T ECHVIEWS
Are You Listening With Your Eyes?
Some people have the idea that controls on the shaft picks off the voltage or current from
a new effect should always be set to the mid- that middle position with a wiper that makes
point or 12:00 on a hypothetical clock. I’ve contact along the resistive path. With linear
heard a variety of answers about why, includ- taper pots, the pot manufacturer makes the
ing that the unit in question gets used up or resistive path as linear and consistent along
damaged somehow by extreme high or low the path as they can for the price. To the
settings. The inaccuracies in controls make extent that the pot wiper contacts the true
this argument somewhat nonsensical. ends of the resistor element, you get a
constant change per degree of rotation of
the shaft. However, the wiper may not go all
the way to the end of the resistive element,
taper pot for a volume control makes the
result seem to be all bunched up at the low
end of the pot’s rotation.
Knobs have a line or dot to show relative position. Some equipment will have a
numeric scale printed onto the faceplate
behind the knob, which suggests some level
of precision in the pot setting. This practice
was immortalized – correctly! – in the movie
Spinal Tap when guitarist Nigel Tufnel talks
about all of the knobs on their amps going
to eleven, based on the idea that a marking
of “ 11” was one more than the same pot
rotation marked “ 10”.
Think of the pot indicator
lines and the numbers on
the faceplate of an effect
or amp as you might think
of the position indicators
on a fretless bass or guitar
– what you get depends
on how you’re playing the
string and it’s your ear that
tells you when it’s right.
In the hundreds of boutique versions of the
Ibanez Tube Screamer circuit, the tone pot
is usually a linear “B” taper, simply because
tapers other than linear and audio are not
available in small quantities. The more
extreme “G” taper compensates for the nonlinearity of the circuit itself, making the effect
sound more linear than the B taper as you
rotate the knob.
You need to set your equipment by what
your ears tell you, not your eyes.
Another example is in one of our pedals, the
Jekyll & Hyde. We used an “A” taper for the
Hyde tone pot originally, but changed to a
“W” taper for a slightly better feel in terms
of audible change per unit of pot rotation.
In both cases the knob had exactly the same
minimum and maximum, but the sound at
the same rotational setting is different and
the feel is more linear to the ear as you
rotate the pot.
Knobs are not generally aligned on the pot
well. The best potential for accuracy and the
worst in practice is the round shaft and knob
with setscrew. It is theoretically possible for
this to be set to the limits of the pot’s own
accuracy. However, setscrews work loose,
knobs twist on the shaft and any hope of
positional accuracy is lost. The fixed-position
press-on knobs do a better job; the D-shape
with a flat and the toothed shape at least get
you to a knob position that does not slip.
Pots come with PIPs on the bushings to lock
them so they won’t rotate on the panel.
However it’s common practice in the boutique effect world to cut the anti-rotation
PIPs off to avoid having to drill a matching
hole in the box for the PIP. So the rotational
position of the pot in the faceplate is again
not always precise.
When you add all of this up, you get a big
uncertainty about what a particular setting
sounds like. It’s much better to set it by ear –
just as it almost always is with musical equipment. Think of the pot indicator lines and
the numbers on the faceplate of an effect
or amp as you might think of the position
indicators on a fretless bass or guitar – what
you get depends on how you’re playing the
string and it’s your ear that tells you when it’s
right. After that, it’s perfectly okay to mark
your preferred positions.
leaving some resistance at the end that the
wiper can’t reach, leaving you with zero not
being zero, and full up not really full up. The
opposite effect, over-travel, can produce
dead zones at each end of the rotation.
Full up or full down on a pot is not necessarily 0 percent or 100 percent of the possible
function of the knob controls, and mid position is usually not exactly 50 percent. High
precision pots do exist, but they cost from
ten to a hundred times more than the simple
controls on most musical gear.
Pots are variable tapped resistors. They have
a resistance from end to end and rotating
The pot’s “taper” can be thought of as how
fast the pot turns electrically as opposed
to mechanically. Pot taper is used to cancel out other nonlinearities. For example,
a volume pot needs to increase its output
slowly per degree of shaft movement at the
low end and quickly at the high end. This
is the reverse of the human ear’s response
to loudness, so the result is that turning the
knob seems to change the loudness an equal
amount per degree of rotation. Using a linear
R.G. Keen
Chief Engineer
Visual Sound
visualsound.net
