly tricky to get all of (1) symmetrical slopes (2) wide range (3) constant slopes over the range and (4) single supply at the same time. I even changed the bipolar to a MOSFET to try to get better and more repeatable slopes and wider range.

It's a neat circuit - but you still have to work for it. 

[Ohioisonfire]

...in the original circuit that "+V/2" on the right-most op-amp would be 3/4 of +V on a single supply version. You've got it set in the centre. Don't know whether this matters.

The two "ground" connections on the tranny and the left-most op-amp you've left at ground instead of shifting them to Vref. It seems to work fine like that, but it's a significant change compared to the dual supply.

I have tried shifting around many of the ground points to Vref as you would do for a single supply design. I will try to trace out the "best" working configuration I have come up with on the breadboard and share today. I end up running into one of two problems:

• I can get a very nice triangle wave and linear response to the control voltage, but the frequency range is quite limited.
• Very wide frequency range, but very non-linear response at voltages roughly lower than Vref. The triangle quickly turns into a very asymmetrical ramp/saw-tooth when reducing voltage below Vref. The frequency starts to rise again before dropping off rapidly at lower voltages. I think I am fighting the input range of the TL072 here but still trying to understand how the components are interacting in the circuit.

A good read about this type of vco:

https://sound-au.com/project162.htm

Thanks for sharing! I referenced this article extensively just to get oscillation in the first place. The pull-down resistor off the base of the transistor was not in the original design and was the key to getting this to work at all on a single supply with a TL072.

I was quite taken with that circuit when I first saw it some years ago. What's not to love? A voltage controlled oscillator on a single supply.

My reaction exactly. As mentioned in my original post, I arrived at this circuit only as a means to make an LFO expression pedal value agnostic, so imagine my surprise when a found a VCO using about the same number of components and no "special parts".

I found that it was fairly tricky to get all of (1) symmetrical slopes (2) wide range (3) constant slopes over the range and (4) single supply at the same time. I even changed the bipolar to a MOSFET to try to get better and more repeatable slopes and wider range.

It's a neat circuit - but you still have to work for it. 

Unfortunately I am still very deep into this stage of discovering this circuit. Recommendations in other groups seem to suggest that going dual supply or modern rail to rail op amp will get this circuit where I want it with minimal grief. I don't want to move on without learning what I can about the limitations/solutions of the single supply/generic op-amp.

[Ohioisonfire]

It is possible to operate the TL072 with a single supply, but the oscillation frequency variable range is limited due to input limitations.
I tried it on such a circuit.
https://mrotqch.web.fc2.com/stomp/OSC/osc_test_40.png

	MIN.	MAX.
LT1013	6.88Hz	189.8Hz
TL072	76Hz	186.2Hz

Thank you for sharing this circuit! Finally getting around to taking a deeper look at it. Couple of questions if you don't mind...

• What is your supply voltage and control voltage with the quoted frequency ranges?
• What purpose do the diodes in the negative feedback loop of the comparator serve?
• I am curious about the Hysteresis on the comparator, the resistor values seem to set the threshold much wider than the other schematics?

[m4268588]

• PSU_volts = 9.9V
• The oscillation frequency depending on the maximum output voltage of Op-Amp and the power supply voltage. (It's different from a general function generator)
  Since the output waveform of the "sqr" is disturbed, it may be necessary to add a shunt diode.
  
  • https://mrotqch.web.fc2.com/stomp/OSC/osc_test_5P.jpg (without NFB diode)
  • https://mrotqch.web.fc2.com/stomp/OSC/osc_test_5Q.jpg (with NFB diode)
• Some resistance were selected according to the limiting(NFB) diodes. (hysteresis, pull-down of base pin at lower NPN...)
  If you do not need diodes, ignore these resistor values.