Question on R.G.'s Secret Life of Pots

[makerie]

Hey folks,

I've a question regarding R.G. Keen's article on Pots : http://www.geofex.com/article_folders/potsecrets/potscret.htm

I'm interested in making a reverse log pot from a linear.

I understand if I add a resistor between the CW lug and the wiper lug, that is a ratio b of the original resistor R, I will get a reverse log pot with its reverse-loggyness increasing as b approaches 6.

My question is will this not also affect the final total effective resistance of the pot? So if I had a B1M and added a 150k resistor, at full CCW rotation of the wiper I'm left with an effective resistance of ~130k (as we now have 2 resistors in parallel)?

Cheers!

[Mark Hammer]

Adding resistors in parallel WILL reduce the effective resistance value.  Note that it will have more effect on the highest resistance value of the pot than on the lowest.

People generally want reverse-log pots for those cases where they want to be able to make a modulation source go "faster" as you turn clockwise.  As such, the reverse log taper distributes the smaller resistance values differently such that fast speeds are more "dialable".  One loses the slowest speeds, due to the overall reduction in total pot effective resistance, but often it's not a particularly noticeable difference, since it takes a lot more resistance to yield a noticeably slower sweep.

But yes, in many instances, a person might wish to start out with a larger pot value when custom-tapering, and simply using the pot as a variable resistance.  If the intent is to use it as a voltage divider (where the relative effective resistance on each side of the wiper matters), that changes things.

[ashcat_lt]

Yeah, that's an important point that many people miss.  This trick works for rheostats, but not actually for potentiometers. 

[makerie]

What I want to do is replicated the C5K pot for the SHO boost https://hotbottles.files.wordpress.com/2012/03/zvex_sho.gif , but I don't have one.

Do I start with a B5k pot and add a resistor between lug 1 and wiper 20% in value, or do I need to start with a larger pot. I'm confussed.

[ashcat_lt]

You need to start with a larger pot.

[PRR]

Reverse-audio _POT_ works with R.G.s technique.

Reverse-audio RHEOSTAT (2-leg variable resistor) can NOT be done that way, or any simple way.

Aside from more searching, the quick-fix is to wire straight-audio backward and number the knob 10 to 0.