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Pedal Insider: Control Freak PDF Print E-mail
Tuesday, 07 October 2008 00:35
By Theo Hartman - GJD Contributor
The first two installments of this column discussed examples of how pedal tone can influence—and be influenced by—electronic and acoustic factors upstream and downstream of the pedal itself.  A third and possibly more subjective criteria in pedal design is us--people, guitar players—and how we expect our gear to operate.

A good point of departure to explore matters more technical is to examine examples of how we set up circuits to be “user-friendly” (or not).  I refer to this part of design as “control logic”.  Do dials and switches work as you expect them to?  Is their range of operation musically useful?  Redundant?  Overkill?  Showstopping?

One of the simplest and most prevalent user controls on any pedal is the Volume control.  There are numerous ways to implement a volume control, but one of the most common (passive) designs is as follows:

Figure 1

The output of the effect part of the circuit appears at the top of a resistor, the other end of which is connected to signal ground.  The voltage difference between the signal and ground ends of the resistor at any given instant is the “full-volume” signal.  If we replace the fixed resistance with a potentiometer (Figure 1, above) and take the output from the wiper of the pot that traverses between the two ends of the resistance, we will get fractions of the full voltage ranging from 1/1 (~100%) at the top, to 0/1 (nothing) at the bottom.

The above model is typical of passive instruments as well as many pedals.  In almost every case, the type of potentiometer chosen has what’s referred to as Audio or “Log” taper.  What is taper?  A 500K linear taper pot and a 500K audio taper pot both have 0-ohms and 500kohms as their extremes, but how they get there is different:

       Logarithmic (audio) Taper                               Linear Taper

Whether the pot is linear or logarithmic, the range of operation is identical.  But because the logarithmic increase in voltage most naturally corresponds how we perceive an (acoustic) increase in volume, that taper is chosen.  The movement of the control matches the perceived change in volume well; the hand and the ear move together.

A linear taper pot in this type of application will work, but it tends to cram the entire usable range of “medium volume” setting into a very narrow band of the knob’s turn, usually right at the end of the sweep.  As a result, much of the control’s range of motion ends up having no corresponding musical function.  The hand’s movement is now out of scale with what the ear hears.

A different application requiring choice of pot taper is the use of a variable resistor (pot) in the feedback loop of an op-amp to introduce voltage gain for clipping/distortion or other purposes.

Figure 3

In this garden variety distortion circuit, distortion is achieved by amplifying the input waveform sufficiently to encounter the op-amp’s limited ability to reproduce the waveform accurately.  The amount of amplification is determined by the ratio of two resistances in the circuit.  (R2/R1)

In Figure 3, above, one of the resistances is fixed.  The other is a user control (often emerging on the outside as “Gain”, “Drive”, or “Distortion”).  By increasing or decreasing R2 with respect to an unchanging R1, the amount of gain is controlled by the user.  It so happens that linear taper potentiometers do just fine in situations like this.  Where the linear taper had a tendency to run afoul of our ear in the “loudness” application, it turns out to be ideal for altering the numerator of our amplification factor.  In fact, log taper pots in this scenario exhibit a “cram all the good settings into the extreme end of the dial” problem similar to the linear taper pots in the first example.

Theo Hartman
Hartman Electronics

Part 2 next time.


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