envelope controlling volume

[Dimitree]

Hi everyone
I'd like to build a simple volume swell pedal, just like the function used on the EH microsynth.. I was wondering if it was possible to make a smaller circuit than the boss slow gear..using opamp.. Could I use the Envelope detector circuit from the mutron 3 and control with it a voltage divider as volume control?

otherwise, does anyone tried to build just the "volume swell" part on the microsynth?

[edd29]

try to build this one

here's the topic link http://www.diystompboxes.com/smfforum/index.php?topic=76457.msg636626#msg636626

here's the schematic http://www.dirk-hendrik.com/temp/dod_fx15.pdf

http://www.youtube.com/watch?v=GWdWrgiCQoY

[Mark Hammer]

Many of the reverse-tape simulators rely on identification of the beginning of a "musical event".  Once the begining is detected - whether accurately or not - a control voltage is produced which ramps up at some predetermined rate.  The PAiA Gator is a good example of this.  It is essentially a noise gate with the capacity to produce very slow turn-on times.

The trouble with such devices is that identifying "the beginning" requires adapting your playing to the circuit, and setting the circuit parameters perfectly so that finger glisses are easily distinguishable from picking.  Seeing the title of the thread go me to thinking: What if volume were envelope controlled, but sluggishly so?

To pursue this, let's think about inverting op-amps for a moment.  The gain of an inverting op-amp is determined by the ratio of the feedback to the input resistance.  If Rfeedback=100k and Rin=10k, then gain =10.  One can adjust gain by increasing or decreasing either of these resistances.  One of the ways in which it can be cleverly deployed is to make the feedback resistance so much smaller than the input resistance that gain drops well below 1x.  Where non-inverting op-amps never descend below unity, inverting ones can and do.

So, let us say that we have an inverting op-amp with a 47k resistor in the feedback loop, and an LDR as the input resistor.  When no light shines on the LDR its resistance goes high, probably much higher than 47k, and the gain drops below 1x.  So, imagine that instead of an event detector that produced a trigger when a new event was detected, we simply had an envelope follower, with a slowish attack and fast decay time, that drove the LED lighting up an LDR.  Play hard enough and the LDR resistance drops enough to produce audible sound.  Play softly, and you hear nothing.

This use of an LDR is the "engine" underlying Craif Anderton's tremolo project: http://www.generalguitargadgets.com/diagrams/catrmsc.gif  No reason why it could not be harnessed to a suitable envelope follower to produce smoother fade-ins than the choppiness of a gate-based circuit.

[Heemis]

I'm very intrigued... Is this something that has been done before on a commercial/diy pedal?  I'm fairly new to envelope follower circuits, but I like where you're going with this Mark.  Do you have any suggested simple envelope follower circuits to start with?  I would love to tinker with this.

[Mark Hammer]

The net abounds with envelope-follower circuits, and schematics of pedals that use them.  What is different here is that we would want something with a very slow attack time.  Normally, the way to produce a slower attack time is to insert a larger resistance in series with whatever diode forms the quick and dirty rectifier.  Unfortunately, that reduces the drive current enough that we'd never be able to extract even a single millicandle from the LED half of the opto-isolator.

Looking over the recently posted schematic for the MFOS Envelope Follower, however ( http://www.musicfromouterspace.com/analogsynth/ENVELOPE_FOLLOWER/ENVELOPE_FOLLOWER.php ), got me to thinking that perhaps the trick is to use suitable buffering.  U1c and U1d are the parts of the this circuit that intrigue ( http://www.musicfromouterspace.com/analogsynth/ENVELOPE_FOLLOWER/images/schematic_pg1.gif ).  R1 and C4 set the attack and decay time.  While R1 is way larger than any attack-time resistor you'll see in something like a Dr. Q, Bassballs, FX-25, or other similar envelope-controlled filter, it can BE larger in value because of U1d, which buffers the envelope voltage (just like the schematic says!).  The buffering makes it so that, by not driving the LED directly, the current lost to that larger resistor will not matter.

If you listen to the sound samples provided, you can hear that they tend to have a longer and more pronounced attack than one normally hears in autowahs.  That's what got me thinking about the advantage of a buffered envelope, and how that might be used to provide a much longer attack time.

So, consider the humble Dr. Quack (seen below).  No chance in hell of it producing an attack time as long or longer than what you hear from the MFOS unit; the reason being that if the 100R resistor is increased to even 2k, you'll get only very modest sweep in the absence of something to take that puny current and transform it.  If there was another buiffer stage between the 100R/10uf pair and the 25k trimpot, THEN you could likely aim for much greater series resistance and longer, extended attack times.  Would such attack times ever be the length of what one gets when a trigger and ramp envelope generator are used as in other types of attack-delay units?  That's an empirical question I suppose, but once one is willing to step beyond the bounds of the simplest envelope-follower design (like that of the Dr. Q/Quack), then more things become possible.

Of course, getting a gain stage to gradually increase gain is one thing.  Getting it to rapidly curtail gain so as to be ready for another ramping up is another.  To do that, I suppose one would want a very rapid decay; something which is relatively easy to do simply by putting a bleed resistor in parallel with the cap to ground.
BUT.....do you always want rapid decay?  For example, if one were attempting to have a slow fade-in of a riff, perhaps there might be brief spaces between notes in the riff that could be long enough to force a re-initiation of the ramping up.  If you don't want a re-initiation for those notes what would you do?  here's an idea.  Let's say we have a 50k bleed resistor for the cap to ground, in series with a 1M pot.  The 1M pot would set the default decay time (more resistance = longer decay time).  Place a momentary footswitch in parallel with the pot, and when you engaged it, the pot would be bypassed and you'd revert to the minimum decay time for whatever notes you wanted to sound like that.  Take your foot off, and you'd revert to the preset extended decay time, where you'd need to wait longer to be able to force a re-initiation of the ramping up.

The more I think of it, the more flexible and interesting this sounds.  Finally, I might point out that one of the traits of true reverse-tape swells is that harmonic content increases over time (because played forwards it normally drops off over time).  Use of something like a single-LED/dual-LDR Vactrol might let you fade in volume and treble simultaneously for a more realistic swell.

[Dimitree]

really thanks for all those things guys.. I'll study now!