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Re: OT linear and log pots

ok, boring off-topic technical discussion of the differences between
amplitude and volume, what your ears hear as loudness, and the differences
in log and linear pots follows:

At 4:47 AM -0800 1/16/00, Jeff Duke wrote:
>Kim Flint wrote:
>> you have that backwards.
>> Audio (or log) taper gives a steady increase in volume throughout the
>> range. This reasonably matches the way your ears respond to volume. 
>> doubling the signal amplitude does *not* sound twice as loud to your 
>> it takes an exponential amplitude increase to sound like a linear volume
>> increase to you.)
>> Linear pots tend to sound like all the volume change happens right in 
>> beginning of the range, then you hardly hear any change through the 
>rest of
>> the range of the pot. They do not sound like a steady volume increase,
>> which is why the other sort was invented.
>> So volume pots are usually audio/log, expression pedal and such are 
>> linear.

>Well I had to turn to my book Electronic Projects for Musicians by Craig
>but here is his definition ;
>"One other charactoristic of pots, taper, might cause confusion. The taper
>of a pot
>is another word for the rate at which the resistance changes. The most
>common is
>linear" "turning it halfway gives half the resistance" "a quarter gives a
>"two-thirds gives two-thirds,and so on."

yes, of course that is true. That is what I am trying to tell you. This
gives you a linear amplitude response on the audio signal. Your ears *do
not* hear a linear amplitude increase as a linear volume increase!

In fact they respond to power, and that power is spread over exponentially
increasing volumes of air as you move away from the source. So it requires
exponential amplitude increases for your ears to hear a "linear" volume
increase. So when you use a linear pot, you will *not* hear a steady volume
increase as you go through the range of the pot because the pot is only
linear for amplitude and not power. And certainly not linear for the power
per square meter that we hear! What you will hear with the linear pot is
all the volume change happens at the beginning range of the pot, and you
will hardly hear any change at all over the remaining 3/4 of the range of
the pot.

This is why volume measurements are made with dB units. (maybe this will
help you understand, by relating it to a dB meter.)  dB is a logarithmic
scale. If you double the amplitude, you get only a 3dB increase in volume
(or a 6dB increase in amplitude, to be anal, but we use power for volume so
it is divided in half for a dB volume scale). 3dB volume change is not
much, and certainly not twice as loud. calculating dB is easy with a
calculator with a log function, dB volume = 10 * log (change in amplitude)

So if you consider a 100K linear pot, you will be at 50K at the middle
point. From there to the 100K point will double the amplitude of your
signal, but only give you a 3dB change in volume. ( 10log(2) = 3dB )  You
will hardly hear this and consider that whole 1/2 range of the pot as
useless. 10dB change in volume is usually considered twice as loud. This is
a 20dB change in amplitude, which is a 10x change in amplitude.

>"A log taper pot, however increases resistance logarithmically" "This
>means that
>turning up the pot halfway covers only about 10% of the total resistance;
>two-thirds covers about 40%" "as you get past this point, each degree of
>covers a progressively greater amount of resistance"

exactly, this is frequently called an "audio" pot. This does more closely
match the way your ears hear volume, since this sort of exponential
amplitude response results in a reasonably linear "power per square meter"
response. So your ears hear this as a steady increase in volume (linear)
through all of the pot range. In any audio product, any knob marked as
volume will use a log (or audio) taper pot.

>I am paraphrasing but I believe that this is what my point was in my lame
>description. I prefer linear for my vol. pedal so I now use an Ernie Ball
>which is

If that was sold as a volume pedal, it almost certainly came with a log pot
inside. Get an ohm meter and check it, you'll see! That log pot is what
gives you the linear volume change you like.

hope this helps,


Kim Flint                   | Looper's Delight
kflint@annihilist.com       | http://www.annihilist.com/loop/loop.html
http://www.annihilist.com/  |