Simple Glide/Slew/Lag/Portamento..

LetterBeacon · Joined: Mar 18, 2008 Posts: 439 Location: London, UK

Thanks for the reply!

Just to be 100% clear -I should be able to hard wire this to the CV Out of a sequencer PCB and, with the Glide pot fully CCW, it would be as if there's no Glide circuit in there at all.

Sorry to state the obvious but I want to be absolutely sure as I'm designing the front panel at the moment.

j.dilisio · Joined: May 19, 2009 Posts: 186 Location: baltimore

Thats how it works over here. You could try it first before commiting.
And you can always substitute a different circuit later if you're not happy.

Mooger5 · Posted: Wed Oct 07, 2009 7:01 pm Post subject:

That´s logarithmic portamento, right? I found this circuit online for linear. Didn´t try it yet.

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Tim Servo · Posted: Thu Oct 08, 2009 2:48 pm Post subject:

Hey Mooger,

Yes, that circuit should produce a linear glide instead of expo. It comes from putting the cap in the feedback loop of the output buffer op-amp. A triangle core VCO uses the same arrangement to get linear slopes. I've also done the same thing with envelope gens to get linear slopes instead of the more common exponential slopes (linear slopes work nicely with expo VCAs, while expo slopes are typically used with linear VCAs). You'll probably need to use a bigger timing cap with the linear setup, as the timing won't be the same.

Tim (hittin' the slopes) Servo

Adam-V · Posted: Thu Oct 08, 2009 8:55 pm Post subject:

Mmmm trapezoid generator!

Cheers,
Adam-V

Mooger5 · Posted: Thu Oct 08, 2009 10:39 pm Post subject:

Mooger5 · Posted: Fri Oct 09, 2009 3:00 pm Post subject:

I think I grok the diode for the optional asymmetric portamento: as it is, it´d bypass the resistor at positive voltage changes, so the circuit would perform only "downward" portamento and "upward" by reversing the diode?
If that´s so, could it also work as saw/ramp waveshaper?

Tim Servo · Posted: Sat Oct 10, 2009 12:02 pm Post subject:

Hi Mooger,

The glide in the expo circuit should start quick and then slow as it gets closer to its 'destination' voltage. The output will look a lot like the typical envelope shapes you see (kinda like a shark's fin).

OTOH, with linear glide you get a constant RATE, but not a constant time. In other words, linear glide doesn't start quick and then slow down. A linear glide from C1 to C5 will still take longer than a linear glide from C4 to C5. Getting a glide that was a constant TIME would be extremely tricky. You'd have to have a VC glide and some circuitry that would compare the start and end voltages and produce a CV that was proportional to the difference. It could be done, but this is the sort of thing that would be much easier with a digital synth or microprocessor-based circuitry. If one of the buffers was out of phase (non-inverting, while the other was inverting), I don't think the circuit would work. The feedback going from the second op amp back to the first is what keeps the output stable once the output voltage = the input. Might be worth playing with if you have this up on a breadboard. You might want to put a small value resistor (<1k) after the glide time pot. This acts as a current limit. Look up some other glide circuits on the web, or check out Barry Klein's "Electronic Music Circuits" book for more info:
http://members.cox.net/barryklein/em.htm

BTW, You might want to verify this, but I recall the Minimoog uses a linear glide, while the ARP Odyssey uses an expo glide.

And yeah, on the Asymmetrical glide, the trick with the "steering diode" is the basis of most of the Tri/Saw variable waveshape oscillators you see around. You could use two diodes and put a glide pot in series with each one to get separate up and down controls. Now you've got a circuit that is roughly the same as an AR envelope gen. It's funny how many synth circuits depend on the basic lin or expo integrator circuit (glide, envelope gens, LFOs, even VCOs). Check out Ken Stone's "Utility LFO" for an LFO example:
http://www.cgs.synth.net/modules/cgs58_lfo.html

Tim (integrating the flux capacitors) Servo

Mooger5 · Posted: Sat Oct 10, 2009 5:45 pm Post subject:

Thank you for taking the time to reply, Tim.

Looks like I mixed up the nomenclatures while reading elsewhere about the lin and expo glides properties!
So the expo simple design is somewhat compromised by the non-linear response of the cap and the lin compensates it with some sort of "negative feedback" loop...
I´ll experiment with the following:
a) a positive feedback loop; b) a "two-pole" expo; c) audio through the lin; just to see what the heck comes out of it Laughing

You´re right about the Mini and the Odyssey. It´s common knowleadge, I think. Been checking a few schematics and the Ody is definitely expo. The Mini features a 1uF grounded cap, but there´s two jfets for up and down glide; too complex for me to understand.
The best news is the expo glide in my Rogue is very much like this simple one. There is no first buffer, but the second is a single dedicated IC. Hoping it´ll sound more "Minish", it´ll be a matter of replacing the single IC with a dual, cutting a few tracks and using the existing components to turn into a lin glide Smile

Thanks for the Klein book tip.

Mooger5 · Posted: Sat Oct 10, 2009 7:33 pm Post subject:

Spacebones · Posted: Wed Jan 30, 2013 7:12 pm Post subject:

I'm having an issue with this glide for some reason. When I feed it square waves, it doesn't seem to work at all... I set it up such that the output of a 40106 oscillator is connected to the input of the glide circuit, and the output of the glide is connected to an led so that I could see visually if the glide is working. When I give it power, the led just stays lit, and doesn't oscillate between lit and off.

However, I tried putting the 40106 output into the first input of a little R2R DAC, and fed the output of that into the glide input. At this point, the led modulates like I had hoped it would with the oscillator alone. What gives? I'm using a TL072 in the glide if that makes a difference.

Any insight would be excellent!

[EDIT] Nevermind, apparently putting some resistance (1 meg) between the oscillator and the input did the trick... Can anyone explain to me why that is?