linear or log

[arawn]

I am building a fetzer valve deluxe from ROG, it calls for a 1meg audio taper pot all i have or have the means for at this moment is a 1 meg linear pot. how much difference is this going to make?

[nexekho]

I don't think it's the end of the world, it'll just be a little more difficult to find a comfortable volume level and (as the human ear is non-linear in terms of amplitude) it might feel like there is a dead zone in the middle of the dial.

[arawn]

good enough for me

[DC9V]

If you solder a resistor with 1/10 the value of the pot between lugs 2 and 3 of a linear pot, you get a curve pretty close to that of a log pot.

[arawn]

even better

[Paul Marossy]

If you solder a resistor with 1/10 the value of the pot between lugs 2 and 3 of a linear pot, you get a curve pretty close to that of a log pot.

First time I've ever heard of this trick! This doesn't affect the overall value of the pot?

[Pigyboy]

^^^
Hey Paul,
Maybe this is a trick that can be applied to the Boomerang pot. Something to think about. Maybe ask Joe...

The trick in general in very interesting. I'd never heard of it either.

Chris

[oskar]

Halfway down this page:
http://www.geofex.com/article_folders/potsecrets/potscret.htm

[DC9V]

If you solder a resistor with 1/10 the value of the pot between lugs 2 and 3 of a linear pot, you get a curve pretty close to that of a log pot.

I found this trick on a very helpful French website (http://www.sonelec-musique.com). The site is a real goldmine for beginners -- and occasionally for seasoned DIYers as well it would seem . Unfortunately there is no English version, but given a little time I could translate at least the pot tricks page if anyone's interested. Just let me know. Or you can try google translate if you're feeling a bit adventurous .

I've never had the opportunity to try it myself, but according to the graph it seems this mod doesn't affect the overall value of the pot, just the curve (BTW, I realize I didn't include a translation of the chart's legend, so here it is: green is the rotation of the lin pot, red is the variation of the resistance of the modded lin pot accordingly, and grey is the variation of a log pot for comparison).
The author warns though :

Impedance to the source depends on the position of the potentiometer, in a ratio of 1 to about 10. This requires that the source has low output impedance (if you're designing the circuit, no problem, you can remedy this with a transistor or opamp set as a voltage follower).

[arawn]

I almost like the modded pots curve better

[Gus]

R.G. has a write up about this in "The Secret Life of Pots" at geofex.   oskar posted about R.G.s write up a few posts above
It is also written about at other sites IIRC.

The following is  a screenshot of a sim of a wha idea.



R11 and R12 are one 100K wha potentiometer.  R15 and R14 are for tapering in the sim you can leave out R15 for the taper this thread is about.

You do need to keep in mind the change in RC time constants as you move the wiper.

[digi2t]

I recently discovered this trick as well, trying to tailor a linear pot to a wah application. Super stuff.

I went to bed that night feeling less stoopid  !

Very good reference as well; http://www.elby-designs.com/documents/tailoringpotentionometers.pdf

[petemoore]

  The pot is two variable value resistors that always add up to the R value of the pot wafer.
  Even though 100k pots usually measure to 94k or so...
   100 is a nice even number to divide, a 100k variable resistive divider [pot] could have two resistances of any value [up to the value of the wafer] that always add to the wafer value.
   70/30, 60/40, or even 44.5k/55.5k...will always add to 100k.
   The two resistances are really 1 resistor [the resistive wafer], but divided by the wiper which taps somewhere between the 0.0ohm and 100K R-range//at some user setting-point on wafer.

[joegagan]

i have spent way too many hours trying to make tapering /limiting resistors work for the boomerang. an embarrassing number of hours. i tried combinations that would make your head spin when the simple ones did not capture the feel and sound of the original.
ultimately what it comes down to is- you take a 100k audio pot and try to limit its overall R to around 35k, the actual sweep gets narrowed down to a much smaller physical range than the stock gear will accommodate ( no matter how rad you get with the limiting resistors, trust me). so, math tells us we need a larger gear to slow down the ramp. a larger gear is a tight  fit and also hard to come by.  bottom line - in dozens of attempts, the wah sweep gets narrowed to about a half inch of foot travel ( crybaby is approx an inch, boomer a little more)

i did  a large number of protos with cam shaped gears as well. this gets me close, but STILL in side by side comparos the stock boomerang gear travels more evenly across the freq range relative to the foot movement, especially when AB testing two boomers while  zac watched my wahfoott from floor level.

i finally concluded that  i can make it work, but it still isn't quite right.

from 2007, there was a large error on this concept, i state in the vid that i choked the 1 meg pot down to 250k, but i am only using 130 k of the travel. that means the wah will not get the range it needs the full value of the pot to sweep full range. i had found 40 NOS AB  RV4 style pots so i was trying to use them


2011, a mechanism that was used to mock up cam action on a 100k trimmed to approx 35k for a boomerang clone . despite the flimsy look, this lasted thru many many experiments. different hole locations were drilled in the  round gear to assess cam sizing

a rad cam on a stock boomer spring loaded rack gear. this put too much strain on the pot ( side load) made noises.

this one actually works, but i made my own spring loaded 'pusher' for the straight gear. the cam is less rad offset than the previous experiment.

there are many more, but these are the highlights.  i am not saying someone should not try, but be warned, it is a long and arduous path.

[Paul Marossy]

After reading Joe's last post, I have come to the conclusion that the real McCoy is always the best! As with most other things in life. Not speaking of Clyde McCoy, however. 

[joegagan]

clyde mccoy, the greatest guitar-guy that was a trumpeter

[Paul Marossy]

clyde mccoy, the greatest guitar-guy that was a trumpeter

Ha ha, yeah, exactly!

[joegagan]

sidetrack alert


in various interviews, del casher is priceless. del  claims that he was the guy who tuned the orignal wah circuit with the "thomas engineer" ( un - named, however brad plunkett mentions having worked with a guitarist as well - un-named  hmmmmm ).

he also claims he put the original wah to use on a demo  record for vox/thomas and still owns the prototype used on the recording to this day.

[stringsthings]

I am building a fetzer valve deluxe from ROG, it calls for a 1meg audio taper pot all i have or have the means for at this moment is a 1 meg linear pot. how much difference is this going to make?

probably not that much of a difference .... in this case, it looks like the resistance value ( 1meg ) is the important factor and you've got that covered .... you can always swap in a log taper pot at a later date

DIY rule #631:  Build now, Modify later !  

[arawn]

yeah it seems to work just fine with the linear and when i can i'll get some logs and swap it out.