30 October 2014
driving the audio cable and amplifier. Actually, it all gets
quite complex with other matters such as signal-to-noise
figures, RF interference, and even the capacitance of the
connecting audio cable complicating the issue. In any event,
the result is usually a poorer tonal response than we’d hope
for even with the controls open full tilt. So, we have several
problems to overcome. Passive circuitry like this: tends to
load the pickups down; has a high output impedance less
than ideal for driving the amp; and offers no more than a
– 6 dB/octave cut, which isn’t very inspiring.
Enter the Active Controls
Pretty clearly, the solution lies in internal active
electronics. Commercial affairs didn’t appeal to me because
of the price, and a search of previous DIY projects failed to
turn up circuits with the features I desired. So, I decided to
go it alone by starting from scratch.
What resulted is a tiny bit of circuitry fitting completely
inside the electric guitar. It offers a whopping 15 dB of both
cut or boost in three separate bands. Many people think of
emphasizing a frequency range to be the touchstone, but
actually the deep reductions possible with this unit are just
as important. It is often the case that pickups on their own
emphasize the midrange which (to my ears) yields a muddy
result. With this circuit, it is possible to flatten out the
response giving a fuller sound at the two extremes. Of
course, with multiple bands, it’s possible to create all sorts
of tonal mixtures right from your fingertips.
The benefits don’t stop there, however. Even if you ran
this device flat (the bass, midrange, and treble controls set
to their midpoints), your guitar would still sound more alive.
The reason for this is that the pickups are now fully
buffered. Loading and attenuation of the highs are nothing
more than memories. Moreover, the output has a very low
impedance, allowing you to drive any amplifier or effects
device without losing crispness. Of course, this is also a
preamplifier, which can boost the signal and may prove
useful for overdrive applications, among other things. If this
all sounds appealing, then let’s get into the circuit details.
A Look at the Circuit
Figure 2 shows the schematic for the active tone
controls. We’ll move the pickup switch to the input of the
rig now, which allows us to buffer any combination
selected; compare the location of the switch in Figures 1
and 2 to see this. The wiring of your guitar may change, but
you shouldn’t have to drill any new holes or leave old ones
unused. (Many electric guitars have four potentiometer
■ FIGURE 2.A three-band internal equalizer.