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 Forum index » DIY Hardware and Software » Developers' Corner
tailoring pots
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scriptstyle



Joined: Jan 22, 2008
Posts: 250
Location: nj

PostPosted: Wed Apr 29, 2009 12:23 pm    Post subject: tailoring pots Reply with quote  Mark this post and the followings unread

i know this subject is alittle beat to death so sorry in advance.

tailoring pots, i have read: http://www.geofex.com/Article_Folders/potsecrets/potscret.htm

http://sound.westhost.com/pots.htm#taper

and also have had a print out of the elby article.

these have been a great help in my learning! but up untill this point i actually havent needed to tailor a linear pot. i would just add them to my list for futurlec orders. just yesterday i realized i had enough 1m linear pots to finish up my yusynth adsr. so i read through my printout of the elby article, and for some reason i got the idea that your fixed resistor value needed to be 1/10th the pots total resistance? this didnt seem right to me. so i did some more research(thinking maybe i just read it wrong). this brought me to the other 2 resources. the first one dosnt give any ideal values so i was still alittle lost. but the begineers guide to pots was real good. they give an example of "The "ideal" value by calculation is 22k for a 100k pot, and this gives a maximum deviation of +1.58 and -1.7dB from a real log curve" and then also state "it really should be 35k for a 100k pot, but the error when using 33k is minimal (about 0.16 dB at centre position)". this made me feel alittle better. i now feel that im more on the right track. given these real world numbers it made it easyier for me to figure out.

so for my application i am using a 1m lin pot with a fixed resistor of the value 1% 350k = 1m log. is this correct or am i missing something?

i wish i had the time or the know how to chart out the resistance.
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scriptstyle



Joined: Jan 22, 2008
Posts: 250
Location: nj

PostPosted: Thu May 28, 2009 5:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

Just getting around to finishing this module up. I have re considered and I'm going to use a 250k fixed res on the 1m pot. This is 25% of the pots value, And is what elby suggest in the article.
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LetterBeacon



Joined: Mar 18, 2008
Posts: 454
Location: London, UK

PostPosted: Thu Jun 04, 2009 8:37 am    Post subject: Reply with quote  Mark this post and the followings unread

Funnily enough I'm also putting the finishing touches to a Yusynth ADSR module and have come across the same problem. The 1Meg log pots I own have a slightly too long shafts on them, although I do have some 1Meg lin pots lying around.

I'll be very interested to hear how you get on....
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fonik



Joined: Jun 07, 2006
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PostPosted: Thu Jun 04, 2009 11:01 am    Post subject: Reply with quote  Mark this post and the followings unread

i used 1/10th value of potentiometer for the fixed resitor.

here is a little java applet calculating the response curve for a lin2log mod:
http://www.mindspring.com/~clist/PotGraph.html

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LetterBeacon



Joined: Mar 18, 2008
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PostPosted: Thu Jun 04, 2009 12:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for the link Fonik! So the aim is to get the closest match between the red and blue line?

For a 1M pot, 100K seems to get the closest to the blue line, although 250K seems to follow the blue line's curve a bit better. Think I might go for 100K.
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fonik



Joined: Jun 07, 2006
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PostPosted: Thu Jun 04, 2009 12:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

LetterBeacon wrote:
Thanks for the link Fonik! So the aim is to get the closest match between the red and blue line?

this is not a game Laughing
but yes, that's it. however, who could say what you want? just try some values and decide... maybe you won't even recognize a diffrence between 100k and 250k.

Quote:
For a 1M pot, 100K seems to get the closest to the blue line, although 250K seems to follow the blue line's curve a bit better. Think I might go for 100K.

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matthias
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scriptstyle



Joined: Jan 22, 2008
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Location: nj

PostPosted: Thu Jun 04, 2009 6:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

I used 270k and I must say 100k would probaly be less touchy.
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LetterBeacon



Joined: Mar 18, 2008
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PostPosted: Sun Jun 07, 2009 11:47 am    Post subject: Reply with quote  Mark this post and the followings unread

My reading of that first article you posted is that you can't turn a linear tapered pot to a log tapered pot if the pot is being used as a variable resistor (i.e. only having two terminals hooked up). You can only do the trick if the pot is being used as a variable voltage divider (i.e. all three terminals are used).

On YuSynth's ADSR module, the log tapered pots are used as variable resistors, so we can't use this trick here, can we?

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scriptstyle



Joined: Jan 22, 2008
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Location: nj

PostPosted: Mon Jun 08, 2009 8:11 am    Post subject: Reply with quote  Mark this post and the followings unread

LetterBeacon wrote:
My reading of that first article you posted is that you can't turn a linear tapered pot to a log tapered pot if the pot is being used as a variable resistor (i.e. only having two terminals hooked up). You can only do the trick if the pot is being used as a variable voltage divider (i.e. all three terminals are used).

On YuSynth's ADSR module, the log tapered pots are used as variable resistors, so we can't use this trick here, can we?



yes this is true ^

you can make a linear pot "emulate" a log as a variable resistor. but you really end up with a reverse log.
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LetterBeacon



Joined: Mar 18, 2008
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PostPosted: Mon Jun 08, 2009 10:10 am    Post subject: Reply with quote  Mark this post and the followings unread

Ah I see. Is the module still usable with the pots set as reverse log? Can you dial in values easy enough?
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scriptstyle



Joined: Jan 22, 2008
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Location: nj

PostPosted: Mon Jun 08, 2009 7:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

It's ok at best
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