Very simple sawtooth VCO (V/Hz)

[-minus-]

CV is a triangle LFO. You can CV by taking a wire from the +V to a CV pot though if you just want to adjust the tone manually.

I've just got back to this circuit. Going to get it off the breadboard and onto some stripboard. Thought I might make two of them. Changing the cap value from 10n to 3.9n is getting a better range for me at the high end. I'll probably be adding a switch to make it LFO as well. 2.2uF seems to work ok. One could be fed into the other if you have two.

[tokyomatik]

i built a couple of this nice vco, now i would like to have both on a single module
with individual outputs and a common output, when i connect them to the "main"jack the wave is "stepped"....is it normal? disconnect the jack and individul output is ok....

due to the small parts and size,i would be great to make a super saw vco with 4 or 6 of these chained together

[oculus]

thanks so much.
iv´e been looking for a very simple vco for a while and this is the perfect as a addition to korg Ms10.
just spent 15 minutes and worked right away as soon as i sent some cv to it.
and btw i used tl072

would it be possible to make this work with 1volt/oct ?
thanks again

[ChrisLody]

Oh man, can't thank you enough for this Nicolas! I'm also using this to add to a Korg Ms-10, or at least i will be when i build it into a box properly, but at the moment its on a breadboard and sounding great. Seems to be most useful as a sub osc or matching the 32' octave on the Ms-10. Also makes a super awesome fast lfo for modulating cutoff etc. I see no reason that i shouldn't build a little Ms-10 expander with this and a few other modules in it. Bloody great!

Cheers!

[ChrisLody]

Works a treat with the Ms10! I added a fine tune knob to the design by inserting a 10k pot between the 100k pot and 68k resistor at the input.

After i'd built the vco into a faceplate i did a little experiment and fluked a square wave out of the circuit by connecting the saw out to a schmitt trigger inverter on a CD40106 chip. I didn't bother to do the maths as i haven't the equipment to measure the saw at the output but powering the CD40106 from 0v and +9v got me a 50:50 pulse obviously oscillating between 0v and 9v, nice and useful for some audio rate modulation! When i build it into the fascia proper i'll bring down the output to the ms10's 5v standard i reckon.

My enclosure needs work as it has no back yet. Also plan on building a +9/-9v power supply based on the one on MFOS.

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[nicolas3141]

Hi Chris

Yep you pretty much understand how it works. The first op amp is being an integrator which gives us the sawtooth ramp, the slope being proportional to the CV. The 1N4148 diode discharges the integrator to create the flyback of the sawtooth. The second opamp is working as a comparator and the back to back LEDs or diodes provide a voltage drop in both directions which turns the comparator circuit into a lazy comparator - a schmitt trigger - where it doesn't switch over until the voltage difference exceeds that drop. That laziness in the comparator allows the sawtooth to swing back and forth between the two voltage limits created by the voltage drop of the back to back LEDs. So they determine the output level of the oscillator. The 47k resistor just feeds some current through the back to back diodes because LEDs don't have a voltage drop unless there is at least a little bit of current flowing through them.

You mention adding a comparator on the output to give you a pulse output. This is fun. If you make the other input of the comparator a CV input and feed it from an LFO or ADSR you can get that classic PWM sound happening.

Hope this helps,
Nicolas

[kjackman]

I've built this on the breadboard. I only get an audible signal when 8.20v < CV-in < 9.28v. Is this expected?

I want to build an AF function generator out of this, so I'm looking for the optimal CV range to supply. (Not an accurate function generator, mind you - just something handy for quick experiments.)

[Edit] NEVER MIND - I hadn't tied pin 3 to ground - so that input was just kinda floating around in outer space. Problem solved.

[ChrisLody]

Thanks for the reply Nicolas3141, i can't believe it has taken me over a year to realise you'd replied to my post on this thread

Thanks for the explanation, that's a great help. I've learned a bit more about op-amps since i built this so its all starting to make sense. I Ended up adding the second op amp to make a pulse output and even realised i could have a movebale pulse width by having an adjustable voltage on the other pin. Didn't think about making it a CV though. Damn.

Here's what i did back in Jan of last year

http://www.youtube.com/watch?v=Bo3ZHFr57_M

[isak]

Sorry for the newbi question..
C2 is connected between +9 and -9?
I just dont see it on the stripboard layout.
Thanks

[nicolas3141]

Yes, C2 is missing from the layout. Just add it across the chip or wherever is convenient. Sorry about that

[isak]

Thank you Nicolas

I hope to build like 6 of them to have super saw.
I have this crazy idea to chine between all 6 so one saw will modulate the second and the second will modulate the third and so on...
This will be a kick ass super saw vco

[jackdamery]

Hi, I've tried this a couple of times and cant get any sound out. I noticed the LEDs don't light up and the batteries are getting really hot, perhaps suggesting a short. Just to clarify i have two 9v batteries with the ground of one and the positive of the other joined together as my 0v bias. Does anyone have a photo of this on breadboard they could show me?

[jackdamery]

Also, it looks like pins 3 and 6 should be joined as there is no cut on the stripboard on that line.

[isak]

As you will see in the scheme pins 3 and 6 are connected to 0V (ground).
This is why there is no cut.

[jackdamery]

Yeh, I've correct that. Still nothing Can you send me a pic of your circuit please?

[isak]

I'm sorry, I didn't built that one yet.
I'm planing to do it in the near future.
Did you tried it with power supply?
Did you chain the battery's like this...
http://www.musicfromouterspace.com/analogsynth_new/SOUNDLABMINISYNTH_PLUS/pdf/slmsplusbatteryconnections_assembly.pdf
Did you put C2 (220nF) between the power rails (+/-)?
Did you checked orientation of the LEDs and diode?
A pic of the strip won't help you much, afaik every one that built it did the same like the stripboard shows.
very minimal components, no reason to have mistakes unless you really shorted something strips.
Did you tried with breadboard?

[jackdamery]

Yeh, I checked all these things. Started from scratch twice over, still no luck. I am building it on breadboard yep.

[isak]

Maybe it's a bad chip?
Did you tried replacing the lm358 with a another lm358?

[nicolas3141]

Just a possibility - sometimes if you don't get a circuit right first time when you power it up, you can blow something, then even when you correct the layout it won't work right. Something is definitely wrong if the batteries are getting hot. If you can't find a short, then it probably means the chip is fried. If you have a multimeter and/or oscilloscope, set it to measure voltage and put earth probe on 0V. Then go round all the pins on the chip. Only 3 and 6 should be exactly zero. And the rest should all be different from each other. Any two pins other than 3 and 6 that read identically should point you to your short. But the short may be internal to the chip if it is dead.

[isak]

Hey Nicolas.

I have a Q...
Is it possible to make it a full range 1V/oct? (or is it 1V/Hrz if I recall?)
Is it to much trouble?
I have some spare 1k Tempcos and a wide value range of mica's if needed.

If not, it's ok, it's special as it is

[nicolas3141]

This circuit is roughly V/Hz, but not very linear because we are using op-amps and the flyback is slow because it is constrained by the bandwidth limiting internal to the op-amps. And we are not doing anything special in terms of the timing capacitor or the triggering diodes so you get all kinds of inaccuracies creeping in that adversely affect tracking.

You could work on improving all that, plus adding a V/Octave converter circuit on the front end, but it would kind of go against the spirit of what we are aiming at here. If you want accurate tracking look at one of the circuits out there that already offers that and either use it as-is or adapt it to your needs. Or if you want completely bizarre response then look at a 555 or 40106 based circuit. This circuit (and the rest of mine) is ideal if you want something that tracks vaguely, but not accurately. Does that make sense?

If you want to convert it to vague V/Octave response (ie not accurate and not worth wasting tempcos on) then the simple V/Oct converters that are often used on the front of VCF circuits could be adapted to this. I haven't tried, but it probably wouldn't be too hard. One or two op-amps and one or two transistors would be all it would require.

[isak]

Yes, I know what you mean about keeping the spirit behind the creation.
You probably right, I leave it as is, it's special enough.

Thank you Nicolas.

[jackdamery]

Hi Nicolas. Here are the measurements I took with a fresh 358. Still not working

Pin 1. 6.07v
Pin 2. 0.04v
Pin 3. 0.02v
Pin 4. -7.90v

Pin 5. 6.12v
Pin 6. -0.00v
Pin 7. 6.40v
Pin 8. 7.65v

[-minus-]

I guess you have the diode placed between PIN 2 and PIN 7?

Can you upload a photo of your breadboard? It may be easier to spot what is going on.

[jackdamery]

Hey its working now! It was because I hadn't connected pin 3 to ground as it's not immediately obvious when translating the diagram to breadboard.