I was browsing through my copy of Walter Jung's old op-amp book (2nd edition from 1981 or so) the other night and stumbled on a circuit I had known about but long since forgotten.  It will be a useful one for anyone building a modulation effect that uses any sort of wet+dry combination.  I inserted it into an envelope controlled filter last night and it worked like a charm.

Works like this.

A unity-gain inverting op-amp has a resistor in the feedback loop from output to the inverting (-) pin.  It also has a same value resistor going to that pin from the stage preceding it.

Normally, the non-inverting (+) pin would be tied to ground or Vref.  In this application, it is tied to the wiper of a linear pot.  One outside leg of the pot is tied to ground and the other is tied to the "far" (i.e., NOT the op-amp side) of the input resistor.  

At one extreme end of rotation, the + pin is tied directly to ground and there is a resistance equal to the full value of the pot from the far end of the input resistor to ground.  At the other extreme end of rotation, there is a resistance (= pot) between the + pin and ground, and the + pin is tied directly to the far end of the "input" resistor.

What this does is provide the same signal to both the inverting and non-inverting inputs of the op-amp.  If the amount of signal directed to each input pin is equal, then these two antiphase signals will cancel each other out at the output of the op-amp.  If you go in one direction, you get more non-inverted signal than inverted.  Go in the other and its the opposite.

So how is this useful?  Basically, it provides an intensity-plus-polarity control in one handy dandy knob.  Electro-Harmonix used a version of this in their old Y-Triggered Filter, although they used it for the envelope signal not the audio signal.  In the middle position there was no sweep, turn it more to the right and you got strongly downward sweep.  Turn it to the left and you got stronger upward sweep.  

In the case of a wet+dry combination, you can change the phase relationship between signals and the mix between them in one control.  So, if you had a stage like this between the last phase-shift stage and the mixing stage in a phaser, you could adjust whether the phase-shifted signal was added or subtracted from the dry signal, and how much phase-shift depth there would be, from subtle to strong.  Same goes for flangers, etc.  I suspect you could also use it for a regen control, such that regeneration could vary between entirely negative or entirely positive.  Again, both intensity and polarity in one knob.  If you're going to use it in a regen path it doesn't even have to take up another pot/knob or panel space!

In my instance, I wanted a flexible envelope controlled filter, suitable for bass, with a small parts count.  Part of the small parts count involved passive mixing, so I stuck the polarity "morpher" in the dry/clean path (although if it had used an op-amp based mixing stage I would have stuck it in the wet path).  In the centre position, all you heard was the output of the single bandpass filter since the two "versions" of the clean signal cancelled each other out so there was no clean signal to mix in.  As you rotated to the side, you got degrees of dry signal added on resulting in the filter being added to the dry (when they were both in phase) or subtracted from the dry (when they were out of phase with each other).  The one produces a peak superimposed on the clean signal, and the other produces a notch.  Very big tonal payoff for little investment.  The whole thing - input stage, envelope follower, filter section, polarity reverser - took 2 dual op-amps and fit neatly in a Pomona box just a little bigger than a 1590B.
I think Anderton uses a version of this in one of the AMS-100 voltage-controlled phaser modules in DEVICE (http://hammer.ampage.org and scroll to the last page)

Jung's example shows 20k input and feedback resistors, and a 10k pot (linear is essential here).  I suppose other values would work fine, but these are easily obtainable.  No particular op-amp is required.  I did this with a 4558.

Sounds cool mark, but how would you go about putting this into an envelope follower like the nurse quacky :mrgreen: Would you put the circuit you described before the transistor? Because I'd like a down sweep mode on it and it sounds like this little circuit will do the trick... :?

how about removing the porn link from your tag smoguzbenjamin  :?

Porn link? What the hell? Mmkay. That's wierd I use a script that they gave me to stop that from happening. I'll dig into it  :oops: Sorry!

Step 1... click on the "Visit My Site" link in your tag
Step 2... get pissed because you are at work and you are getting a bunch of "Click yes for free porn...xxx..." etc.  boxes

:evil:

Thats ok... as long as you aren't doing it on purpose

:shock: My password's been changed! I've been hacked! :shock: Sorry 'bout that triffid  :oops:

edit: I haven't been hacked, my account has been deactivated because I wasn't updating enough. Bugger. New account it is. That makes sense though, now that my content is gone, so is the anti-porn script. Okay I'll be updating as soon as possible.

Sorry for messing your thread up Mark

eYeah sorry :oops: This wasn't the idea. Now to my question, could I put this inbetween the envelope follower and the transistor in the dr quacky?

Hmmm... This sounds like something that might have a lots of nice uses.

One thing came to my mind... I have been thinking of making a distortion box that has two different dist-circuits and I could mix between the two. Antiphase mixing could do some weird sounds.

I have actually tried mixing two disorted signals antiphase in software. I got some really interesting results when the distortion plugins had otherwise similarly but one was symmetric and the other asymmetric. It would probably sound even cooler with analog stuff.

That sounds really smart, Mark! There's a ton of good stuff in those Device scans.

:roll: Anyone remebers or has heard this trick: Connect two almost but not just dead on tuned bandpass filters to + and - inputs of opamp connected as difference amplifier. It should give very sharp and strong bandpass characteristic.

OK, I've been thinking. Wouldn't the pot have to be twice the size of the resistors, because to have an equal amount of signal in both inputs, I guess you'd have to have the same value resistor preceding the opamp.

And what about biasing the opamp :?

Elektrojänis,

I don't know if you are familiar with the Gretsch Controfuzz (part of Jamie Heilman's original Leper's archive of schematics), but it uses an interesting subtractive arrangement where a straight signal and Blue-Clipper/Dist+ type diode clipper are each fed to different inputs of a mixing stage (one goes to the - input and the other goes to the + input).  The output level control of the fuzz/clipper section determines how much fuzz is subtracted from the straight signal.  Because of how fuzzes work, and the compression they introduce, the straight signal decays faster than the fuzz signal so the fuzz appears to "creep into" the output.

The schematic shown in the book, and the circuit I used without any difficulty whatsoever, used resistors double the value of the pot, not the other way around.  I was using it in a bipolar-supply pedal (what can I say, Anderton brainwashed me!  :lol:), so biasing was simply sidestepped.  Your question is a good one, though.  I have absolutely no idea how one would use this arrangement with a 9v supply and a derived Vref.

The intensity/polarity control described above provides a means of accomplishing that particular result in the Controfuzz, but in producing something equivalent to the Voodoo Labs Sparkle Drive as well.  That is, the fuzz signal can be added to or subtracted from the straight signal in varying amounts.  I suppose you could insert it either between the fuzz and mixer or between the straight and mixer.

for reasons I don`t wanna explain here, I`ll stay out of this thread...

Mark, that might be what I'm doing in my Funk.a.Duck, using a -1 to +1 gain amp ckt from Horowitz&Hill "Art of Electronics".

Mark, whatabout using half of a dual opamp, would the biasing of the other half be sufficient to bias the other 1/2 opamp? Whatabout adding 4.5v DC to the signal and filtering it out with a cap before sending the envelope signal on its way? That way you'd float the signal on the DC... :?

Oh by the way I figured out how to put this thingy in a dr quack by reading marks initial post carefully. I'd have to put this little gadget in before the clean gets mixed with the wet signal from the bandpass filter, after the 470k resistor before the output. Gimme a sec to create a cheapo website and I'll show it on a schemmo.

Ben,

Biasing is not for the device.  It's for the *path* (i.e., centering the signal in the middle of the range of potential voltage swing).  The chip/device just happens to be along for the ride.  Since the +1 to -1 stage (to use the term Paul mentioned) is an audio stage, the signal needs to "sit in the middle", like any audio stage in the pedal.  The same bias voltage *could* be fed to more than one stage requiring biasing; I just don't know how to make the decision about how to do it.

In an earlier post, I had mentioned using a sort of blend pot to mix in some straight signal with the DQ filter output.  I also mentioned that the filter output was out-of-phase in relation to the straight signal, and that this caused a sort of problem with some signal reduction.  The plus-to-minus-1 stage helps to address this by keeping the straight signal in phase with the filter output for at least part of its range.  The drop in output when the passband turns into a notch can be "fixed" by having an output stage with some gain that can be trimmed back with a volume control.

So what I posted would work?