I was looking through a schematic for an exciter circuit last night, and came across a "timbre" control that seemed very familiar in its implementation.  It was absolutely identical in form and spiirit to the Stupidly Wonderful Tone Control I posted here a few months back.

That circuit, if you recall, was a pot with the wiper tied to a cap going to ground.  The resistive element of the pot would impose a constant resistance, but where the cap tapped the resistive element would result in different RC lowpass corner frequencies.  Stupid and wonderful.

In this case, same idea was pursued with the pot serving as the input resistance to an inverting op-amp gain stage (10k pot as input resistor, and 47k feedback resistor), but instead of tying the wiper to ground through a cap, it was tied to ground through a diode.  Of course, what led up to that pot was a fairly complex set of allpass and highpass stages, but the gist was that this "timbre" control would provide variable asymmetrical clipping.

Haven't tried it out, but interesting idea, and not necessarily restricted to a one-diode setup.  For instance, let's say we have a Distortion+ clipping stage, followed by an op-amp gain-recovery stage. Just for the hell of it, let's make the initial gain stage inverting, and the gain recovery stage inverting as well, so that the output is in phase with the input.

Okay, now we have a hot output going to a pair of diodes (and fizz-taming cap) to ground, through a resistor.  BUT, the resistor is actually some portion of a pot, whose total resistance forms the input resistor to the second gain stage.  As you vary the "tap-point", you change the quality of the clipping via two means: the current-limiting of the resistance, and the lowpass filtering of the cap and pot-segment.  HOWEVER, the gain of the second stage does not change.  Pretty clever, though who knows if it would yield useful or pleasing variations in tone.

It's funny, sometimes what happens when you decide to give the symbol for a pot on a schematic a quarter turn and see it in a whole new light.    Incidentally, that pot-as-tappable-resistance doesn't have to be just the input.  It can be the feedback resistor too.

Mark, I've been using that same idea on my Magnolia overdrive pedal for a couple of years now.  It started as a TS-xxx type overdrive, but using the PCB from my Sweet Cheetah 2 distortion.  A few judicious changes in values, ditch the clipping amp stage and substitute diodes in the feedback loop and voila, new pedal.  Turns out the PCB layout had a couple of pads for a jumper to connect or disconnect the diodes from the loop (originally for testing purposes) so I ran a wire from the diodes to the unused lug of the drive pot.

It's not as noticeable as you might think, at least not in my circuit.  The extra resistance of the pot acts like a bounding resistor for the diodes, and basically blends in more diode clip at higher drive settings.  As the resistance in the feedback loop increases (and thus the gain of the opamp) the bounding resistance decreases (thus the clipping diodes are brought more into play.)  The practical upshot of all this is that at low drive settings, the pedal functions as a more-or-less clean boost, but retains the familiar TS type clipping as you increase gain.  Kind of like a Sparkle Drive without the added versatility of the clean boost, but easier to dial in with just three knobs.

Matt Farrow

So that's the "gain" control of the Magnolia?  Clever.  You probably know my fondness for single knobs that do several things at once.  That'sa nice move on your part.  You're back in my will, now. 

Hmmm, cool idea.  I guess you could also use a 100K pot and hook the cap to ground from the loop, to the other side.

The Marshal Blues Breaker uses the resistance in the loop and the resistance between op amps with this.   

Mark, I've used that set-up in designs of my own long before you posted it.  I don't think I came up with it myself either...in fact I'm pretty sure of it!  Must have copped it from some magazine article or something.

Great minds think alike though (congrats for discovering it yourself)! 

The schem I saw it in is at least 23 years old.  Hard to imagine the idea lay dormant all this time.  Congrats to you for stumbling onto it earlier!

Quote from: Mark Hammer on November 01, 2005, 11:38:52 AM
So that's the "gain" control of the Magnolia?  Clever.  You probably know my fondness for single knobs that do several things at once.  That'sa nice move on your part.  You're back in my will, now. 

Haha!  What will I inherit, I wonder...  I appreciate the compliment, it's been years since someone thought "I" was clever.  Anyway, it's easy to keep on adding knob after knob, more toggle switches, etc. to a box.  At some point, ease of use has to balance out with versatility.

My FAVORITE overdrive sound is the "distortion" section of the Cort MIX-10 multieffector.  This charming box features three gigantic rubber switches, crappy analog delay and chorus, and a distortion control with one knob.  It sounds great, total rock city.  You can keep adding buttons and lights to things but at the end of the day it's the music you make with it that matters.

I simply found a simple way to do something new with a box I was building, and went with it.  BTW, one of the sound clips on my website features a 1978 TS808 vs. my Magnolia pedal.  Interesting comparison.

http://www.pharaohamps.com/Pharaohamps/Sounds/magnolia.mp3

Matt Farrow

QuoteI was looking through a schematic for an exciter circuit last night, and came across a "timbre" control that seemed very familiar in its implementation.  It was absolutely identical in form and spiirit to the Stupidly Wonderful Tone Control I posted here a few months back.

You must be talking about the clipping control in the Aphex patent (long since expired)...  a single diode from the wiper of the pot to ground.    This method of clipping adjustment gives more even harmonics, which is advantageous for the exciter circuit.

For anyone interested, there are more controls for warping the  timbre of a signal in my article at:

http://www.muzique.com/lab/warp.htm

regards, Jack

Actually, it was a schematic for a full-featured exciter from an old issue of a Japanese magazine called Professional Crafts.  Could it have been their adaptation of the Aphex schem?  I don't know enough Japanese to say.

I'm always pleased anytime there is a reason to refer people to the warp article.  That's a nice piece of work, and still remains relevant and seminal.

BTW: Stumbled onto the little item in EM last night (Joe Zawinul cover, 1986 or something) where you detailed how to add a limiting indicator LED to an EPFM compressor.  Wow.  I rmember when things like that were big news.  We've come a long way since, haven't we? 

Quote from: Mark Hammer on November 01, 2005, 07:18:15 PM
Actually, it was a schematic for a full-featured exciter from an old issue of a Japanese magazine called Professional Crafts.  Could it have been their adaptation of the Aphex schem?  I don't know enough Japanese to say.

I'm always pleased anytime there is a reason to refer people to the warp article.  That's a nice piece of work, and still remains relevant and seminal.

BTW: Stumbled onto the little item in EM last night (Joe Zawinul cover, 1986 or something) where you detailed how to add a limiting indicator LED to an EPFM compressor.  Wow.  I rmember when things like that were big news.  We've come a long way since, haven't we? 

Hi mark,

The Aphex patent is at http://www.pat2pdf.org/patents/pat4150253.pdf

You can compae it to the magazine article. The clipper is on page 5.

I haven't thumbed through the old Polyphony/EM copies in a long time...  sounds like a good project for a rainy day!    At least the USA has a magazine or two for diy hobbyists back then... 

regards, Jack

Quote from: amz-fx on November 01, 2005, 07:37:12 PM
I haven't thumbed through the old Polyphony/EM copies in a long time...  sounds like a good project for a rainy day!    At least the USA has a magazine or two for diy hobbyists back then... 

Thanks for the patent link.  Me, I thumb through my Polyphony collection at least a few times a year.  Never ceases to provoke my thinking.  EM kind of stopped being interesting to me around 1994 or so, when it seemed everything went digital.  Still find back issues here and there in the used mags piles.

Quote from: PharaohAmps on November 01, 2005, 04:20:36 PM

My FAVORITE overdrive sound is the "distortion" section of the Cort MIX-10 multieffector.    It sounds great, total rock city.

aaahem....thought you could slip that by? In this forum?
got a schematic?
;-)


-Eric

Quote from: Eric H on November 02, 2005, 02:18:01 AM

Quote from: PharaohAmps on November 01, 2005, 04:20:36 PM

My FAVORITE overdrive sound is the "distortion" section of the Cort MIX-10 multieffector.    It sounds great, total rock city.

aaahem....thought you could slip that by? In this forum?
got a schematic?
;-)


-Eric

  You caught me.  It's a huge PCB, with everything mashed in together.  The chorus sounds like vomit, the delay isn't much more than reverb, but the compressor is real squashy and the distortion is awesome.  I'll take it apart this weekend and poke around.  No promises on a schem of the whole thing, but I MIGHT be talked into adapting the compressor and distortion parts into a new project.

Matt Farrow

It's always cool to see how a good sounding distortion works, especially if it has something new/or different to it.

The Warp article is way cool, perfect for Rat mods, as are the other articles on Jack's site.

I've played around with a "T-tone" control consisting of a pot w/wiper-to-cap-to-ground, and thought it was promsing. I started getting some really interesting results by bootstrapping the cap instead of sending it to ground, but this requires use of a extra active stage to follow the network.