Anderton Remote Wah circuit explanation ?

[aab0mb]

I've i've implemented the Anderton remote wah and pot de-scratcher mod to my wah.  All in all it sounds good but the sweep has a little but of bunched up high end in the toe down position.  Basically i need the LED to get to full brightness sooner in the sweep of the pedal.  Does this mean i need to adjust the sweep of the pot?  Limiting resistors?  I've rotated the gear towards the high end as far as possible without breaking the pot when i engage the switch.  It's allllmost there but not quite.

I've tried a 100k trim on both sides of the LDR and across the LDR.  Nothing seems to be doing what i'm looking for. 

Would a smaller current limiting resistor allow the LED to reach max brightness sooner in the pedal sweep? That's basically what i need.  Smaller total resistance LDR?

I feel like it's one of those things that's been rattling around in my head for a week and I'm losing perspective....

Perhaps someone could help me understand the circuit block and how to fine tune the response a little better.

[Mark Hammer]

"Bunching up" means the taper of the pot you are using to control the LED is not fully compatible with what you want the LED to do, or rather the LDR to do.

I recommend reading the GEOFEX document on The Secret Life of Pots, which may have some ideas and formulae for you to play with to get exactly what you need, or as close to it as possible.  http://www.geofex.com/article_folders/potsecrets/potscret.htm

[aab0mb]

Thanks, Mark!  I was hoping someone like you might chime in. 

I'll take a closer look at the Secret Life of Pots and see what I can work out. 

Be back with good progress I hope. 

Really hoping this project turns out satisfactory.  Most of the elements are coming together nicely. 

Side note:  For anyone that likes the light plates or "light emitting boarder", I attached the light plate to my VC wah pot in parallel with the LED/LDR combo.  Makes for a plate that glows brighter when the pedal is depressed.  Kind of a cool look.   

[aab0mb]

Testing continues:

Trim pot from wiper to either hot or cold produced little of value.

Trimmer in parallel with wah pot to lower resistance seemed to have some effect on the treble amount but it requires high parallel resistance. 

I'm thinking i'll have to play with the current limiting resistor again.  That was my last option as i didn't want to unroll the home made optocoupler.  If it means tuning this thing in to my liking it'll be worth it.   

The spaghetti nest i have going in this shell is a spectacle all it's own...   

[aab0mb]

Progress. 

I rewired a lot of the spaghetti mess and while i was at it i used a brighter LED.  Got everything put back together and it's working much more like I wanted it to.  More quack out of the top of the pedal range.

The changes made to the mod were:

1) Omitting the 100k resistor on top of the LDR.  I assume this is for limiting.  Not sure but my circuit worked better without it.

2) Lowered the value of the current limiting resistor.  Might be something like 470 ohms.

3) Didn't place the polarized cap across the pot from 9v to ground.  What is this for exactly? Perhaps it's more critical than i realize...

4) The .22 cap to ground off the wiper is now a .01 i think.  It's all i had lying around.  Seems to be ok. 

I guess I'd like to know what the caps on and around the pot do as far as functioning of the LED? Is this a part of tuning the sweep or range of the LED?  I'd like some of the theory behind the circuit as i'm not about to rip this thing apart for the third or fourth time for more testing.      However, if i knew what the parts where doing a little better and I could go in with a purpose...  I'd consider more tweaks to optimize the sound to my needs.

[aab0mb]

Ok.  Made the final tweaks and i think it's finally done. 

I connected the 100k half of the voltage divider to the CORRECT spot this time as well as mounting the 100uf cap across the pot from 9v+ to ground. The sweep got less bunched up and it mostly sounds like it did stock if i dial in the controls right. I have a Bunch of other sound options as well.

What started as a stock v847 wah now has external Q control, gain control, 5 position sweep cap selection, mid boost switch, true bypass, light emitting boarder plate, and the remote wah LED LDR no scratch mod.

I'd still like to know what that .22 cap to off the wiper of the pot is doing....

[R.G.]

I'd still like to know what that .22 cap to off the wiper of the pot is doing....

It blocks DC from the pot wiper (which is at ground for DC) from affecting the DC bias on the second transistor.

The second transistor is purely a buffer, which buffers the AC signal from the pot. The DC conditions on the second transistor are set by the 470K resistor from the collector of the first transistor. The 470K bias resistor does not contribute significantly to the AC operations.

The way this wah works, in a nutshell, is that the wah pot is just a volume control. It divides the AC signal on the collector of the first transistor, as coupled by one of the 0.22uF caps to the top of the wah pot, to the base of the second transistor, coupled out of the wiper of the wah pot by the other 0.22uF cap. The second transistor feeds this variable percent of the output signal back through the capacitor on its emitter and into the inductor. This makes the capacitor look variable, for the reasons explained in the article.

The range of the usable resonance is determined by the fixed inductor and the electronically variable capacitance. The frequency varies as the square root of the LC product, and since L is constant, the square root of the variable capacitor.

The human ear thinks that frequencies that vary exponentially sound like they're changing "linearly" according to the musical scale. So if the frequency varies as the square root, it can't match "exponential" very well. The taper of the wah pot can only crudely match these two together, which is one reason there's so much dithering, myth, and advertising about what pot taper is "magic".

The Anderton LED/LDR mod uses a fixed resistor and a variable LDR to make a voltage divider which stands in for the wah pot's variable voltage division. This works well because the wah circuit is not terribly sensitive to the absolute value of the wah pot, only the voltage divider ratio the pot position makes. So a fixed resistor and a variable one make for a variable divider, and it works well enough.

The "taper"of this rig is odd, and hard to figure out. LEDs tend to have nearly linear light per current once they turn on. LDRs have responses that are at a very minimum non-linear; I haven't seen a description of the curve they have in terms of resistance per light-intensity, only example curves. So the whole chain of pot->LED current->LDR resistance->voltage attenuation is a chain various responses that fortunately seems to work OK for some values of pot/LED/LDR.

There's probably a way to figure out the equations, decide the "best" curves to get, search for parts that do that, and then test and re-test. But testing, substitution and retest have always gotten me a working unit in a short time, so I never did the math.

[aab0mb]

I'd still like to know what that .22 cap to off the wiper of the pot is doing....

The "taper"of this rig is odd, and hard to figure out. LEDs tend to have nearly linear light per current once they turn on. LDRs have responses that are at a very minimum non-linear; I haven't seen a description of the curve they have in terms of resistance per light-intensity, only example curves. So the whole chain of pot->LED current->LDR resistance->voltage attenuation is a chain various responses that fortunately seems to work OK for some values of pot/LED/LDR.

There's probably a way to figure out the equations, decide the "best" curves to get, search for parts that do that, and then test and re-test. But testing, substitution and retest have always gotten me a working unit in a short time, so I never did the math.

This was a huge part of the battle.  Dead on.  There are enough links in the chain that variables are many. 

Thanks for breaking it down, R.G.  It was the last thing i needed in this quest to understand a little deeper.

Wah works well as of today!

[aab0mb]

Just another note that might help someone out.  Get a bright LED for the LDR to see if you're rolling your own optocoupler.  Bright LED drives the resistance low.  Sometimes if you aren't driving the resistance low enough on the LDR, the sweep range, pedal feel, etc can be lacking.  If i'm not mistaken it would account for lack of high end quack in the wah and overall would make it seem muddy or dark or not move enough towards the treble in toe down.  I think that was one of my issues.  The other would be making triple sure all the connections go to the right spots.   

I hope someone else tries this out.  Think of all the wah pots that could be put back into use! As far as the taper arguments and myths, I think that a circuit like this ultimately could lend itself to a very tunable and responsive device.  Just needs the work put into development. Perhaps taking the needed measurements and figuring out what values and part value ranges would work could kick start a new phase of wah tweakers and add yet another variable to the pot taper mystery. I'd be inclined to think that people would be more apt to venture down this road if the path was a little more clearly defined.

I took the v847 over to a friends house tonight and he put it in place of his morely wah for a test.  In the end he was really excited about the range of sounds and overall "feel" of the pedal.  He's not someone that cares and LED/LDR controlled doohickies so I figured if he liked it with a pair of fresh ears that it basically passes the  test.  Said it was better than the morely and liked that it could do fat synthed out wahs and thin funky wahcka wahs. 

Color me satisfied. Many thanks for the help gentlemen.