Faking a log taper with a log pot of larger value

[Ripthorn]

Hi guys, I searched around for this here and at geofex, but didn't find anything.  What I am wanting to do is have an spdt on/off/on switch that can take a pot like a 1MA and at the flick of a switch, turn it into a roughly 500kA or 250kA.  Can this be done, and if so how?  The only way I can think of is to use two equal valued resistors, one from each outside lug to wiper.  Is this all there is to it?

[rousejeremy]

Check this out http://www.diystompboxes.com/analogalchemy/emh/emh.html

[Ripthorn]

I've been there and that only gives a how to when the source pot is linear, but I have a log that I want to maintain a log taper, just of a lower value.  I will have to see just what strapping two equal resistors across the lugs does, though tracking the taper might be tough.  My gut says that it will stay roughly log, which may well be enough for me.

[R.G.]

Not knowing what you're really trying to achieve is a problem with providing help.

Are you trying to fake the loading on the signal source or the change in impedance on the wiper output, or just trying to make do with a pot you have in hand?

Faking the loading is as simple as adding another resistor in parallel with the pot. Faking the change in impedance at the wiper is harder.

[Ripthorn]

My end goal is to use a 1M pot for a guitar's tone and volume and have switches to get the effect of using a 500K or a 250K pot.  I would like the taper to stay close to log, but I am not sure I will be too picky.

[zyxwyvu]

My end goal is to use a 1M pot for a guitar's tone and volume and have switches to get the effect of using a 500K or a 250K pot.  I would like the taper to stay close to log, but I am not sure I will be too picky.

For a tone control, you can just use a 1M tone pot. A 250k tone control is exactly the same as a 1M tone control set lower (at least for a typical guitar tone/volume setup). I don't know of a way to make the resistance smaller without giving it a more reverse log taper. If you still want to switch it, and can handle a change in taper, a parallel 1M resistor will give you a 500k pot, and a 330k resistor will give you a 250k pot.

For the volume control, you can parallel a resistor across the outside of a 1M volume pot. This will change the loading on the guitar, and should also leave the taper the same, which is ideal. Putting resistors from the wiper to the outside lugs will change the taper significantly (see http://www.diystompboxes.com/smfforum/index.php?topic=70732.0).

[Ripthorn]

Well, I decided to simulate it in Matlab and found the results interesting.  If trying to divide the original pot value by half, the taper stays acceptable, though the first 3/4 pot rotation has a slightly steeper slope.  However, when trying to simulate a pot of 1/4 the original value, the resulting taper is very unacceptable.  There is no way to reproduce any of the smaller resistance values at the beginning of the pot travel.  Just thought I would share.