How to turn log pot into linear pot ??

[miqbal]

Hi guys,

I want to build a 7-band EQ. I just can not find a small package of 10kB linear slider potentio, the stores only sell a very very big version in size, not small enough for a Hammond 1590BB.
I just bought a dozen of the smaller version, (should be) almost similar to BOSS GE-7, they are so cheap, but... so LOGARITHMIC. Any idea on how to turn this LOGARITHMIC pot into a LINEAR pot? The main goal is, I can get about half of the total resistance if the slider is at the center position.

Even a link to a web address will be highly appreciated. Thanks in advance.

Edit: they are dual/stereo pot..

IQBAL

[ik6gpy]

You can't do it. You can only turn a Log pot into Linear with an external resistor. Why don't you use Alpha 9mm or 12mm pots instead of a slide pot? They fit nicely in a Hammond BB. Check them here: http://www.banzaieffects.com/Potentiometers-c-290.html

I used 6 in line of the 16mm ones in a Hammond BB for some projects. So you can easily fit 7-8  9mm pots into that kind of enclosure. Hope it helps.
Cheers

[Paul Perry (Frostwave)]

The good thing is, all there is to know is here: http://www.geofex.com/Article_Folders/potsecrets/potscret.htm thanks to R.G. Keen.

Note it doesn't tell you EXACTLY what you need to know, but obviously there is a particular value of resistor which, if connected from the wiper to one end (I can't say which) will give you a pot that acts as though the centre of the rotation, corresponds to the resistance being halved at that point.

BUT..... the total end to end resistance will change as you use the wiper, this might or might not affect the circuit. And, the resultant pot you have made, has a LOWER overall resistance. So if you started with a 100K log pot, you might end up with a 20K approximately linear pot, for example.

[miqbal]

You can't do it.

Thanks, ik6gpy. Actually, the first words that I was going to put in this thread are: IS IT POSSIBLE TO TURN bla bla bla... .  In-line slider pots would gave us a quick information, at a glance, to the frequency response, and... I already bought them anyway

Possible, with some exceptions (thank God), like Paul said. I did have read R.G's Secret Life of Pots, it is already in my local harddrive, and I thought it was only about LINEAR to LOG. Looks like I'm not a good reader heh ,.

if you started with a 100K log pot, you might end up with a 20K.

100k falls down to 20k... Wow!. Actually, I'm starting with 20k by the way. I think it is a high time for some algebra.....

Thanks anyway, guys!

[petemoore]

  Ouch..that's a lot to have to deal with, but/.
  When that' pot doesn't do what I want, I put resistors over and around it 'till it does, and...
  Especially if the actual value isn't setting 'some other parameter' [like gain pot in FF] like bias or something which is on or near a 'border'...then...
  If the section I like the sound of is in a really finicky spot on the pot taper [no fine tuning] you can limit the range and 'increase' the fine tuning feel using a tapering resistor, of course you'll lose some range in the tone, and it'll be easier to get the setting you want dialed in...so...
  Of course that can take some diddling/figuring to find the outside lug the resistor goes to from the wiper, and the value of the resistor used [put clip wires on a pot used as variable resistor] to make the action of the final taper choice do what you choose [of course that has to be within the parameters of what's possible, Generally staying away from tapering resistors that are more than say 2x smaller value than the pot..'unless'..
  If where I like the pot set falls on a 'coarse tune' portion of the pot, maybe put a resistor 'under' or 'over' [stop resistor] the pot so 'that' setting falls in a finer tuning portion of the pot range...IOW the range starts above '0' [compared to how low it was going] and goes above '10' putting the setting I like at '6' instead of '9'...where the pot is easier to dial more precisely...hopefully.
  "And, so, then, unless, and hopefully"...are easier to figure out with a VR testpot with testclips and a DMM [and whatever you have to test on] than to explain...but a pot is two variable resistors [1 ea. between wiper and an outside lug] hooked together in the middle [wiper].
  It's easy to test a reduced pot value [tapering resistor outside lugs... or wiper/outside lug, etc.] on the pot, stop resistors of course require insertion / lifting a connection.

[miqbal]

Yeap, Pete. I played with an Excel spreadsheet last night, I think I can see those "And, so, then, unless, and hopefully". Pot is much more like a living creature to me right now. As you know, my main goal was to get hal of the resistance in the center, and I come up with this conclusion: It is.... (probably almost) IMPOSSIBLE to do that . I decide to live with those pots until I can get the proper ones.
The consequence is, the signal will be cut at the whole frequency range, as soon as the pedal is engaged.

[Paul Perry (Frostwave)]

my main goal was to get hal of the resistance in the center, and I come up with this conclusion: It is.... (probably almost) IMPOSSIBLE to do that

If you have a 100K log pot, there definitely does exist a resistor that, put from the slider contact to one end, will give you equal resistance to each end.

yes, it will be a lot lower than 100K from end to end. Will it be OK in the circuit? that depends what the circuit is like.
Is the pot simply acting as a variable resistor? then you are lucky.

[miqbal]

Now I got it, Pete.

Stolen from RG's:




As in the beginning of the story, my main goal is to get half of the total resistance at the center pot position (thats the usual position to get a flat frequency response of an EQualizer).

At the center, my pot reading is: R1=16k, R2=4k.

I should take R3=250 ohm, parallel to R2, to get the half of pot value at center. The total pot resistance will be 16.2k, and will vary from 250 ohm to 20k if we slide/turn the pot.

If I take R3=4k, parallel to R1, I will also get half of the pot value. The total pot resistance will be 7.2k, and will vary from 3.33k to 20k.

Will the pot be usable for such project, considering such a wide variation in total pot resistance, and considering that I have to put one resistor for each pot?