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pot problem


infest
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Hi all!

finally finished my midibox64 and set it up. for financial purposes i bought only 2 pots in case nothing works and connected them to the AIN module. the pots & buttons work but the pot rotation is not linear to resistance, e.g. in knob values from 0-127 when the rotation is at half the pot value is like 10 and afther that it goes relatively liner up to the end but it freaks me out. i think that it's cause i use some cheap pots (1 EUR)?

can somebody tell me what pots have a good linearity/price realtionship? afcourse 4 eur/piece is the top price, and that can be ordered via farnell or bought somewhere in the ex YU area?

thank you

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Hi.

You just have bought log. pots. You'll need linear ones. I think you should find those too.. maybe from the same place that you bought the last ones. I wouldn't search for "really good quality" stuff for this but more like something with good enough "touch"/ "feeling". Usage in MB64 isn't really that critical for the quality.

Moebius

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This is something that I was wondering about too - I plan to build a mixer for when I have my SID using 4 mono outs, and I was wondering whether I should use linear or logarythmic pots/faders... My understanding is to use lin for data values - like controlling a midi value, and log for volume or gain. Is this correct?

Also, anyone know where I can get a center-detented pot to use for stereo panning?

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This is something that I was wondering about too - I plan to build a mixer for when I have my SID using 4 mono outs, and I was wondering whether I should use linear or logarythmic pots/faders... My understanding is to use lin for data values - like controlling a midi value, and log for volume or gain. Is this correct?

Yup. You are correct.

Also, anyone know where I can get a center-detented pot to use for stereo panning?

That's a hard one - I've got 10k Alps from the a VCR -- I've heard that Panasonics has similar. (these are 9mm pots or something with 6mm shaft that's marked with center).

And "mee electronics shop" sells these "guitar" parts with 10k CD.. cheap pots, big price.

Moebius

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i sorted it all out, my pots were log.  :-\

now, dont EVER use log! only in special situations!

reason:

let's say a pot has a turning radius of 270 deg. in that turn resistance varies from 0 to 10K ohm. translated to midi signals, the digital range can be fom 0 to 127.

now using a linear pot means that if i turn it by half of full turn radius (135 deg), the resistance will be about 5K ohm, and the midi value about 63. this means that the relation between the degrees turned and the resistance changed is linear, e.g. if turned by quarter (25% of full rotation, clockwise or counterclochwise from any position), the resistance changes by quarter (25%, + or -), and so the approx midi value (e.g. from 63 to 100). this pot will give most accurate representation of the "virtual" knob in an audio software - there will be no difference in ther positions trough all of the turning range.

the log pot simulates logarythmic function in relation turn amount and resistance. the result will be that youll turn the pot by half, and the virtual pot will move just by few deg. and when turned further, the virtual knob will start to rotate faster than the "real" pot and theyll meet at 100%

               _________LINEAR________          ___________LOG_________

rotation    : 25%  :   50%  : 75% : 100%      : 25%  :   50%  : 75% : 100%

resistance : 2,5K  :   5K     :  7,5K : 10K         : 0,8K  :   7,9K     :  9K : 10K

midi         :  32    :   63     :  100  : 127         :  10    :   30     :  115  : 127

this means that the log one is more sensitive in the one turn area, and less sensitive in the other. the only thing i can think of using log pots is in example if you need to controll (lets say) volume, but in a such way that you need very fine adjustments allmost at the maximum of it, but allso be able to turn it down completley - in that situation the half knob turn area uses as fine adjustment, and the other for coarse adjustments. afcourse if you connect the power to the pot otherwise, fine adjustments will be at the 0 - 30% of the volume. oh yes, there are not 2 strict "fine" and "coarse" parts of turn area, but they blend at some point in the turning area.

i hope everyone can get a hint how a log pot works now  ;D

also, is someone needs, the pots can be custom made to represent any function/relations you desire, but those cost bout 2000 EUR  ;)

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this means that the log one is more sensitive in the one turn area, and less sensitive in the other. the only thing i can think of using log pots is in example if you need to controll (lets say) volume, but in a such way that you need very fine adjustments allmost at the maximum of it, but allso be able to turn it down completley...

Not bad analysis - BUT!

Log pots are there for reason, one of them Human Hearing being log curved :D

It means, that we sense log control curve as linear volume rise in our ears.

Moebius

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