TECHVIEWS
Exploring Tone Controls
Practically every piece of musical electronics
we use has one or more tone controls. We all
use them intuitively, but let’s look closer.
To make frequency selective networks, we
must use capacitors and/or inductors. Unlike
resistors, both caps and inductors discriminate against some frequencies in favor of
others. Capacitors preferentially pass higher
frequencies; inductors pass lower frequencies
more easily. Resistors help us set how much
gets through, while caps and inductors select
which frequencies get through. Inductors are
big, heavy and expensive, so almost all tone
controls use only caps and resistors.
The simplest tone control is the one inside
practically every guitar. That knob is a single
potentiometer set up as in Figure 1.
The signal from the pickup coil goes through
the internal impedance of the pickup itself,
then to the output jack. The capacitor C and
resistor R are in series to ground from the
guitar signal. C shunts signals above some
cutoff frequency to ground. R prevents this
by resisting the signal flow to ground. As R is
made smaller, more and more treble is lost.
However, the bass level remains at the same
volume as it was before the treble cut.
One step up is the combination of a treble
cut and a bass cut, with a single knob to
select between them, like the one in the
Electro-Harmonix Big Muff (Figure 2). The
knob selects a blend of highs left over from
the bass cut side and lows left over from
the treble cut side. If you make the cut
frequencies of the treble and bass sides
far apart, you get a persistent midrange
scoop, as in the original Big Muff. You
can also have the bass and treble sides
overlap in the middle and get a midrange
hump in response. The near-endless variations you can get by tinkering with the relative values of the parts and the need for only
one knob make this a favorite in effects.
Another tone control we almost all come in
contact with is the amplifier tone stack, as
sketched out in Figure 3. A sequence of evolution at Fender led up to the 1957 Bassman
becoming the prototype for most amplifiers’
Treble/Bass/Mids control knobs. Marshall
and Vox used a similar system. The amplifier
“tone stack” is just that – a stack of two or
three potentiometers which provide treble,
bass, and sometimes midrange controls.
From a perfectionist/engineering standpoint,
there is a lot wrong with this circuit. The
controls are complexly interactive. There is
no really flat setting, as there is a residual
midrange scoop unless the Treble and Bass
controls are fully down. The action of the
Bass control is very uneven for normal taper
controls. While the Mid control appears to
affect only the mids, it is actually a form of
volume control that affects all frequencies,
but is the only control that also affects mids.
Although it has several imperfections, this
tone stack actually helps some of the quirks
of guitar, so it works well.
From the standpoint of theoretical perfection, the Baxendall tone control is the
opposite of the Fender tone stack. With
bass and treble variations that are mostly
independent, the frequency response is quite
flat when both controls are set to the middle
of their range using linear pots. Using the
opamp/feedback form of the controls means
that there is no signal loss that needs be
made up elsewhere. This is virtually the standard for hi-fi tone controls. However, there
are some guitar amps which use this form of
control, tweaked a bit to match the guitar
frequencies.
There are a huge number of different style
tone controls in musical equipment; this
hardly scratches the surface. If you’re interested in what your tone controls do to your
sound, agood place to look is at Duncan’s
Amp Pages, where there is a tone stack calculator that shows frequency graphs of several different types of tone controls. Check
out http://www.duncanamps.com/tsc/ for
more info.
R.G. Keen
Chief Engineer
Visual Sound
visualsound.net
