Listen while I play my Green Tangerine...

[Mark Hammer]

Following up on a thread from the last few weeks, I whipped up a hypothetical limiter-compressor, based on the Orange Squeezer that I have dubbed the Tangerine Peeler (I'm running out of citrus metaphors).  It's posted at http://hammer.ampage.org (the link works) with some comments.

The thing I wanted to experiment with was being able to choose between feed-forward OR feed-backward envelope control.  As was drawn to my attention, both by the kind and wise folks here, as well as a stroll through my big binder of dynamic control schematics, feed-forward is generally used for limiters, while feed-back is used for compressors.

You would think that an envelope follower tapping the input would behave the same as one tapping the output, but that's where you would be wrong.  If you tap the signal for envelope following AFTER the point where the signal has been altered, then you are essentially rectifying a signal whose dynamics have already been altered.  In effect, you are altering the dynamics based upon how the dynamics have already been altered.  The net effect is that the dynamic alteration tends to linger a while.  If the signal envelope is tapped *before* the signal dynamics get altered, the envelope signal derived tends to decline very quickly because nothing is artificially propping it up the way it is in feed-back systems.  The net effect is that the maximum dynamic alteration occurs on initial peaks.

The unit also includes a tone control that is further explained in the accompanying notes.  This is a completely untested unit.  I needed to draw it out first to help me build it.  If there are any quirks you can spot (and there are bound to be some), please let me know, and I'll edit the posted schematic.  Many of the values are complete shots in the dark.  Sometimes my instincts are right and sometimes they aren't.  No unjustified compliments, please.

As an aside, I realized that one of the reasons why the classic OS sounds as good as it does is the use of 2n2 cap and 470k resistor between the drain and gate of the FET that makes up the ground leg of the "virtual attenuator".  It didn't click until tonight that this is precisely what is done in some FET-based phasers to increase the FET's immunity to distortion.  Normally, it raises the threshold of clipping quite a bit, though it impairs fast tracking of the control signal.  How much, I don't know.

[moosapotamus]

[ watching this space ]

Looks very interesting, Mark.

~ Charlie

[bioroids]

Hi!

This is very interesting, Mark, thanks!

I dont see any obvious kirk (though I'd made the 220K input resistor higer )

Too bad I'm out of breadboarding space to give it a try right now

The switch between feed-back and feed-forward looks like it follows your principle of more versatility with less knobs 

Miguel

[Mark Hammer]

Thanks.

Of course, I suppose the paradox is that when something is supposed to be "transparent" in the first place, do you or can you actually notice whatever flexibility is built in?

I don't know.  Once the High Holy Days are over, I'm gonna have to throw one together and find out.

[Eb7+9]

Mark,

nothing preventing you from sticking a 10kLIN pot between FF and FB and sweeping any mix ratio to your side-chain ... also, have you ever tried tacking on an extra order of filtering on that averaging stage ? ...  helps minimize pumping from a varying envelope - a 3k9 res and another 4.7u cap should do (look at the difference on a scope) ... same first order idea as the PAIA Infinity+ sidechain - with similar improvements in that area ...

~jc

[RedHouse]

Just to pick you brain a little ...what's up with that output buffer? and why did you use it's inverting input?

Looks like original phase is maintained through the compression amps therm gets flipped right at the end.
(phase is a pet-peeve of mine)

[Mark Hammer]

As far as I can tell, phase is maintained throughout.  Maybe the drawing may LOOK like something gets inverted, but I know I was aiming for phase coherence throughout.

[bioroids]

It looks non inverting to me.

Luck

Miguel