replacing a stepped attenuator with a potentiometer basic math

[pucho812]

Basic math here  but  if I was to replace a stepped attenuator with a potentiometer, you add up all the resistors and pick a pot closest to that value. Ideally if they are all the same resistor value for each step it is a linear pot and if not then we are looking at an audio pot or some other taper.

I am trying to wrap my head around the stepped attenuator below. I redrew it from elsewhere and it is described as an 11 position gain switch. Yes in the original schematic they drew12 positions and skipped position 1 and position 12. The confusing part for me is the fact  that the last position  is 2 resistors in parallel.  Would I get their resistance first, and then use that value with all the other resistors in series to get the value I need?

 

[gyraf]

gain switches are most-often reverse-log

in this case probably a 2k2 or 4k7

/Jakob E.

 

[abbey road d enfer]

Basic math here  but  if I was to replace a stepped attenuator with a potentiometer, you add up all the resistors and pick a pot closest to that value. Ideally if they are all the same resistor value for each step it is a linear pot and if not then we are looking at an audio pot or some other taper.

I am trying to wrap my head around the stepped attenuator below. I redrew it from elsewhere and it is described as an 11 position gain switch. Yes in the original schematic they drew12 positions and skipped position 1 and position 12. The confusing part for me is the fact  that the last position  is 2 resistors in parallel.  Would I get their resistance first, and then use that value with all the other resistors in series to get the value I need?

I believe there's an error. This schemo does not make sense. Check the connections of those two resistors in parallels.

 

[JohnRoberts]

Unlikely to find a pot (rheostat) with that taper.

JR 

 

[jacomart]

I agree with abbey road d'enfer, the schematic must result in some sort of voltage divider. There is something wrong in your schematic.

 

[pucho812]

I may have redrawn it wrong, but I double checked my work  Here is the original.

the two connections would make it variable resistor...

 

[abbey road d enfer]

I may have redrawn it wrong, but I double checked my work  Here is the original.

the two connections would make it variable resistor...

OK, so the taper is CCW 6.05 ohm, CW 2776 ohms, and 119 ohms at mid rotation.
That's an A3 taper. I don't believe it exists in standard form. Most A tapers are between A10 and A20, the common standard being A15.

 

[pucho812]

OK, so the taper is CCW 6.05 ohm, CW 2776 ohms, and 119 ohms at mid rotation.
That's an A3 taper. I don't believe it exists in standard form. Most A tapers are between A10 and A20, the common standard being A15.

So for example A10 means 10% and  A15 15% audio taper. Basically the resistance is 10% or 15% measured at mid rotation of the pot, correct?

I am seeing A3 taper also listed as 15A on this bourns data sheet.
Would this be the same?
https://www.bourns.com/docs/product-datasheets/PDB24.pdf

 

[john12ax7]

I believe A3 was meant to mean 3%. The Bourns 15A:A3 is 15%, wouldn't be the same.

You would also probably want it to be reverse log taper,  if going position 1-12 is CW. A reverse log >90% is a common need for smooth gain range in mic pres.  Many products, however,  make due with the common reverse log 85%, the consequence is the gain range is not evenly spread out.

Perhaps a custom part group buy is in order.

 

[pucho812]

I believe A3 was meant to mean 3%. The Bourns 15A:A3 is 15%, wouldn't be the same.

You would also probably want it to be reverse log taper,  if going position 1-12 is CW. A reverse log >90% is a common need for smooth gain range in mic pres.  Many products, however,  make due with the common reverse log 85%, the consequence is the gain range is not evenly spread out.

Perhaps a custom part group buy is in order.

I cant imagine anyone needing this other then me and it was for a specific thing. might rethink this

 

[john12ax7]

I cant imagine anyone needing this other then me and it was for a specific thing. might rethink this

All of the common mic pres people like to build need it,  THAT 1510, API 312, etc.

You could try one of the readily available tapers,  might be an ok  compromise for the application.

 

[jacomart]

So for example A10 means 10% and  A15 15% audio taper. Basically the resistance is 10% or 15% measured at mid rotation of the pot, correct?

I am seeing A3 taper also listed as 15A on this bourns data sheet.
Would this be the same?
https://www.bourns.com/docs/product-datasheets/PDB24.pdf

in any case it’s a really strange type of attenuator, made with a variable resistor in series with the signal path...

 

[abbey road d enfer]

So for example A10 means 10% and  A15 15% audio taper. Basically the resistance is 10% or 15% measured at mid rotation of the pot, correct?

I am seeing A3 taper also listed as 15A on this bourns data sheet.
Would this be the same?
https://www.bourns.com/docs/product-datasheets/PDB24.pdf

A3 is a brand specific denomination.
It's really an A15, in accordance with common usage.
Their C1 (actually a 5C - 5% at mid-rotation) would be an acceptable compromise.

 

[abbey road d enfer]

in any case it’s a really strange type of attenuator, made with a variable resistor in series with the signal path...

this subject came out recently.
https://groupdiy.com/index.php?topic=65345.msg973344#msg973344
https://groupdiy.com/index.php?topic=65345.msg973675#msg973675

 

[Martin Griffith]

Some antilog pots from alpha, here https://www.taydaelectronics.com/potentiometer-variable-resistors/rotary-potentiometer/anti-log-reverse.html

 

[abbey road d enfer]

Some antilog pots from alph

Their C type is a C15, their X type is a C10. Does not come close to C5, even less to the ideal C3.