how to fake 250K rev log pot ?

[lowstar]

hi everybody,
i have read through r.g. keen´s article "the secret life of pots" quite often now, and it all makes sense to me, but maybe i´m too dumb but what i cannot find is the "formula" to make my reversed log pots. i would need it for the ce-2 chorus, where the pot is use as a variable resistor (lugs 1+2 connected).
could anybody tell me

• which value resistors i connect where on which value pot
• and how to calculate it myself the next time ?

thanks in advance,
lowstar

[theman]

Why don't you pick up a few at www.smallbearelec.com? They definitely stock the 250K C pot you need for the CE-2 clones.

[lowstar]

i´m using european pots in the ce-2 right now which nedd a bigger hole in the casing than the alphas.

plus, this post is primarily about understanding how to do the thing with the tapering resistors.
i know that i can buy them.

cheers,
lowstar

[Seljer]

I'm not good at the math but you can take a regular 250k log pot and wire it in reverse (lugs 2+3 connected instead), the sweep will be only mirrored (if that doesn't bother you too much)

[Jeremy]

Put a 250k (or 220K) resistor across the wiper and another lug of a 1M linear pot.  Your variable resistance will be across the leads of that resistor. 

The problem is that your max resistance is only 180K with that.

With a 2M linear pot and 330k resistor, the taper will have a little harsher knee, but your max resistance will be around 280k, and the taper won't be too bad, I think.

This should work, unless I read R.G.'s article wrong. 

[theman]

Sorry -- from your post, I assumed you needed a reserve audio pot because you didn't have one handy, but did have other components.

If you take a 250K A pot in reverse, it still doesn't do what you need. Audio taper pots start out (assuming CCW) with a slow gradual change, then change very fast as you get close to the CW end. Reversing the pot lugs only changes the magnitudes of the values (either starting low, ending high, or starting high, ending low) but not the taper.

Reverse audio taper pots, on the other hand, start out (assuming CCW) with a fast change, then change slower as you near the CW end.

From the article "The Secret Life of Pots" ... look at the plot labeled "Reverse Log Series Resistor"  This shows the way you would hook up a linear taper pot and taper resistor to simulate reverse log resistor (2 terminal).  The max resistance is Rt in parallel with Rpot. Assume you need this to be Rtotal. Also, Rt = Rpot/b. You now have two equations which can be reduced to relate Rpot to Rt, with b as a variable.   It is

Rtotal = Rpot/(b+1)

Assume you want Rtotal = 250K. Then you need to choose a Rpot size larger while at the same time choosing a large b to simulate the log effect. Choose Rpot = 1M (usually largest available). Then b=3. So Rt = 333K. So make it 330K resistor.

Using these values, plot out the equivalent resistance and you'll see it's kind of like reverse log.

As a check. What is the resistance at middle? Rpot = 500K, Rt=330K, so resistance is 198795. This is about 80% of the max value of 250K. Which matches the plot ...

[calpolyengineer]

If you take a 250K A pot in reverse, it still doesn't do what you need. Audio taper pots start out (assuming CCW) with a slow gradual change, then change very fast as you get close to the CW end. Reversing the pot lugs only changes the magnitudes of the values (either starting low, ending high, or starting high, ending low) but not the taper.

Reverse audio taper pots, on the other hand, start out (assuming CCW) with a fast change, then change slower as you near the CW end.

I'm pretty sure that hooking up a 250K audio pot in reverse will give the same response as a 250K reverse log (hense the reverse part of the name)

If you have an audio pot hooked up backwards, the resistance does start with a fast change. Also it does go low to high as well because you have moved your "reference" point from lug 1 to lug 3. It is pretty difficult to visualize, but it really does work.

-Joe

[Toney]

Yep.
Just turns anti clock wise.

[cakeworks]

where is this article about which everyone seems to be talking?

[calpolyengineer]

Go to GEOFEX (there's a link at the top of the page), find "tech tips" and the article is in the right hand column.

-Joe

[lowstar]

thanks everybody for your answers.
the hooking up backwards works very well, i´ve done it many times, but this is about tapering resistors...
...and the man explained now how to calculate it. (of course, r.g. keen explains it in his article, but i didn´t get it without some help  )
i´ll try it out today.

cheers and thanks again,
lowstar

[Joe]

http://www.diystompboxes.com/analogalchemy/emh/emh.html

Go to tapered pot section.

[lowstar]

cool link.
how does the percentage of taper translate ?

cheers,
lowstar

[theman]

If you take a 250K A pot in reverse, it still doesn't do what you need. Audio taper pots start out (assuming CCW) with a slow gradual change, then change very fast as you get close to the CW end. Reversing the pot lugs only changes the magnitudes of the values (either starting low, ending high, or starting high, ending low) but not the taper.

Reverse audio taper pots, on the other hand, start out (assuming CCW) with a fast change, then change slower as you near the CW end.

I'm pretty sure that hooking up a 250K audio pot in reverse will give the same response as a 250K reverse log (hense the reverse part of the name)

If you have an audio pot hooked up backwards, the resistance does start with a fast change. Also it does go low to high as well because you have moved your "reference" point from lug 1 to lug 3. It is pretty difficult to visualize, but it really does work.

-Joe

The response for a log and reverse log are NOT the same. You can get the same values and taper, but the rotation direction will be OPPOSITE. This is why they make reverse taper pots and why you cannot get the same response from log pots with the same rotation direction.

As you turn from CCW to CW, log pots start out gradual, then change fast. Taking the resistance from either the CW lug to middle or CCW lug to middle does not change this fact -- only the start and end point of the resistance values. An audio pot hooked up forwards or backwards does not start out changing fast if you start out from a CCW position and move CW.  It only has this characteristic when starting CW and moving CCW.

Reverse log pots (turning CCW to CW) start out changing fast, then change slower. You cannot get this behaviour from a log pot no matter how you hook it up!

Thus, reverse audio and audio pots can exhibit the same behaviour from a resistance point of view, but with opposite rotations. This is why they are "reverse."

[calpolyengineer]

That was what I was telling you, but I guess you knew that. It is hard when certain terms mean more than one thing. But yeah, I get it, you get. A log pot hooked up backwards will behave like a reverse log pot, except that the rotation direction is opposite.

-Joe

[SolderBoy]

A log pot hooked up backwards will behave like a reverse log pot, except that the rotation direction is opposite.

... sort of defeats the purpose...

You can also make your own out of a dual gang 16mm pot and a log pot of the right value and the same brand...

I posted this a few month ago.

http://www.diystompboxes.com/smfforum/index.php?topic=46762.0

[Toney]

NICE ONE!   

[jimbob]

They definitely stock the 250K C pot you need for the CE-2 clones.

The Ce-2 has a 250 kc pot? I thought it was a 100k pot? I used a 100k and used the layout from Tonepad.

[Darren N]

the Maestro phaser:


has an example of something I posted a while ago here:

http://www.diystompboxes.com/smfforum/index.php?topic=1061.msg7100#msg7100

If you put the knobs on the side of the enclosure you can use a log taper pot wired backwards and the sense of "turning up" will be clockwise for the right side of the enclosure and counterclockwise for the left. (interestingly, I would have expected the Maestro to have the speed on the left to take advantage of this but it doesn't)

[lowstar]

the 250kc pot is for the bass mod - effect level control

cheers,
lowstar