One-Knob Wide-Range Tone Control

Copyright 2001, Roy Mallory

Experiments I conducted have convinced me that a reverb channel does not need
a complicated tone control with multiple knobs. However, a control with a wide
range of adjustment is required. The following circuit that I developed meets
all requirements quite nicely and is simple to build and also to modify if so
desired. It allows symmetrical cut/boost of bass and treble as the graph shows.
The graph depicts the output with the control set for full treble boost and full
bass boost. When the control is centered, the response is ruler flat with a gain
of one. This circuit would normally be placed at the output of the signal
recovery amplifier.

As shown below, the circuit consists of a single inverting op-amp stage and
one linear potentiometer. I think it's easy to see that with the pot centered
the input and feedback networks are identical, and therefore the circuit's gain
will be one, with no frequency-response manipulation. As the pot's wiper is
advanced toward the top, the upper network, which is a band-limited integrator,
more controls the stage's response. With the wiper at the bottom, the lower
network, which is a band-limited differentiator, controls the response. Because
only the op amp's bias current flows through the pot's wiper, the circuit is
largely insensitive to changes in the wiper contact resistance. The gain of the
circuit extends to D.C., so if the input to the circuit has a D.C. offset,
capacitive coupling should be used– a 0.47 uF capacitor works nicely. Someone
skilled in tube electronics will readily see how this circuit can be adapted for
use with tubes.

Many modifications to this circuit are possible. You will note that the
frequency response graph shows that the circuit tilts about 1 kHz. I chose this
frequency because it didn't cause the perceived volume of the reverb signal to
change much as the frequency response control was adjusted. This effect will be
modified by the frequency response of the rest of the amplifier's circuitry and
speaker(s), of course. If one were to use this circuit for a simple guitar amp,
it would make sense to lower the tilt frequency to around 500 Hz. Changing the
0.022 uF caps to 0.047 uF accomplishes this task.

Lowering the 2.2k resistors increases the adjustment range. The amount of
treble boost/cut can be changed with respect to the amount of bass boost/cut, by
changing the value of the potentiometer. Be careful, because many of these
component changes are not orthogonal, i.e., changes may affect more than one
aspect of the circuit's characteristics. If you would like to play with
component values, one good way of doing so is to download the free student
edition of Pspice, available at http://www.orcad.com/. Spice can be a bit
frustrating to learn, but is well worth the effort for those who enjoy playing
with circuit design.

As always, the selection of op amp will affect the sound quality and noise
the circuit produces. A good choice would be Burr-Brown's OPA134, 2134 series.
Other good choices would be Analog Devices' AD823 or Burr-Brown's OPA604, 2604
series.

When I developed this circuit, I had never seen this topology used, and so
thought it was original to me. Several months later, I saw essentially the
identical circuit, though with different values, in "Audio Xpress" magazine with
a reference to its design in 1991–so much for originality!

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