Pot tapers, always confused

[martin griffith]

duh, one of those days.
just looking for some rev log pots for a S+K HP filter, I can never
remember which curve is which. But does law C look wrong
http://www.omeg.co.uk/lawe16ct.htm
when compared to
http://www.bourns.com/pdfs/gapno_panel_controls_taper.pdf




martin

After the first death, there is no other.
(Dylan Thomas)

 

[Walter Harley]

"martin griffith" <martingriffith@XXyahoo.co.uk> wrote in message
news6tc61pakr3g6jb0uojkpps3j4idrhhhrt@4ax.com...

duh, one of those days.
just looking for some rev log pots for a S+K HP filter, I can never
remember which curve is which. But does law C look wrong
http://www.omeg.co.uk/lawe16ct.htm
when compared to
http://www.bourns.com/pdfs/gapno_panel_controls_taper.pdf

Unless your confusion has infected me as well, the Bourns chart looks like
it uses different terminology than I've seen from other mfrs; and Omega's C
is what I would call reverse log. I'm used to thinking of A as log, B as
linear, C as reverse log, and D as a steeper variant on A.

Panasonic makes some reverse-log pots, but in the Digikey catalog I don't
see dual ones; just combined A/C, to be used as a stereo balance control.
Noble sell them, but I imagine you'd have to order 10,000.

 

[Joel Kolstad]

Speaking of audio taper pots...

....does anyone have an equation for what the relative resistance of an audio
taper pot is supposed to be vs. position? Empirically I came up with
R(pos)=Rmax*(1.4^pos-1)/(1.4^10-1), where "pos" ranges from 0-10, but I
imagine somewhere this must be defined.

 

[martin griffith]

On Wed, 20 Apr 2005 09:33:18 -0700, in sci.electronics.design "Joel
Kolstad" <JKolstad71HatesSpam@yahoo.com> wrote:

Speaking of audio taper pots...

...does anyone have an equation for what the relative resistance of an audio
taper pot is supposed to be vs. position? Empirically I came up with
R(pos)=Rmax*(1.4^pos-1)/(1.4^10-1), where "pos" ranges from 0-10, but I
imagine somewhere this must be defined.

Do you mean relative resistance or attenuation, as in Volume Control?

Its difficult to define because of the bad cut off at the bottom of a
typical pot. Do you consider the 40dB,60dB or80dB atteuation point as
the other measuring point, compared with 0dB att.

Thats what I like about having a linear pot driving a log VCA, you can
so easily define the end points!
I cant honestly see any point in having any attenuation, below -60, it
makes the scale too cramped. A 40dB range would be better

but i'm sure J.Woodgate will know


martin

After the first death, there is no other.
(Dylan Thomas)

 

[Joel Kolstad]

Hi Martin,

"martin griffith" <martingriffith@XXyahoo.co.uk> wrote in message

Do you mean relative resistance or attenuation, as in Volume Control?

I meant resistance; I'm using one to tune some op-amp parameters rather than
as a volume control.

Its difficult to define because of the bad cut off at the bottom of a
typical pot.

Yeah, I was hoping for the theoretical curve. For my purposes, if a 1k pot is
still 1 ohm at the far end, that's close enough to zero for mean even if it
only is 60dB attenuation.

---Joel

 

[martin griffith]

On Wed, 20 Apr 2005 11:21:15 -0700, in sci.electronics.design "Joel
Kolstad" <JKolstad71HatesSpam@yahoo.com> wrote:

Hi Martin,

"martin griffith" <martingriffith@XXyahoo.co.uk> wrote in message
Do you mean relative resistance or attenuation, as in Volume Control?

I meant resistance; I'm using one to tune some op-amp parameters rather than
as a volume control.

Its difficult to define because of the bad cut off at the bottom of a
typical pot.

Yeah, I was hoping for the theoretical curve. For my purposes, if a 1k pot is
still 1 ohm at the far end, that's close enough to zero for mean even if it
only is 60dB attenuation.

---Joel

have a look at one of my defunct websites...

http://www.wavefront.mcmail.com/pot.htm




martin

After the first death, there is no other.
(Dylan Thomas)

 

[John Woodgate]

I read in sci.electronics.design that martin griffith
<martingriffith@XXyahoo.co.uk> wrote (in
<u83d61tj2ua9mbiu25e4ij61oenh9iivu2@4ax.com&gt about 'Pot tapers, always
confused', on Wed, 20 Apr 2005:

but i'm sure J.Woodgate will know

That's very flattering, but WHAT will I know? The relevant IEC standard
is IEC 60393-5, but I don't know whether it defines non-linear
resistance 'laws'.

Note that the 'log' laws are actually exponential!
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Pooh Bear]

Joel Kolstad wrote:

Speaking of audio taper pots...

...does anyone have an equation for what the relative resistance of an audio
taper pot is supposed to be vs. position? Empirically I came up with
R(pos)=Rmax*(1.4^pos-1)/(1.4^10-1), where "pos" ranges from 0-10, but I
imagine somewhere this must be defined.

In practice, the usual inexpensive pots are made by screening 3 layers of
resistive ink, so merely approximating to a log / whatever curve. See the
manufacturers' curves, the change of slope is very obvious.

There are better ways to get good log accuracy but they cost more.


Graham

 

[Pooh Bear]

Joel Kolstad wrote:

Hi Martin,

"martin griffith" <martingriffith@XXyahoo.co.uk> wrote in message
Do you mean relative resistance or attenuation, as in Volume Control?

I meant resistance; I'm using one to tune some op-amp parameters rather than
as a volume control.

Its difficult to define because of the bad cut off at the bottom of a
typical pot.

Yeah, I was hoping for the theoretical curve. For my purposes, if a 1k pot is
still 1 ohm at the far end, that's close enough to zero for mean even if it
only is 60dB attenuation.

Depends on the exact law. JIS audio tapers A ( volume control ) and C ( 'reverse
log' ) have 15% of total track ressitance at the central position. In practice
anything between about 10% and 20% actually.

That 1 ohm is normally called the 'hop on' resistance.

Other laws are available - usually desired for the 'reverse log' taper. e.g 10C -
means 10% total R @ centre. Manufacturers haven't defined these curves well and
they may vary from one manufacturer to another. The 'RD' curve from Alps is
perhaps best knows - I think it's similar to the 10C. I use a 'Y' taper from
Taiwan Alpha - or 05C pot in one of my mic pre-amps. If you're sweeping frequency
with a pot used as a series element - the ideal Centre value depends on the range
of control. E.g. I use a 20C type part for a swept mid frequency EQ circuit.

You'll only get these parts when making at least modest quantites though.


Graham

 

[Precious Pup]

martin griffith wrote:

have a look at one of my defunct websites...
http://www.wavefront.mcmail.com/pot.htm

Dude,

That kicks ass. Thanks.