I saw the light!!

[Mark Hammer]

Experimental guitarist David Torn is coming to town for our local Jazz Festival in late June.  I like him, and I thought I'd like to maybe present him with a pedal as a token of appreciation.  Nuthin' fancy, but something that he would probably not find anywhere else.  So, I started thinking about it.

Through a variety of associations, I was reminded today of a Jen Fuzz III I had made a couple years ago where I included a few changes, one of them inspired by something that Zachary Vex has been incorporating into some of his pedals that use a Fuzz Face "engine".

The Jen Fuzz III (also going under "JSH Fuzz") is essentially a silicon Tonebender, which is a Fuzz Face configuration with a buffer on the front end, and some clipping diodes on the output for added sizzle.  You can drive yourself nuts, attempting to use the exact same parts, but I just used the first three 2N3904s that came out of the bag.  They work just fine.  It IS a noisy fuzz after all, with nary a trace of subtlety, so use anything with modest-to-respectable gain.  I also used 100nf caps, rather than 10nf.  Other than that, ectly what the schematic shows.

But here is the twist...

The 150k feedback resistor from Q3 to Q2 in this diagram can control the "texture" of the distortion.  It might be reducable to 100k, now that I think of it, if a person wants a little bit of subtlety.  But if you increase it beyond 150k, you get into glitchy territory.  ZVex has a 500k pot there on the Wooly Mammoth pedal and calls the control "Pinch".  Good term for it.

I have been pondering various forms of expression control these days, and thought to myself "I wonder what would happen if I put a photocell in series with that 150k resistor?".

So, I perfed up a Jen Fuzz III, hooked up an LDR and waved my hand over it as I strummed.  Neat!!  Well, actually, messy from an auditory standpoint.  I have to tweak things a bit to get values that achieve usable sounds.  When the LDR resistance adds too much to the 150k, sound cuts out.  Torn already has momentary killswitches on some of his guitars, so he doesn't need that function.  What I'm aiming for is something that would allow a sustained note to get grainier and more jittery as you wave your hand or foot while sustaining the note.  It's quite achievable.  I'll play with parallel resistor values to be able to set the maximum added "dark resistance".

I've also plugged a photocell mounted to my guitar-top as an "expression-pedal" on my Line 6 M5, and it can work quite well.  VERY different feel than a foot-pedal.  You may need to wire up stereo jacks, using only the tip and ring contacts, to make use of external LDRs for control of pedal parameters.  BUt it is worth monkeying around with.  For instance, that photocell could be used to control current to an LED which control other LDRs.  Or maybe use it to control LFO speed.  Lotsa uses.

Experiment!

[BetterOffShred]

   I love this

[Mark Hammer]

I ended up dropping the 150k feedback resistor down to 100k (as per most Fuzz Faces), and used a 330k fixed resistor in parallel with the much higher-resistance photocell, to set the maximum dark value.  Lots of interesting effects possible, in addition to more "normal" sounds.

Now I want to stick a photocell on the top of a phaser, in parallel with the speed pot, or maybe the offset/manual pot.  On a P90, this would be the 1meg fixed resistor coming off the bias trimmer.

[BetterOffShred]

What LDR are you using?  I have been using the GL55 series on most of my photocoupler applications, they range from about 150K ish dark to 10M dark.   I believe the 5516 is about 500K

Just to follow what you're doing here, are you saying you put in a 100k in for the 150K between Q2:B and Q3:E, and then put your LDR in parallel with a 330K after the 100k? (100K)-(330K/LDR) ? 

Once again, Interest peaked (piqued?)   
-Brett

[amptramp]

How do you find the photoresistor in fluorescent light?  Does it add any hum or is the response too slow?

[Mark Hammer]

What LDR are you using?  I have been using the GL55 series on most of my photocoupler applications, they range from about 150K ish dark to 10M dark.   I believe the 5516 is about 500K

Just to follow what you're doing here, are you saying you put in a 100k in for the 150K between Q2:B and Q3:E, and then put your LDR in parallel with a 330K after the 100k? (100K)-(330K/LDR) ? 

Once again, Interest peaked (piqued?)   
-Brett

Yeah, you captured it.  The idea is to configure one or more components added in series with the feedback resistor to provide a suitable range of resistance values.  Too little and nothing really audible happens.  Too much and the sound cuts out completely.  Adding something that lets you add up to maybe 250-300k on top of the 100k, for me at least, provides a usable range of tonal variation.

The LDR I used has a dark resistance of something between 5 and 10meg.  Of course, the difficulty with using LDRs for real-time control under gigging circumstances is that you can't predict how bright things are going to be or how much you're going to be able to occlude any light with your finger, hand, or foot.  In which case, perhaps the wise thing to do is parallel a 1meg trimmer with the LDR, and use it to set the appropriate range of possible values, contingent on the ambient light.

I guess this is why ZVex used a proximity sensor for the "Probe" pedals.  Your lighting conditions may vary but your distance is your distance no matter where you put the pedal down.  Still, I think using LDRs like this provides a cheap and relatively easy and simple way to incorporate real-time expression control on pedals that might not normally include it.

[Kipper4]

I've used this in a SVF arrangement before. Not sure it's going to help Mark.



You know me I'd control the vactrol with an envelope.


I too am Intrigued.

[Mark Hammer]

You know me.  I love envelope control every bit as much as you do, Rich.  But sometimes, transitions that happen across notes/strums are more interesting or appropriate for the moment than changes happening on a per-note/strum basis.  I suppose in more sophisticated and thought-out pedals, one could have sweeps/transitions of a one-shot variety, such that the user hits a momentary and instigates a transition of their desired trajectory and duration.  Or, in other pedals, one simply works an expression pedal.  Light-control presents another opportunity for slower, more deliberate changes.

[BetterOffShred]

I was messing around with the Devi Ever LDR feedback loop and was about to order a bunch of the little clear lenses you can mount in enclosures with the dished top .. but the min order was 56 and I didn't want 56 so I deleted them from my basket.  Now a week later I'm wishing I had just ordered them hah!

Well maybe they have clear Fresnels..

Edit:  https://www.mouser.com/ProductDetail/VCC/CMC_321_CTP?qs=sGAEpiMZZMvVdBlXEaMJhEGb7EUgsFWXYMA5qEjkp5w%3d
probably would work