Nautilus vibrato???

[WorkBench]

I built the nautilus from madbean and it sounds good, but when I let a note or chord ring out there is a vibrato like effect?  I suppose the envelope filter itself keeps opening and closing real fast?  I feel that it has something to do with the gain/sensitivity part of the circuit?

Any thoughts?  Thanks.

Chris

[Mark Hammer]

That's envelope ripple.  You can reduce it by either shortening the decay, so you don't year it, or lengthening decay so it's smoothed out.

[Blitz Krieg]

The LED shining on the LDR is flashing. Adjusting the trim pot can help.  also, different LEDs behave differently.

[duck_arse]

you don't want to year that decay, mark, that's far too long. even dead astro wouldn't go that far.

[deadastronaut]

hmmm i dunno, a light year maybe..

[Mark Hammer]

 

That's what I get for trying to answer on a tablet while I'm watching the Habs-Rangers game.

Yeah, yearing it would likely require a cap that wouldn't fit the board, anyway, so bad idea on many levels.

[WorkBench]

Thanks guys.  I will try adjusting the trim pot, but I think I already tried that?

How do I go about adjusting the decay??

Chris

[Mark Hammer]

R14 sets the attack time - how long it takes to reach whatever the opposite end of sweep is (going upwards or downwards).  That speed also depends on the value of C9 (4u7).  R14 limits the current, which determines how fast C9 charges up, but that charge-up speed also depends on how much there is to charge up.

R15 determines how quickly/slowly C9 discharges - i.e., the decay time.  If C9's big enough to accumulate a lot of charge, it will not only take more time to charge up, but more time to drain off that stored charge.  So arriving at any particular drain-off/decay time will depend not only on the value of R15, but the value of C9.

If C9 is made a little smaller - like, say, 3u3 - and nothing else is changed, both the attack and decay times will be shorter, simply because we have altered what there is to charge up, and how easy it is to drain that off, all at once.  This is partly why caps like C9 are often referred to as "averaging" capacitors.

But not so fast.  Hold up at 2nd base, there.

Since this is an optically swept filter design, the sweep is not instantaneous and absolutely identical to the calculated times.  The LED certainly is instantaneous, but the speed of the LDR can play a role.  Some respond faster than others.  LDRs are like your retina.  Some will react very quickly to sudden illumination, and reach minimum resistance fast, but recover from that illumination, and go back to maximum resistance, much more slowly - not unlike the way your eyes can take a while to recover from a sudden direct flashlight burst and take what seems like a few minutes to be able to see in the dark again.

So, depending on the parameters of the optoisolator or LDR used, you may have to play with the component values to get the ideal sweep feel.