VCLFO ic or easy circuit?

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floris
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VCLFO ic or easy circuit?

Post by floris » Wed Jul 12, 2017 6:19 pm

Dear,
for a new project I need around 40 VCLFO's.
With TRI of SINE outputs.
Is there a small and simple circuit around?
Or an that has this IC?

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Post by nigel » Wed Jul 12, 2017 7:12 pm

What about this? viewtopic.php?p=2279405#2279405

Each of those little boards has two VCOs, using a 13700, 074, and 072. Triangle outputs. I can send you the PCB details if you like.

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Post by Paul Perry » Wed Jul 12, 2017 8:39 pm

Agree, the simplest "VCO sin plus triangle"is a LM13700 triangle oscillator as per data sheet for the triangle, followed by an overdriven LM13700 section for the sine.

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Post by SoundPool » Thu Jul 13, 2017 5:49 am

Do you need them to be analog? I'm not so computer/digital savvy but I imagine that it might be easier and more cost effective to whip up a design using something like a Teensy/Arduino/Pi with some I/Os?

There is also the Electric Druid VCLFO - in some ways its overkill for what you want (maybe you would find the extra features helpful?), but depending on your total design and implementation maybe it makes sense. Def not as cheap as bulk op-amps, but maybe you could get a price break at 40 pieces if you ask nicely?

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Post by floris » Thu Jul 13, 2017 9:15 am

Thanks guys!
Nigel, it would be great to have the PCB!

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555x555
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Post by 555x555 » Thu Jul 13, 2017 11:16 am

There are two very simple circuits for this. First, you use an LM13700 looped into a shmitt trigger. You can use the buffer on the LM13700 since the amplitude switch point is controlled by the trigger, not the LM13700. This uses a single opamp and 1/2 LM13700, so with a dual opamp and a LM13700 you get 2x LFOs. The second circuit uses three opamps. One opamp is wired with an FET to invert depending on the FET input. The fet input is fed from a shmitt trigger, which is fed from an integrator, which is fed from the conditionally inverting opamp. If you don't have a V-to-I converter in mind, this is simpler.

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555x555
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Post by 555x555 » Thu Jul 13, 2017 11:22 am

Also, 40 is a lot. You have piqued my curiosity. What are you planning?

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Post by Starspawn » Thu Jul 13, 2017 1:58 pm

Moog source LFO is nice and gets you an added vca as well for the other half of lm13700 or just two VCLFOs.

Image

so a lm13700 and tl074 with 6 transistors gets you x2 as previously pointed out. [/img]

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Post by floris » Fri Jul 14, 2017 2:20 am

I'm building a kind of loudspeaker orchestra and want to control all the speakers separately in multiple ways.
[/img]http://hetbos.be/project/69

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Post by Jarno » Fri Jul 14, 2017 2:38 am

It is probably easier now than it was in 1958 :D

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Navs
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Post by Navs » Fri Jul 14, 2017 3:05 am

If the LFOs have to be analogue and you need that many, how about very simple CMOS inverter oscillators + filtering?

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Post by flts » Fri Jul 14, 2017 3:51 am

http://electricdruid.net/product/vclfo-10/

digital, but i don't think analogue was demanded, and that's voltage controllable + has sine and triangle outputs.

buying 40 of them will be somewhat expensive but probably not more expensive than building 40x of any of the designs mentioned so far.

you could probably run 40 separate tri/sin lfos easily with one of the newer teensys for example, but you'd still need enough ad/da to interface them and probably some protection and level shifting...
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Post by nigel » Fri Jul 14, 2017 7:58 am

Here are the PCB and component overlay diagrams for my dual VCO board. It's basically the same as the nonlinear circuits Dual OTA VCO circuit, but stripped back to suit my requirements (triangle output, three expo VC inputs, suitable for home PCB etching, and compact). I've made two drone modules with them (one with two modules / four oscs, one with three modules / six oscs) and they track reasonably well over at least a few octaves. For an LFO, you'll need to increase the timing capacitor - I'm not sure what effects that will have.

Some notes: the board has five wire links on the component side, one running underneath the 072. The 1k resistor between the two transistors is a tempco, using a regular resistor may or may not make a difference. The 68k resistors which set the minimum frequency were a little low for audio, and I changed them to 150k in my builds. The blank square pads are for 100nF SMT caps across the power rails. The inputs and outputs are all on one side of the board - I put connectors on that side to mount multiple boards onto a backplane.
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tojpeters
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Post by tojpeters » Sat Jul 15, 2017 1:49 pm

The MFOS VCLFO meets most if not all of your requirements.
I'm not sure of you would find it small/cheap enough but it does have triangle and sine outputs and VC control.
The PCB artwork is on the website if you want to try and make your own maybe a bit cheaper. Two sided but the bottom has very little on it and you could probably just use links.

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Re: VCLFO ic or easy circuit?

Post by donalddokken » Fri Feb 12, 2021 7:13 pm

hi -
i came to this thread because i was looking for a simple VCLFO schematic and sure enough there are some posted.
however, having experimented with both the lm13700 datasheet schematic and the marjan moog vclfo circuit, i am slightly perplexed.
i am not getting a triangle output. looking closer at the schematics, i wonder why i would expect to get a triangle output. there is only one capacitor in the design and it seems to primarily change the frequency. i'm not sure where the oscillation is being shaped into a triangle - normally, wouldn't there be an integrator stage?
i was hoping someone could either confirm a design error or let me know that the error was my own.

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Re: VCLFO ic or easy circuit?

Post by donalddokken » Fri Feb 12, 2021 7:26 pm

noticing now that changing the capacitor value to much smaller value (as in the 137000 datasheet) produces a triangle-like waveform - but it is at the cost of the low-frequency oscillator.

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Re: VCLFO ic or easy circuit?

Post by bgreeves » Fri Feb 12, 2021 7:54 pm

donalddokken wrote:
Fri Feb 12, 2021 7:13 pm
i am slightly perplexed.
i am not getting a triangle output. looking closer at the schematics, i wonder why i would expect to get a triangle output. there is only one capacitor in the design and it seems to primarily change the frequency. i'm not sure where the oscillation is being shaped into a triangle - normally, wouldn't there be an integrator stage?
(for reference, I'm looking at the circuit Starspawn posted. looks about the same as the datasheet circuit).

The capacitor at the output of the OTA is acting as an integrator, if I understand correctly. The OTA converts the input voltage (differential voltage between + and - pins) to a current, with the gain proportional to the control current Iabc.

So, when the voltage on the + pin of the OTA is positive, the OTA outputs a positive current proportional to Iabc. The more current, the faster the cap charges.

When the cap charges to a certain point, the inverting comparator (U2B) flips its output negative, which feeds into OTA+, and so OTA starts sinking current proportional to Iabc. That gives you the falling part of the triangle wave.

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Re: VCLFO ic or easy circuit?

Post by donalddokken » Fri Feb 12, 2021 8:09 pm

So, that would mean that the shape and the frequency were interdependent?

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Re: VCLFO ic or easy circuit?

Post by bgreeves » Fri Feb 12, 2021 8:17 pm

donalddokken wrote:
Fri Feb 12, 2021 8:09 pm
So, that would mean that the shape and the frequency were interdependent?
Hmm, I'm not sure where you're getting that. The waveshape should be a triangle no matter what frequency is set by Iabc. The OTA outputs a constant amount of current, and a constant amount of current charges a capacitor linearly.

It is curious what you described about the waveshape not being triangular when you had a larger capacitor in there. Could you be more descriptive of what type of shape you were seeing? What was the value of the cap? Were there any other modifications you made from the datasheet circuit?

Caveat emptor: I'm not a god at this and am just feeding out of my own limited experience.

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Re: VCLFO ic or easy circuit?

Post by guest » Fri Feb 12, 2021 8:29 pm

how are you viewing the triangle wave? with a scope? is the scope set to DC couple?
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donalddokken
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Re: VCLFO ic or easy circuit?

Post by donalddokken » Fri Feb 12, 2021 8:47 pm

I'm definitely not a god at this either, so thank you for your feedback.

About modifications to the circuit, I removed everything after the 6.8k resistor at U2B since I'm not currently building the VCA or the square wave. I'm also using a simple 100k pot as a voltage divider through a 10k resistor to Iabc, instead of the whole transistor stage, since my understanding is that is only to produce an exponential response.

Changing the 12k resistor to 100k gives me the low-frequency response that I'm looking for...but the shape is still un-triangular.

Attached is an image of the scope.

If you've built this and it works as expected, it sounds like I must be doing something wrong.
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Re: VCLFO ic or easy circuit?

Post by donalddokken » Fri Feb 12, 2021 8:48 pm

Scope is set to AC-Coupling. When switched to DC, it looks like a square wave.

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Re: VCLFO ic or easy circuit?

Post by guest » Fri Feb 12, 2021 9:00 pm

which point are you probing? if set to AC couple, it wont be able to pick up really low frequencies.

[EDIT] you should be looking at the output of the buffer that directly follows the capacitor.
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Re: VCLFO ic or easy circuit?

Post by donalddokken » Fri Feb 12, 2021 9:16 pm

Ha! I was monitoring at the wrong buffer output. That's some impressive invisible trouble-shooting. Thank you.

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Re: VCLFO ic or easy circuit?

Post by guest » Fri Feb 12, 2021 9:24 pm

no worries, glad its working!
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