We started discussing pot tapers last
month, and we’ll discuss that a little more
this month, but first we’ll have a look under
the hood, so to speak, at a pot’s innards.
You’ll recall that a pot is a variable resistor
but you may wonder how the amount of
resistance is varied. You’re in luck my friend,
because I’ve been busy drawing pictures.
Let’s have a look at an illustration showing
a typical pot on the left and one with the
cover removed on the right.
You can see that the two outer solder lugs
are connected by a ring-shaped strip of
carbon (dark grey), while the center lug is
isolated from them. The carbon strip acts
as the resistor, and if you read between the
two outer lugs with a meter, you’ll read the
full value of the pot plus/minus its tolerance. So measuring a 500k pot with a 10
percent tolerance would give you a reading
of somewhere between 450k and 550k.
REARDeIaNdGin TgHtEheFUfuLlLl vVaAluLeUEofOaF pAoPt OT
When you turn the shaft, you’re moving
a little “bridge” (called a wiper) that connects the middle lug (the inner ring) to the
carbon strip. The position of this bridge is
determined by the position (the rotation) of
the shaft. You can see the wiper in the following illustration as the yellow rectangle
– it has been much simplified in the interest
WIPEWRip( YeE r L(yLeOll Wo w) )MmOoVvEeSs TtoO dDifIF feFrEeRnEt N T
POSpI ToIsOitNioAn Sa P s Op T otSsHhAaF ft TisISroRtOa TteAd TED
You can see the consequence of rotating
the shaft below; the signal path (shown as a
red line) passes through more or less of the
carbon resistor, depending on the shaft’s
SIGNSAigLnPaAl SpSaEsSse TsHtRhOroUuGgHh mMoOrReEoOr lResLsESS
OF THofEtChAe RcBaO rbNonAS asSsHhAaF ft TisISroRtOa TteAd TED
As mentioned, pots come in audio taper
(also known as “log” – short for “
logarithmic”) and linear taper variants. Audio taper
pots are typically preferred in those applications where they’ll perform some volume-controlling function. The reason is that we
humans don’t hear changes in volume in a
linear fashion; we hear them in a logarithmic fashion and so audio taper pots are
designed to mimic this.
Consider the experience of a hypothetical
guitar-playing friend named Bob, who has
recently been surprised to discover that a
10-watt amp is pretty loud. Having reveled
in the prodigious volume of his 100-watt
amp, but having also alienated his family
and the neighbors, Bob decided to buy
a 10-watt amp, thinking that it would be
only one tenth as loud and thus would be a
great practice amp. The surprise came when
he discovered that the 10-watt amp was
roughly half as loud as the 100-watt amp.
Run through his trusty 4x12 cabinet, the
10-watt amp continued to alienate his family until they would no longer allow Bob to
dine with them.
Half as loud? But how can that be?
Because humans perceive volume changes
in a logarithmic way, a tenfold increase
in power (watts) equals roughly twice the
volume to human ears. It follows then that
a 1-watt amp would be half as loud as a
10-watt amp and a quarter as loud as a 100-
watt amp. It probably seems fantastic (read:
impossible) that a 1-watt amp would be a
quarter as loud as a 100-watt amp, but it’s
true nonetheless, with the caveat that this
is all theoretical; in the real world there are
other mitigating factors that impose themselves into the equation. Still, plug into the
average 18-watt amp and you’ll find that it’s
completely capable of producing big volume, despite its diminutive output.
Such is the surprising power of exponents.
Next month, we’ll look at some illustrations
that will show the difference between an
audio taper pot and a linear taper one. I’ll
get busy drawing.
Founder, Acme Guitar Works