Best way to hack a 10K Reverse Log pot?

[benallison]

Reverse log pots are hard to find, and I understand it's possible to approximate the curve using a tapering resistor across lugs 2 and 3 of a normal pot?

My questions are:

1) What should I start with to have the best of approximating a rev. log taper? 25k? 50k? 100k?
2) What should the taper of the pot be, linear or audio?
3) For 25k, 50k, and 100, what would be the resistor values be?

[rousejeremy]

All your answers are here http://www.geofex.com/article_folders/potsecrets/potscret.htm

[aion]

And all your calculations can be done here:

http://www.diystompboxes.com/analogalchemy/emh/emh.html (the Tapered Pot link on the sidebar)

Keep toying with it until you find a combination that lets you use a common value pot with common value resistors.

[crane]

10k rev log alpha can be easily found. alpha is all i use in pedals so - not a problem. I built several colorsound overdrivers which need 10k rev log.
fooled around a littlbe bit before i ordered 10k rev log. you can use 10k log and wire it the other way - but that feels odd.

[Govmnt_Lacky]

And all your calculations can be done here:

http://www.diystompboxes.com/analogalchemy/emh/emh.html (the Tapered Pot link on the sidebar)

Keep toying with it until you find a combination that lets you use a common value pot with common value resistors.

Lovely little link. Only one question....

What % represents the different pot tapers 

Is the pre-filled 90% indicative of a Rev Log taper?

[benallison]

Haha, I was just going to ask the same thing.

I think something like 80% is a standard rev log? At least looking at the Geofex page it seems that way.

What % represents the different pot tapers 

Is the pre-filled 90% indicative of a Rev Log taper?

[Govmnt_Lacky]

I think something like 80% is a standard rev log? At least looking at the Geofex page it seems that way.

I kinda figured that from the Geo article as well however, it is not specified whether or not the taper % pertains to the 1st half of the pot rotation (80-90% = Rev Log) or the 2nd half of the pot rotation (80-90% = Standard Log)

Just want/wanted some clarification 

[Mark Hammer]

The other option is simply wire a log pot in reverse, so that the parameter of interest is dialed in opposite to standard.  I've used log pots this way for LFOs, such that clockwise is slower.  Works just fine.  Only needs a slight change in expectations...and sometimes legending.

[garcho]

Small Bear has a bunch but they're twice the price of 'forward log' pots. These are the values listed:

1K
5K
10K
50K
100K
150K
250K
500K
1Meg
20K "W"

[benallison]

Just want/wanted some clarification 

Oh me too! It's not clear what that calculator site means exactly.

[Govmnt_Lacky]

The other option is simply wire a log pot in reverse, so that the parameter of interest is dialed in opposite to standard.  I've used log pots this way for LFOs, such that clockwise is slower.  Works just fine.  Only needs a slight change in expectations...and sometimes legending.

Thanks Mark. I understand about using a Log pot in reverse but, the value I need is a 2M Rev Log. Also, it needs to be wired as a variable resistor (not a voltage divider)

After looking for a while, it seems that 2M Log pots are as difficult to find as a 2M Rev log 

I figured I mine as well go with the original value. I have a 5M linear pot. Now I just need to turn it into a 2M Rev log 

[garcho]

Dremel! 

[Mark Hammer]

The other option is simply wire a log pot in reverse, so that the parameter of interest is dialed in opposite to standard.  I've used log pots this way for LFOs, such that clockwise is slower.  Works just fine.  Only needs a slight change in expectations...and sometimes legending.

Thanks Mark. I understand about using a Log pot in reverse but, the value I need is a 2M Rev Log. Also, it needs to be wired as a variable resistor (not a voltage divider)

After looking for a while, it seems that 2M Log pots are as difficult to find as a 2M Rev log 

I figured I mine as well go with the original value. I have a 5M linear pot. Now I just need to turn it into a 2M Rev log 

My bad..  I've been cut off from forum access at work, so I end up getting a quick scan of what's new when I come home, and maybe leap into the fray a little prematurely.

[benallison]

And all your calculations can be done here:

http://www.diystompboxes.com/analogalchemy/emh/emh.html (the Tapered Pot link on the sidebar)

Keep toying with it until you find a combination that lets you use a common value pot with common value resistors.

So does anyone know how the taper percentage on this site corresponds to the graph on Geofex?

[PRR]

> it needs to be wired as a variable resistor (not a voltage divider)

Can't be done. Use Linear, find Reverse Taper, or wire Audio Taper backward.

Ideally the "taper %" is what you want at half turn. This Depends. In a radio volume-control or mike preamp with 100:1 (40dB) range, the center should be 10:1. In a tremolo with 10:1 range (1Hz-10Hz) the center should be near 33% to give 3.3Hz.

No designer "likes" to use odd tapers. So when one is specified, and the designer was thinking right, then there is no easy way to avoid a reverse-taper (except to use a Linear with most of the action at one end, or use a forward taper backward).

[Govmnt_Lacky]

Strange outcome 

So I got my 5M Linear taper pot in an attempt to turn it into a 2M Reverso Log taper with resistors.

To start, I strapped a 3M resistor across lugs 1 & 3 and with this I was able to get a 2M reading across lugs 1 & 3 (in effect, changing it to a 2M pot)

The funny thing is, with this setup, I decided to check to make sure the sweep was even with my DMM. I connected the DMM between Lugs 1 & 2 and read resistance while sweeping the pot and this is what I got:

Full CCW: 1.5 ohm
Mid rotation: 1.7M ohm 
Full CW: 1.95M ohm

Souldn't I see about 1M at the mid point?

I took another reading WITHOUT the 3M resistor across lugs 1 & 3 and I got the expected results:

Full CCW: 2 ohms
Mid: 2.55M ohm
Full CW: 4.9M ohms

[PRR]

> Mid rotation: 1.7M ohm

Correct. 1.7187Meg

Draw it out, add it up.

> I strapped a 3M resistor across lugs 1 & 3

Why 1-3? Why not 1-2?

[amptramp]

Forward log pot plus:

[Colonel Angus]

Is that a crimper for Ribbon Mic ribbons?

http://futurlec.com/PotRot.shtml

I bought Antilog pots here

[benallison]

So again, and idea how the ratios here: http://www.diystompboxes.com/analogalchemy/emh/emh.html

Corresponds to the taper % here:

http://www.geofex.com/article_folders/potsecrets/pottaper.gif

?