How to make a 150K Reverse log pot from a 250K linear pot?

[soggybag]

I know this has been covered more than a few times. I seem to remember a couple good discussions. I think there was a calculator also?

I need to make a 150K reverse log pot. The best candidate I have is a 250K linear pot. I seem to remember the short answer as placing the pot value across pins 1 and 3 makes a reverse log pot at about half the value.

I tried the Analog alchemy calculator. This gave some really odd answers for values of 250K and 150K.
http://www.diystompboxes.com/analogalchemy/emh/emh.html

[John Lyons]

Basically if you put a 250K resistor across the outer lugs you'll get a reverse 125K pot.
Make the 250K larger and you'll get closer to 150K but with less of a reverse taper.
It depends on how the pot is wired as well. Voltage divider/volume or variable resistance?

[R.G.]

"The Secret Life of Pots", Geofex.com

[soggybag]

I'm rereading the secret life right now. The secret life states:

Rt = Rpot/b
Where b = 0, 1, 2, 3, 4, 5

b = Rpot/Rt

I'm working on getting the 150K reverse log for a Neutron. Seems like a 470K or 500K pot  with a 220K resistor might work well. My math fu is weak, but I'm getting the impression the smaller the value for b the more extreme the reverse taper.

[Thomeeque]

I'm working on getting the 150K reverse log for a Neutron.

For this Neutron? There's normal log 150k pot only, isn' it? T.

[soggybag]

Yep that's the one. Reading the notes you'll see that the pot should be reverse log.

[Thomeeque]

Yep that's the one. Reading the notes you'll see that the pot should be reverse log.

I'd say you read it wrong - notes concern only different value of proposed Mouser part replacement (A100k vs. A150k) in case of PEAK pot.. but I may be wrong, I've read it briefly only.. T.

[soggybag]

Look at the parts list and then read the notes with ** below on the next page.

[Thomeeque]

Look at the parts list and then read the notes with ** below on the next page.

That's what I did, actually. So we just understand it differently. My understanding is: original circuit calls for log/audio 150k pot (PEAK) and for reverse log 1M pot (GAIN), but both of these pots are hard to buy so R.G. suggests to use log/audio 100k for PEAK and log/audio 1M for GAIN instead. In case of PEAK you will loose part of the original range and in case of GAIN you will have to solder it and use it mirrored (CW and CCW sides switched). Then yet gives you other possibility for PEAK pot if you don't want to loose part of the original range (dual A100k with sections in series ~ A200k pot). Nothing about need of reverse log for PEAK.

T.

[PRR]

> use it mirrored (CW and CCW sides switched).

That will always work.

People think a "backward" gain/volume knob is awkward; however all (US) kitchen stove knobs make "larger fire" with a counter-clockwise turn, and we manage.

The direct fix is to move the pot to the other leg of the gain stage. The drawback is low input impedance at all gains (not just at max gain); and when the pot wiper loses contact it gets very loud. (If you never use it full-up you should increase the first resistor to 50K or 100K.)

The PEAK control is trickier. It feeds back some fraction of the filter signal. If the feedback is zero to half, almost nothing happens. But as you go from 90% to 100% it changes a LOT (and ultimately rings). So we want to spread-out the top of the potentiometer. In variable-resistor connection, as in the original, this is NOT possible by "loading the pot". If we re-think the feedback as a potentiometer, it is possible to load-up, although it is so wasteful of power that it may want an odd-value pot or something more than a 19 cent opamp.

[soggybag]

Thanks for the replies.

It looks like I did not read that closely. Looks like the 150K should be a log pot.

I'm building this from R.G. PCB so I think I'll stick with the circuit as is for now. I etched this board, many years ago, tested it a few times, then threw it on the pile un-boxed effects.

I recently dug up and thought to box. It appears the current version of the PCB is different from the one I built.

[soggybag]

Seems I could still use the B250K pot. With a resistor to create a reverse log pot and wire it backwards.

Looking at the Analog Alchemy custom pot calculator. Looks you can calculate a 150K pot with a taper. But it doesn't say if the taper is log or reverse log.

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

[smallbearelec]

FWIW for anyone doing a Neutron, 150K reverse audio is available:

http://www.smallbearelec.com/Detail.bok?no=116

Somebody had asked me to get it for the input "pad" mod on Dynacomp-type compressors. It also works fine in Neutron builds, though direction of the Peak control is reversed from the original.

[soggybag]

RRR: Your suggestion for the Peak pot is looking better the more I look at it. I may have to give this a try.

I tried a B250K for Peak and the usable range is all bunched up at the very end of the travel. Which is what everyone predicted.

[gtown_caps_fan]

Has anyone tried Paul R's (PRR's) peak pot wiring redesign? Does it work well?

PRR, did you try this, or you just looked at the schematic and applied your knowledge?

Thanks all for your help. I am planning to build this circuit very soon.