Simple sine wave lfo

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Waronheaven
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Simple sine wave lfo

Post by Waronheaven »



Hello everyone, I need to make a sine lfo with just speed and maybe depth, but can’t really achieve it, I’ve tried to use a fixed voltage OTA with a triangular wave in but comes out squarish, anyone can maybe help me with this?


Thanks in advance
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Re: Simple sine wave lfo

Post by jorg »

Looks like a decent triangle there. Your image is way too small to read though.
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Re: Simple sine wave lfo

Post by Waronheaven »

jorg wrote: Wed Jan 12, 2022 9:41 am Looks like a decent triangle there. Your image is way too small to read though.
I have a decent triangular, but I need a sine out.
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Re: Simple sine wave lfo

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A web search with the keywords "sine shaper circuit" will give you various solutions.
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Re: Simple sine wave lfo

Post by JacoPastorius »

I haven't done it this way myself, but what I know about using an LM13700 as a sine shaper is that you need to heavily attenuate the input signal so that it is swinging within the non linear region, providing a more sine like shape at the output. Getting a square output suggests that you are hitting the OTA much too hard and are just getting clipping. Also it is hard to see from the photo of your computer screen, but did you forget to hook up ur LM13700 to the negative supply rail? There are other sine shaper circuits using cheaper and simpler components, like Jfets or 1n4148 diodes. The buchla sine shaper, for instance, uses a jfet. Also if you're using a lm13700 you might as well use the second channel to turn your LFO into a VCLFO
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Re: Simple sine wave lfo

Post by pugix »

The Synthesis Technology MOTM-390 has a VC LFO with sine wave. You can download the build manual from Synthcube which includes the schematic you can study.

https://synthcube.com/cart/makers/synth ... -micro-lfo
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Re: Simple sine wave lfo

Post by fg »

2 x simple tri 2 sine schematic
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http://pjmnc.free.fr/files/tri2sine.bmp
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Re: Simple sine wave lfo

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The PAIA 2720-5 control osc module uses a different classic means to give a variable sine directly.
Here's its build manual w schematic and circuit explanation. Note the limited range of this approach.
Which is why most modern SIN oscs use waveshaping or SVF -both with their own limitations- to do this.

The old HP audio oscs -famously used in Disney's Fantasia and HP's seminal sale- are similar to the PAIA, but use a more complex circuit. And an incandescent bulb for limiting that allows them to have a larger range than the el cheapo goal of PAIA would allow.
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Re: Simple sine wave lfo

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notmiserlouagain
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Re: Simple sine wave lfo

Post by notmiserlouagain »

KSS wrote: Wed Jan 12, 2022 3:15 pm Which is why most modern SIN oscs use waveshaping or SVF -both with their own limitations- to do this.
Short OT: What would be the downsides of using SVF for sine vco, besides the added complexity?
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Re: Simple sine wave lfo

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Slow rates can be hard to achieve.
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Re: Simple sine wave lfo

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notmiserlouagain wrote: Wed Jan 12, 2022 4:47 pm
KSS wrote: Wed Jan 12, 2022 3:15 pm Which is why most modern SIN oscs use waveshaping or SVF -both with their own limitations- to do this.
Short OT: What would be the downsides of using SVF for sine vco, besides the added complexity?
At very low rates, amplitude and frequency accuracy can get pretty bad.
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Re: Simple sine wave lfo

Post by notmiserlouagain »

Ok, thanks, KSS and jorg!
My favourite is Ian Fritz´s 6/8 quadrature vco, but not exactly simple...
PS Just to mention it (haven´t built but idea sounds interesting): MSkala´s sine wave bank
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Re: Simple sine wave lfo

Post by guest »

do you want CV control of amplitude and rate? does it need to be volts per octave on the rate?
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Re: Simple sine wave lfo

Post by JacoPastorius »

notmiserlouagain wrote: Wed Jan 12, 2022 5:02 pm Ok, thanks, KSS and jorg!
My favourite is Ian Fritz´s 6/8 quadrature vco, but not exactly simple...
PS Just to mention it (haven´t built but idea sounds interesting): MSkala´s sine wave bank
I really like MSkala's designs and the technical information that he provides in his documentation is undervalued in the sdiy community IMO. His sine wave bank uses inductors and cannot be frequency modulated iirc. definitely still worth looking at though
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Re: Simple sine wave lfo

Post by guest »

i sketched up a quick, low parts count LFO with CV control of ampiltude and frequency, and switchable square/sine:
https://tinyurl.com/yc4zfb64

theres a spare opamp in there if a quad package is used, so some room for embelishment if desired. the 100nF would need to be increased (100nF made the time scales of the simulation work out), and a trimmer or two might be needed to get the sine right where you wanted it.
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notmiserlouagain
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Re: Simple sine wave lfo

Post by notmiserlouagain »

Very nice and effective. Sine shaper is modulation depth too. And simple.
Two small questions:
not acquainted with simulations, why is there an ac-(tri)-generator going into the Iabc input on the OTA before the integrator?
Any reason this could not be used in the (audio-)VCO range?
Last edited by notmiserlouagain on Sat Jan 15, 2022 9:27 am, edited 1 time in total.
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Re: Simple sine wave lfo

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Waronheaven wrote: Wed Jan 12, 2022 9:51 am
jorg wrote: Wed Jan 12, 2022 9:41 am Looks like a decent triangle there. Your image is way too small to read though.
I have a decent triangular, but I need a sine out.
I can't read the shaping circuit in your picture either, but is it possible that it's just massively overdriven? Hence the squarish shape. :hmm:

My favourite for this is the sine shaper ascribed to Thomas Henry (I think there are previous similar setups):

https://www.birthofasynth.com/Scott_Sti ... ne_Shaper_

It works well but might be overkill for your circuit (unless you want a nice sine). You could look at the sine-ish shaper used on the A-110 here, diodes as suggested by Jorg:

https://doepfer.de/a110_tec.htm

Our Tim Stinchcombe has given the subject some thought here:

https://www.timstinchcombe.co.uk/index.php?pge=a110

https://www.timstinchcombe.co.uk/index.php?pge=trisin

I know this is getting long, but just for completion ;) you can also get an amount of rounding from simple wavefolder circuits. So, if you fancy making the LFO more complicated, you could get a sine and beyond with something like the Lockhart simple wavefolder or a stage or two of Serge Middle Wavefolding:

http://www.synthpanel.com/modules/cgs52_folder.html
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Re: Simple sine wave lfo

Post by neil.johnson »

Waronheaven wrote: Wed Jan 12, 2022 8:25 am Hello everyone, I need to make a sine lfo with just speed and maybe depth, but can’t really achieve it, I’ve tried to use a fixed voltage OTA with a triangular wave in but comes out squarish, anyone can maybe help me with this?
What frequency range do you want?
How often will you be changing the frequency?
What output waveforms, other than sine, do you want?
Do you want voltage control of frequency, and/or output level?
How good a sine are you looking for?

For audio range the classic Wien-bridge-based oscillator is hard to beat: very simple, very good THD, but at low frequencies the amplitude tends to bounce when you adjust the frequency (there are tricks to get round this, but the end result is it takes a while to settle at very low frequencies).
State variable approach is a good way to make a VCLFO. For lowest distortion you use the bandpass tap to control the amplitude, but again that can take time to settle. Most of the analogue audio distortion analysers (HP, AP) used this technique to get very low THD.
If you want to go very low in frequency but less concerned with sine shape then diode limiting gives you a reasonable shape and fast frequency change.
Here is one example I designed a few years ago: https://www.njohnson.co.uk/index.php?me ... bsubmenu=2
Which demonstrates two sine generation techniques: a three-phase SVF-based diode-limited ring oscillator, and a triangle-with-dode-shaper oscillator.

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Re: Simple sine wave lfo

Post by Waronheaven »

thanks everyone for the help!
at the end i went for a triangular sincei need for modulation the result is not too different.
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Re: Simple sine wave lfo

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:tu:
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Re: Simple sine wave lfo

Post by KSS »

Waronheaven wrote: Sun Jan 16, 2022 5:10 am thanks everyone for the help!
at the end i went for a triangular sincei need for modulation the result is not too different.
The ARP3620 delayed 'sine' uses a very simple 'sine' converter of two back-to-back diodes. This only soft clips the top and bottom off the triangle so it's not anything near a true sine and much like your use it's intended as a sine-ish modulation source.
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