Changing Potentiometer value??

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signalflow

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I know strapping a resistor across a pot will alter the value but will it also alter the taper?

Also, since the matching of gangs on a multi-gang pot has a variance of 10-20% per gang in some cases, would it be better to use a trimmer to fine tune each gang? Or just use the trimmer to find the value and use the closest matching 1% resistor there is to achieve the goal? (example: 4 Gang 10KA pot needs all 4 gangs to change to a 5KA.)

Double Also, Does the application have any bearing on performance? Say, like in a LP filter?

-Casey

 
signalflow said:
I know strapping a resistor across a pot will alter the value but will it also alter the taper?
It depends if the pots are used as voltage divider or rheostat.
A resistor in parallel with a pot (from top to bottom) does not change the taper, but AFAIK does not do anything.
A resistor across wiper to bottom or across wiper to top definitely changes the taper.

Does the application have any bearing on performance? Say, like in a LP filter?
Depends on the topology.
Typically a sallen & Key LP filter uses rheostats. A resistor across a rheostat-connected pot tends to cramp the higher frequencies.
 
Rough rules of thumb for slugging a linear pot between wiper and ground to approximate an audio taper, assuming no other appreciable load:

A resistor value of 50% of the pot's value gives a 10dB reduction at the halfway point (this is what Neve did - 10K Lin and 5K1R))

A resistor value of 25% of the pot's value gives a 12dB reduction at the halfway point. 

True audio taper pots are closer to 20dB at the halfway point. 

Slugging a linear pot for audio will usually guarantee better channel to channel matching, unless you use very poor linear pots, or conversely, are prepared to use very good log pots (expensive)


Make sure your preceding circuit can drive the parallel load of the slug resistor and pot.



     
 
Well maybe I should repeat my standard caveat.... Typical potentiometer specify 20% bulk resistance tolerance, the taper tolerance can be much better (a function of screen operation accuracy).

If slugging pots for anything important "measure  the actual pot" or the 20% bulk resistance will give variable results.

Back in the 80's I had my technician measure and slug some quad pots at mid rotation to improve tracking in a 4 pole analog crossover frequency adjustment. Even a premium Alps quad pot is not good enough to track adequately over the full frequency range without slugging based on empirical measurement.

JR 

PS: Large scale manufacturers buy the specific taper they need and at Peavey we had several custom tapers fabricated (like for mic gain). A luxury when you use that many pots.
 
JohnRoberts said:
 
If slugging pots for anything important "measure  the actual pot" 


Yes sorry, I forgot to specify that bit.  Slug the pot with 25% or 50% or X %  of the actual pot value for critical matching. 





 

 
Thanks everyone! "Slugging potentiometers" is definitely a better search term. Way more info came up. I'm not trying to alter the taper just the value. But it's sounding like it doesn't work that way as a voltage divider which is the way it's being used.

Probably less of a headache to try and order samples from a manufacturer.

-Casey
 
It's the HPF/LPF section of the AMEK CIB. Schematics are in the Tech Doc section.  Filters are at the top of page 2 and the pot wiring is at the top right side of page 7.

I have the BX_9098EQ plugin and really like it. Would thought it might be cool to have a hardware version to track with.

Right now I'm just trying to recapture the schematic in EAGLE and make sure that there aren't any un-obtainium parts in the circuit. Right now it's just the triple deck 5KA LPF pot that seems hard to come by.  Bourns PTD90 series works for all other pots but can't find a supplier that carry's the 5KA.

-Casey
 

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signalflow said:
It's the HPF/LPF section of the AMEK CIB. Schematics are in the Tech Doc section.  Filters are at the top of page 2 and the pot wiring is at the top right side of page 7.

I have the BX_9098EQ plugin and really like it. Would thought it might be cool to have a hardware version to track with.

Right now I'm just trying to recapture the schematic in EAGLE and make sure that there aren't any un-obtainium parts in the circuit. Right now it's just the triple deck 5KA LPF pot that seems hard to come by.  Bourns PTD90 series works for all other pots but can't find a supplier that carry's the 5KA.

-Casey
Actually, it's quite easy to adapt the circuit for 10k pots. You need to double the resistor values and halve the capacitor values that are marked with a star.
However it raises the question of the taper.
If the legend is CW increasing frequency, you need Reverse-Log pots ("C" taper). If the legend is CCW increasing frequency (rare but not unseen), you need Log pots.
 

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