Talk to me about bridging pots with resistors...........

[frank_p]

Josh, what have used for tracing those curves.  Pretty clean.  But weird that there is no numbers on X and Y...

[frank_p]

Do you guys think that the B option would be a good idea to use on those Fender amps where you have a lot of volume at minimum settings ( I think of my siverface Princeton for example)  ?

[calpolyengineer]

Ok, lets see if we can take this one step further. Is is possible to take a 100k linear pot and turn it into a 1k linear? I ask because that seems to be the original intention of the thread. Also, what happens to A when the resistors are not equal?

-Joe

[zyxwyvu]

Josh, what have used for tracing those curves.  Pretty clean.  But weird that there is no numbers on X and Y...

I used some Java code I wrote for graphing. No axis labels because there isn't a clearly defined scale. I used 0<x<1 and 0<y<1 (an arbitrary choice).

Ok, lets see if we can take this one step further. Is is possible to take a 100k linear pot and turn it into a 1k linear? I ask because that seems to be the original intention of the thread. Also, what happens to A when the resistors are not equal?

-Joe

In circuit A, when the resistors are not equal, the center is skewed to one side like this (Ra is the top resistor, Rb is the bottom one):

[John Lyons]

Very nice josh! thanks for taking the time to make these up for us.

So I'm still trying to work out "Max".

For Example "D"
The pot is "P" and the resistor across it is "R"
Lets say, 100K pot 100K fixed resistor across it.
That makes "R" 100% (100% of the 100K pot) so max is 50K
(max resistance of the pot and fixed resistor) but what is the % 50, I assume?

Is that correct....made more sense to me as I typed it out.
Hopefully that's right though.



john

[zyxwyvu]

Very nice josh! thanks for taking the time to make these up for us.

So I'm still trying to work out "Max".

For Example "D"
The pot is "P" and the resistor across it is "R"
Lets say, 100K pot 100K fixed resistor across it.
That makes "R" 100% (100% of the 100K pot) so max is 50K
(max resistance of the pot and fixed resistor) but what is the % 50, I assume?

Is that correct....made more sense to me as I typed it out.
Hopefully that's right though.

john

Yes, that looks correct. The 50% is just what you got there - the max resistance of the pot-resistor setup. Similarly, for R = 200k (200%), you would have max = 66% of 100k, or 66k.

[slacker]

Also, to slacker: I was considering adding the load resistance of A,B, and C, but I couldn't figure out where to put it! It depends on the pot position, so another graph would be needed.

I think it would be enough just to put the value of R and P in parallel. That would probably be enough information to enable you to choose suitable values.

[John Lyons]

Bump for a good thread.

[frank_p]

Bump for a good thread.

Yes indeed, equivalent value of a resistances net is one of the first thing you learn in school.  It proves you can have some fun with only a couple of Rs.

[jacobyjd]

wow...all of this is very relevant to a project I'm working on--however, I'm having trouble wrapping my head around the graphs (very nice, +1 btw...).

I'm looking at stuffing a wah circuit into a crybaby shell, but the circuit uses a 50k pot value instead of 100k. I know wah pots have a fairly unique taper, but I'd like to apply some of these principles to avoid shaft-sanding, gear-fitting craziness...

...so...

According to what you guys are finding here, would any of the above methods be applicable to my situation (i.e. will the change in taper be noticeably affected by adding parallel resistors?)?

It looks promising.

Also, can would we be able to run simulations on parallel resistors added to log pots?

[John Lyons]

Jacob
Yes, this will help you.
Depending on the circuit and how the sweep of the pot affects the playability of your pedal you could try a few of these tapering diagrams.
Not all wah pots have a special taper. I'd shoot for a liner response unless you know what6 you are looking for.
Then you can experiment from there. A cool rig to try would be a 100K wah pot with a temporary pot between wiper and one outer lug and temp pot from the other outer lug to the wiper.
This way you will be able to tune in the wah pot to exactly what you want on the fly with no soldering.
Then measure the resistances and swap the temporary pots with fixed resistors.
I'd start with something like 100% of the wah pots resistance as in "A" graph in the second example (second set of graphs)
this will get you either close to liner or an "S" (log.antilog)  curve which is similar to an icar taper.
It depends on the pot you have in the wah right now though as well. Do you know the taper?
You can chart it with multimeter by sweeping though the resistances at 1/8th increments or so.
Pete goes into this above...

john

[jacobyjd]

Excellent--It's your standard crybaby wah pot. I'll measure the taper as best I can, then experiment with parallel resistors--probably starting with the simplest methods first.

Nice to know that it can be done.

[John Lyons]

Do you guys think that the B option would be a good idea to use on those Fender amps where you have a lot of volume at minimum settings ( I think of my siverface Princeton for example)  ?

Frank
I would think that the pot in your amp would be Log taper already but you could add a resistor as in "B" and see if that helps.
20-50% of the pots value should be about right for the resistor.

john

[frank_p]

here's an article with some graphs. A little hard to read but useful.
http://www.elby-designs.com/documents/tailoringpotentionometers.pdf

Cool Thanks !

[arizonabay]

Do you guys think that the B option would be a good idea to use on those Fender amps where you have a lot of volume at minimum settings ( I think of my siverface Princeton for example)  ?

Frank, how did you get on with the pot mod for the Fender? I'd like to try it on my Performer 1000 (which, on mimimum volume, causes all the birds to leave my neighbourhood).

Cheers
Matt

[soggybag]

OK here's my first shot at a graphic for tapers. Let me know what you guys think.

Wow great work! I was just asking about this on the forum this afternoon. I get the basic idea but am a little fuzzy on the details. I have one question. Is B reverse taper or is C a reverse taper?

I'm also a little confused as to which pin would be 1. For example let's say I wanted to make a 5K reverse taper pot. I'm guessing I'd use a 10K linear pot and add a 10K resister between pin 1 and 2. This would be the two pins on the left looking at a pot from behind with the pins pointed down?

[zyxwyvu]

Wow great work! I was just asking about this on the forum this afternoon. I get the basic idea but am a little fuzzy on the details. I have one question. Is B reverse taper or is C a reverse taper?

B is log, and C is reverse log.

I'm also a little confused as to which pin would be 1. For example let's say I wanted to make a 5K reverse taper pot. I'm guessing I'd use a 10K linear pot and add a 10K resister between pin 1 and 2. This would be the two pins on the left looking at a pot from behind with the pins pointed down?

Lug 1 is on the bottom, and lug 3 is on the top, as shown here: http://www.diystompboxes.com/wiki/index.php?title=DIY_FAQ#POTS . To make a reverse log taper pot, you would connect a resistor between lugs 2 and 3. 2 and 3 are the middle and right lugs if you're looking at the pot from the top. It was suggested earlier in this thread that a good value to use for the resistor is 20% of the pot value, so a good place to start is a 5k resistor (4k7 or 5k1), and a 25k pot. The smaller the resistor compared to the pot, the more 'log' the taper, as you can see in the graphs.

[Naz Nomad]

B is log

http://www.youtube.com/watch?v=i9hQIrsHaS4

[petemoore]

  Fiddle with a linear pot 100k.
  Hook a DMM to appropriate R setting to the wiper [something just above 100k].
  Notice the pot measures less than 100k, something above about 87k...we'll say 92k for this discussion-pot.
  The wiper controls two variable resistances, each outside lug, should measure about 46k / 46k when the pot is set 'so'. 
   After that it matters what circuit it is controlling.
  Does reducing the total 92k matter ?
  Would starting with a 200k pot matter ?
  Try intuitively seeking a 'good' value and taper...use what you got or try to get what you need.
  The other pot used as a variable resistor [wiper and 1 outside lug] each of the two used lugs getting a wire and insulated testclip. Stick that on the pot you want to change the taper of, try to find the 'correct' side to start fiddling with.
  Depends greatly or doesn't really matter, depending on...what circuit and the other values.

[Hanglow]

http://www.diystompboxes.com/analogalchemy/emh/emh.html most useful for this kind of thing