Simple Glide/Slew/Lag/Portamento..

[j.dilisio]

Here's a beginner question...
Someone gave me the following schematic for a simple glide circuit.
Should this work?

I tried it and found that while it does add some Lag to a CV signal, it also attenuates it.
I'm using a 100k pot and a 1uF cap just because that's what I had lying around so there's not much lag effect.

I've also attached an image of my attempt to test it.

Any ideas?

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[Tim Servo]

Hey JD,

Well, that circuit is really close. What you need to do is add an op-amp buffer to the input, and another one to the output. The idea behind the buffers is that they isolate the circuit. Otherwise, the load that the glide circuit places on your CV source changes a lot depending on what the glide setting is.

What you're looking for is a "voltage follower" or "unity gain buffer." The circuits are REALLY simple, and you can get a single 8-pin IC that contains the two op-amps you'll need. Of course, you're going to need a power supply, but that's something you'll need for almost any synth circuit. Do a Google search for those buffers and I bet you'll find a ton of useful info.

Tim (sometimes useful) Servo

[synthmonger]

I'm not sure how you have your jacks grounded but they don't look like they have wires tied to ground. Don't forget those!

[j.dilisio]

well heres another question..
If this works I'm planning on ultimately using Banana jacks hense the lack of ground to the jacks.
I've never tried Bananifying anything before, but thats how it works right?
You just don't ground the jacks?

[synthmonger]

Right. Though it never hurts to have a banana to 1/4" 1/8" inch jack module. ;D If you do plan on running the banana output to a computer/amp or whatever then you must have a common ground obviously.

[j.dilisio]

ok, thanks,.
I think I'm going to go with the glide circuit that's attached.
It's taken from Ken Stone's Psycho LFO.
This one makes more sense right?

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[widdly]

not quite. The first opamp is non-inverting, and the second opamp is inverting. So you end up with an upside down cv. If you ditch the 100k pot and wire the first opamp like the second you will be in business.

[fonik]

[j.dilisio]

Thanks for the help.
One last question..
Is it the capacitor value or the potentiometer that would increase/decrease the range of glide? or both?

*edit. I just saw fonik's post. I'll probably just be using one input. Is it better practice to do it the other way with both inverting?

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[synthmonger]

The cap size controls how long the sampled voltage or 'glide' will last while the potentiometer just fine tunes it. With a 1M pot and that cap size you'll have a nice range.

[fonik]

[haxster]

I like this one, it's very simple
This one has (2) outputs.
You could just use (1)

http://electro-music.com/forum/topic-32272.html

[widdly]

Why do you need an opamp buffer for each output? I would have thought you would be better off using one of those buffers on the input.

[Dave Kendall]

Hi guys.

The one below from Osamu Hoshuyamas "lunchbox synth" has worked well for me. It's buffered with voltage followers both on input and output, and is non-inverting. R2 doesn't have to be 1K2. 1K is fine. I didn't wire up or test the switch, but it looks useful.

cheers,
Dave

(edited because *ahem* I just checked, and the lockbox synth is a Scott Bernadi synth, Sam's is a lunchbox modular.... Oh well....it was a nice day when I posted, I had a beer ...
Glad the circuit was of use to you Creekree - it was my first ever perfboard circuit, used to fix up a paia midi2CV8's CV zipper noise, which it did nicely using a 62K resistor instead of a 1M pot... )

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[creekree]

Dave,

thanks for posting that.
I quickly soldered it on stripboard - works like a charm!

Chris

[j.dilisio]

I'm having a little trouble getting this to work.
The CV signal I'm feeding it ranges between .5 and 3 volts.
The output from the first op-amp(pin1) is around 10.8V and stays around there for the rest of the path so the output frequency is really high.

I figure there's either something wrong with both of the LM358's I've tried,. or I've most likely made a really dumb mistake with the wiring.

I've attached a picture of the breadboard.
Any suggestions?

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[haxster]

From looking at the schematic above, you have the inversions wrong. (-)inv should be pin 2, pin 3 is (+)non-inv. Op-amp (B) has the same label error. Also you do not have a freedback resistor or jumper on Op-amp (A).

[Tim Servo]

Whoops, you've got that first op-amp wired as a comparator. When the input goes just a teeny bit above or below ground, the output will jump between (about) +12 and -12V. No in between for the output of a comparator. If you look at the diagram above, you'll see there's a feedback path on the op amps from the output back to the "-" input. What you want is to have the first op-amp wired as a buffer or "voltage follower."

Tim (just a bit above ground) Servo

[j.dilisio]

Thanks for replying so quickly!
It looks like I forgot to switch the + and - when I switched the numbers on the first op-amp. The second one should already be correct.

So all I'm missing is the feedback jumper.
Which of the following two schematics is right?
Does the jumper always go back to the - or whichever is the input?

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[j.dilisio]

I just noticed that pin 4 is marked as ground on the data sheet.
Should I go by that or does it matter?

[j.dilisio]

After some fiddling around I got it working.
I'm using a linear pot right now but it seems like a log pot will make a world of difference in controlling the subtle effects.

One last question..
I'm still getting a slight change in pitch when I turn the glide up.
It seems like the effect is too heavy for the CV and pulls it down about a quarter tone.
Are there any fixes for this?

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[Tim Servo]

Okay, good work, you're really close now. What you need to do is remove that connection to ground on the + input of the first op-amp. My guess is that's what causing the glide circuit to load down whatever is hooked to its input and mess up the output. Also, the LM358 can be powered by +V and Ground or +V and -V. My choice would be +V and -V (or "bipolar supplies") because then the glide circuit will be able to process any negative CVs that come its way.

Tim (wasn't there a dance named "The Glide"?) Servo

[j.dilisio]

Well now its gliding along just fine..
Thanks for your time Tim.

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[LetterBeacon]

I'm thinking about building that circuit above - when the Glide pot is turned completely CCW, does that mean there is a 0 amount of glide, or is there still a little bit of glide going on?

Thanks!

[j.dilisio]