Linear pot taper...not so linear

john12ax7

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I have some center detent linear pots for an EQ installed but behaving strange. According to spec they should be linear and reaching min or max resistance at 5% and 95%, fairly typical. But mine are more like 20% and 80%, essentially approaching close to max cut and boost at 3 o'clock and 9 o'clock of the pot position. The response is fairly symmetrical.

Obviously this is not good. The question is whether it is poor performance due to process variation, or some strange taper that got mislabeled?
 

gyraf

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Sure they are linear? Many stereo balance and eq are MN-curve, like you desctibe, two opposite pots..
 

john12ax7

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Bourns PTD902-2215K-B203 from Mouser. Attached is the taper graph. Possible they made a batch of B2 and called them B? That would sort of explain things, but kind of  a big mess up.
 

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JohnRoberts

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I recall an "S" (?) taper from alps that was 50% at mid rotation but slowed at both extremes. I used it in place of conventional B taper (linear) for my old kit company back in the 70s.

JR
 

john12ax7

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I was able to find and measure an uninstalled pot,  and it too exhibits the same behavior.

So the question is really how do I find out who messed up,  Bourns or Mouser?  And how to prevent it happening in the future?
 

Brian Roth

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Find the part number printed on the body of the pot, then check that vs. the Bourns data sheet and the part number on your Mouser invoice.

Bri
 

john12ax7

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The only labeling is B20k.

I found a BI pot,  also labeled B20k, and it measures the expected 5% to 95% linear taper.
 

JohnRoberts

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john12ax7 said:
The only labeling is B20k.

I found a BI pot,  also labeled B20k, and it measures the expected 5% to 95% linear taper.
B20k sure looks like B taper which IIRC is linear...

JR
 

john12ax7

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I got an answer for this after sending some parts back to Bourns.  It turns out that the dual gang pot is actually an S type taper (B3). They did this to improve tracking.  The single gang pots use the normal linear taper (B).

The problem is they never updated the data sheet to reflect this, both single and dual gang simply use "B" in the part number to indicate taper.  Rather confusing and frustrating imo,  glad I checked them before ordering 500 pcs.

They did say they would update the datasheet in the next revision.
 

abbey road d enfer

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john12ax7 said:
  It turns out that the dual gang pot is actually an S type taper (B3). They did this to improve tracking.
Doesn't make much sense to me. Non-linear tapers are less reproductible.
 

pucho812

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john12ax7 said:
I got an answer for this after sending some parts back to Bourns.  It turns out that the dual gang pot is actually an S type taper (B3). They did this to improve tracking.  The single gang pots use the normal linear taper (B).

The problem is they never updated the data sheet to reflect this, both single and dual gang simply use "B" in the part number to indicate taper.  Rather confusing and frustrating imo,  glad I checked them before ordering 500 pcs.

They did say they would update the datasheet in the next revision.

so was that their answer, they will date the data sheet. no we will replace them for you with correct ones?
 

john12ax7

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pucho812 said:
so was that their answer, they will date the data sheet. no we will replace them for you with correct ones?

Pretty much yeah.  I'm guessing a big customer wanted the S taper so it became the standard,  so a straight linear would probably be a custom order at this point.

The parts cost itself is rather small. All the time lost is much more costly.
 

abbey road d enfer

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I re_read your original post.
Actually, S-taper pots make sense in a swinging-input EQ. When I designed graphic EQ's I switched from 10k lin to 50k S-taper; it resulted in instant noise improvement and smoother gain control a the travel ends, so it could make sense to replace all the faders. Boost/cut indications would not be accurate anymore, but who cares?
 

JohnRoberts

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abbey road d enfer said:
I re_read your original post.
Actually, S-taper pots make sense in a swinging-input EQ. When I designed graphic EQ's I switched from 10k lin to 50k S-taper; it resulted in instant noise improvement and smoother gain control a the travel ends, so it could make sense to replace all the faders. Boost/cut indications would not be accurate anymore, but who cares?
+1 I used S taper pots for my kit business back in the 70s. Nice for Baxandal tone controls, but i also slugged them for volume controls.

In my experience real*** customers get whatever taper they want... even in my kit business days I had to buy 1k or 5k pots at a time each value (from alps), but I got full semi-custom pot configurations (shaft , bushing, taper etc).

JR

*** FWIW while at Peavey we had one major pot manufacturer decline our business (he said we were too picky).  :eek: :eek:
 
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