change LFO +/- modulation to + only (like ADSR)?

Ojd · Joined: Feb 22, 2008 Posts: 66 Location: Ua Audio files: 1

hello. I've build LFO for my synth, but i'm somewhat disappointed with the overall feel. +/- modulation feels very unnatural to me (+5V/-5V), + only modulation (like 0/10v) would be much preferred. Bipolar modulation on VCA really annoys me, as i cannot fade tremolo slowly using only LFO output attenuator. So is there a way to change that? I'm sure there is - i just need to offset LFO's output, right? (but I need schematic)

bubzy · Posted: Wed Sep 14, 2011 5:26 am Post subject:

what LFO did you build? do you have a schematic for it? this might help a bit.

Tim Servo

Hey Oj,

There are a couple of different ways to do this. I did a search for for "non-inverting op amp with offset" and came up with a circuit that I tweaked for +/-5V in and 0 to +10V out. This should get you pretty close. Wink

Tim (tweaked) Servo

non-inv_op_amp_with_offset.jpg

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Thomas_Henry · Posted: Thu Sep 15, 2011 9:05 pm Post subject:

Hi Gang,

Let's get rid of a resistor. See the attached schem.

Actually, you can get it down to only four resistors if you're willing to put up with odd values. The five resistor version shown here uses very ordinary values however and is dead-on for accuracy. You'll all recognize it as a variation of something which appeared in my VCO Chip Cookbook.

But if your original LFO is of the Schmitt trigger/integrator type, then you can jettison all of this and get what you want with no additional parts. That topology lets you easily set the center line and amplitude. I did this recently in a new VCO design in which I needed a 0 to +5V output---I changed a few resistors and away I went!

Thomas Henry

Shifter.jpg

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Reduced Parts Count Version

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Ojd · Joined: Feb 22, 2008 Posts: 66 Location: Ua Audio files: 1

Thank you all for the reply.
Here's the schematic.
Posted Image, might have been reduced in size. Click Image to view fullscreen.

And output voltage is not 5v bipolar.
But i've found this: http://www.musicfromouterspace.com/ELECTRONICBUBBLEGUM/TOOLBOX/toolbox.html
(thanks Servo for the keyword tip, didn't know how to put it into google)
For the lfo inversion doesn't mean much, so i'll stick with MFOS version (though i need to reverse LED on the panel)

Thomas_Henry · Posted: Sun Sep 18, 2011 12:45 am Post subject:

Hi again,

Okay, here's the usual Schmitt-trigger/integrator with the comparator input offset as I mentioned earlier. As shown, this will put out a 0 to +10V triangle instead of a +/-5V signal. The alteration is easy and if you really need precision for the baseline, just replace the divider with a trimmer.

As I guessed, the circuit the original poster asked about is of this type. Obviously, this mod can be done with the Woollaston schem posted above. But Peter Lutz's original circuit, which predates the one above, is more versatile since it allows variable offset. You'll find it in Electronotes, Volume 10, Number 88, April 1978, pp. 24, 25.

By the way, the LFO shown here has a 1000:1 range which I suspect is greater than either the Lutz or Woollaston circuits.

Thomas Henry

LFO2.jpg

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0 to +10V LFO

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