Best way to hack a 10K Reverse Log pot?

[benallison]

Yeas?

[GGBB]

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

You've hooked it up wrong.  To get a reverse log series resistor from a linear pot, you only use lugs 1 & 2 (you can short lug 3 to 2 if you wish).  What this means in practice is that you can only use this technique for increasing resistance clockwise.  This does not work if you want to use lugs 2 & 3 to decrease resistance - it will behave like a normal log pot.  See the bottom graph on RG's pots page: http://www.geofex.com/article_folders/potsecrets/potscret.htm.

[GGBB]

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

?

As I understand it, with log pots, as you rotate the pot clockwise, the amount of resistance change per increment goes up.  So from "0" to "1" is a small change in resistance, and from "9" to "10" is a much larger change.

With reverse log it is the opposite - from "0" to "1" is a large change in resistance, and from "9" to "10" is a much smaller change.

Remember that I'm talking about resistance here, not voltage or output level, because a pot is a type of resistor.  Also note that I'm talking about the amount of change (delta), not the amount of resistance.  The direction of the change may be either positive or negative depending on the use of the pot (between lugs 1 & 2 is positive, between lugs 2 & 3 is negative).

RG's "ratio" values on the y-axis are what correspond to Analog Alchemy's "taper" values.  Most readily available "log" pots have a 90% taper.