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50kB to 25kC Pot Mod?

Started by Bio77, December 17, 2018, 10:10:37 PM

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Bio77

Hi all,

I need a 25kC pot for my next build, but I'm having trouble finding one.  Can I solder a 50k resistor across two legs of a 50kC pot as a work around?   

cooder

BigNoise Amplification

Bio77


Rockhorst

Some info on the particular circuit would be helpful.

Bio77

#4
Quote from: Rockhorst on December 18, 2018, 09:24:10 AM
Some info on the particular circuit would be helpful.

Here is a shot of the part of the circuit in question, I thought that would be more illustrative.  I can upload the whole schematic if necessary. 

I plan to place the resistor across pin 1 and 3 of the pot.

Bio77

I did some searching on pot tapers and learned that most audio pots are not truly logarithmic, they are a combo of two linear tapers.  I'm thinking that placing a resistor in parallel will actually give a more anti-log response.  I'm excited to give this a try. 

Here is a picture of different pot tapers and also some plots I made for tapers expected from different combos of pots and resistors. 

cooder

Very cool, do you have a software to calculate the taper changes or a link to a calculator?
And yes the proof is in the pudding. Give it a try.
BigNoise Amplification

Bio77

I made the taper plots in excel, I can send you the file if you want, PM me.

cooder

Quote from: Bio77 on December 18, 2018, 11:50:51 PM
I made the taper plots in excel, I can send you the file if you want, PM me.
Very kind and cool, will send you PM! Cheers and Merry Xmas!
BigNoise Amplification

bryanrichie

This is very interesting! Would it be possible to simulate a 50kC pot out of a 250kB pot?

EBK

If you can find a 50kC dual gang pot, you can wire both halves in parallel and not have the taper change.  I did something somewhat similar on a build once.
"There is a pestilence upon this land. Nothing is sacred. Even those who arrange and design shrubberies are under considerable economic stress in this period in history." --Roger the Shrubber

Bio77

Quote from: bryanrichie on December 28, 2018, 09:19:37 PM
This is very interesting! Would it be possible to simulate a 50kC pot out of a 250kB pot?

I haven't tested my idea out yet, but the math works.  Actually, you just made me realize that the OP was for converting a C pot to a C pot, which isn't nearly as cool as converting a B to a C.  That's what is modeled above. 

Here is a few plots of the R tapers 50kCs made with a 100kB, 250kB, and a 500kB.

 

cooder

That's very usefull stuff, thanks! Also thanks for sending me those excel files, top notch!
BigNoise Amplification

Bio77

So, I tried this and it didn't work.  Does any one have any ideas where my logic is flawed?

I used a 100kB pot and ran a 33k resistor in parallel with lugs 1 and 3.  So R14 should always see 25k from the pot (100*33/(100+33)=24.8).  The voltage divider at lug 2 should work the same no mater the value of the resistors used.

Does that seem right?


Bio77

So, I switched out the work-around (100kB with 33k) for a 25kB pot, the mids control is still not appreciable.  Maybe the work-around was working after all. Any thoughts?