Talk to me about bridging pots with resistors...........

[frequencycentral]

I'm quite sure the answer to this question is hidden in the great graphs earlier in this thread but here goes. I have a Phase 45 type LFO which call for a 470k rev log pot. The pot is configured as a variable resistor. The Tonepad PDF says 'or lin', but a lin kinda bunches things up a bit. So to make the taper rev lin I bridge lugs 2 and 3 with a 470k resistor?

[MoltenVoltage]

OK here's my first shot at a graphic for tapers. Let me know what you guys think.

Bump for a really helpful thread.

[Dan N]

What % taper are the standard log pots we buy?

[Kearns892]

The percentages have nothing to do with the pots themselves. The percentage reflects the value of the resistor being run parallel to the potentiometer relative to the potentiometer i.e. 100k pot run parallel with a 10k resistor would be the 10% line. See page 1 reply #17.

[Dan N]

Right. Actually, I'm referring to the link posted earlier:

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

I just want to know the % of our bog standard log pots to plug into Joe's calculator. Sorry if this is the wrong place.

[aron]

Then there is this:

http://www.geofex.com/article_folders/potsecrets/potscret.htm

[trad3mark]

sorry to bump such an old topic, but i've spent the last hour and a half trying to figure out how to go from a 1M linear pot to a 500k Rev Audio pot. I'm making an MXR Microamp, and in the schematic, the 500k has lug 1 connected to the op amp, and then 2 and 3 connected eventually going to ground. I've tried just about everything, and my head is fried. can someone shed some light on my situation?

[Morocotopo]

Another old topic bump, it´s a very relevant topic.
I´m trying to replace the tone pot of a TS, the original is I believe, a "S" taper pot. After much testing, I have a 25K linear with a 47K from 3 to 2, and a 6K8 from 1 to 2. It sort of works, but it´s not as smooth in change as the original (I have a reissue TS9 to compare), it has a sort of "flat spot" in the middle where there´s not much change. I´m starting to believe it´s not possible to get a real replacement with resistors...

Anyway, Besides the useful info here, I think there should be a graph to show what happens with two unequal resistors on each pot side. And a comment on how this tricks work in regards to the input/output impedance, load, etc.
I´m not really up to the task... anyone?

[ayayay!]

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

Play with all the pot & resistor links. Try Linear Pot, Tapered Pot, and Parallel Resistors.    

Oh and since you're playing with the TS tone, I think nothing beats a 5kB pot in there.    Lay a 6.8k across the outer lugs of the 25k and there you go. 

[Morocotopo]

I´ll try that. Doesn´t having a linear pot bunch up the treble at the last 10% of rotation in your experience?

[ayayay!]

IME no it doesn't.  I do it on my SD-1's, TSx's, and TS clone/variants.  I love it.  Mehbe it's just meh.   

[vendettav]

that EMH is helpful as @#$%! thanx everybody for posting the link

[MoltenVoltage]

What % taper are the standard log pots we buy?

Bump for this helpful thread.

Also, I am wondering the same thing - what % taper are standard log pots?

From the graphs on RG's page, I'm guessing 50%
http://www.geofex.com/article_folders/potsecrets/potscret.htm

But when you put in 50% taper on the Tapered Pot calculator, it says to use equal resistors, which seems wrong:
http://www.diystompboxes.com/analogalchemy/emh/emh.html

My gut tells me that you should have one of the resistors twice as large as the other (or something like that), and that equal resistors will simply lower the range of the pot...but my gut isn't always right!

[R.G.]

Pot companies (no, not medical MJ) have several tapers.

Actually, real pot tapering has pretty much gone the way of the dodo for commercial pots. They tend to use two-section linears to approximate a log or reverse log. Kind of.

Old textbooks show the various pot tapers as smooth graphs.

But there are many, and they're not all that standardized. You can special order X% log (for example) if you special order enough pots.

[flyshooter]

Great thread, inspired me to (join, &) run my own tests to compare against the "C" graph, for Reverse Log and to see what I could substitute for some hard to get parts in the form i'm looking for. 

Some rough measurements below, made using factory 5K RevLog pots and a 10K linear with various resistors.  Knob positions are approximate, and Vout varies enough with slight nudges to call this rough.  However, graphed & smoothed, the result looks useable.

I might try a 1.5K, 1.8K, or just use the 2.2K with the 10K linear.  Depends on how this loads the circuit, so i might bump up to 50K pot & find the right resistor.  This Linear is a TT/BI part.  Don't know who makes the RevLog pots I have, but in case someone might know I'll take pictures and upload in a subsequent post. 
.

[ashcat_lt]

Just to mention that it is much easier to get finer adjustment in pot rotation (for these kinds of measurements above, etc) if you either use a really big knob, or stick (verb) a stick (noun) perpendicular to the shaft.

[flyshooter]

nice tip about the stick.  will try that next time.

on part sourcing, i just noticed an ALPHA stamp on the new C5K part.  these are 4 pin (1 dummy), snap in, side adjust.  a manufacturer might list a nice wide range of parts but then none of the suppliers carries the range for in-stock & single/low quantity ordering.  so it's coming down to mixing between suppliers and manufacturers.  getting all the same shaft type, length, rotation angles, metal bushing, height, etc, is another aspect besides the foot print.

for the curious, i've been looking at these pots (through mouser, digi-key, newark/farnell, etc)

Alpha Taiwan:  RV110F-20E1-(L)A  http://www.taiwanalpha.com.tw/english/p_e_27-1.htm
Alpha USA: 3RP/1112N  http://www.alphapotentiometers.net/html/11mm_pot_3.html
Bourns: PTV111  http://www.bourns.com/data/global/pdfs/PTVPTT.pdf
CTS: RK11K http://www.alps.com/WebObjects/catalog.woa/E/HTML/Potentiometer/RotaryPotentiometers/RK11K11/RK11K11_list.html
Panasonic: EVU (this is 3 pin) (saw these on mouser)
Changzhou Yuhao:  WH111A-2  http://www.penyu-cn.com/2_prod062.htm (from web search, no supply)

limited sourcing on the above, and in some cases none found so far.   then there's the dual-pots...  and etc.

[gambit07]

so, has anyone tried this to change the taper of the pot? i've tried it, graphed it, but there seems to be no change (audibly) for me..

and how can you apply this to change a speed control? what's the best taper to use? I tried linear and the rate just mashed up on the top (full counter) portion.

[armdnrdy]

I just modified a 50K linear taper pot into a 25K 18% log taper for the rate pot on the Mutron Flanger build.

The tried a 25KC pot, but the slow rate was too small an area, and the fast rate too large.

Mutron used a custom pot according to the factory parts list. My partner on the build, Dino (digi2t) supplied resistances at different rotation of the original flanger's rate pot, and I was able to emulate it.

This does work, although there are limitations.

[zombiwoof]

I've read the secret life of pots a few times.
The concepts are there but more examples are needed.


There are (at least) a couple ways to alter pot values with respect to taper.
One resistor across a pot, one resistor across the wiper and each outer lug.
Depending on the fixed resistor's value you can get a different response.

Thanks for the graphs zyxwyvu
I don't have any simulation software.
Would you be interested in making some graphs with different fixed resistances for the other
configurations? It would be great to get this in the wiki.

Here are some schematics for other configurations.




Sometimes I use make the resistance higher in the log voltage divider to get a "super audio" taper level control
It's at unity when half way up, then after half it ramps up a good bit to boost an amp etc.

The rev. audio configuration is great for Fuzz Face and speed controls or tremolos and oscillators.

john

I've read that the original Vox wah pots had something like an "S" taper, if so you could buy one of those cheap linear taper wah pots that AES sells in a few values, and use the "S" tapering resistors to approximate the old Vox wah pot taper.  I might try that, to see what happens.

Al