Exploiting Virtual Tone-Control Capacitors

April 24, 2014
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Most guitarists know the “turn down your guitar’s tone control before going into distortion” trick to obtain a smoother distortion sound. Reducing high frequencies can lessen harsh, buzzy overtones. This trick works with amp sims as well as conventional amps, but an amp sim advantage is you can precede it with a filter that emulates the sound of different tone capacitors—no soldering required.

A conventional tone control connects a capacitor (typically 0.047μF or 0.022μF) from the guitar’s hot output to ground through a potentiometer. The standard explanation for how a tone control works is that the capacitor rolls off high frequencies— which is true, and is also how tone controls work in a device like a hi-fi amplifier. But guitar pickups are special critters. They’re primarily inductors (coils) that already incorporate some capacitance due to the windings, as well as the resistance from the wire used in the pickup.

Before we get bogged down in Mr. Science Land, the bottom line is that all pickups have a resonant peak toward the upper midrange (around 2kHz to 4kHz) whose amplitude and frequency varies according to the pickup type and how it’s connected. When the tone-control capacitor interacts with the pickup, it lowers this resonant peak and reduces highs. You can verify this for yourself. Turn down your guitar’s tone control, and you’ll hear the highs start to diminish. But at the tone control’s absolute minimum resistance, you’ll hear a subtle lower midrange boost.

Fig. 1 

We’ve oversimplified a bit, because the tone-control pot’s value, cable loading, the capacitor’s reactance, and other factors all influence the sound. However, those effects aren’t as significant, and they become less relevant when feeding an audio interface’s high-impedance input (which provides virtually no loading).

 Fig. 2

Let’s start emulating. Fig. 1 shows a spectrum analysis of Gibson’s Dusk Tiger guitar (middle pickup switch position, with Burstbucker 3 neck and P-90H bridge). Fig. 2 shows the same analysis, but with the tone control rolled all the way down. You can see the high frequency rolloff, but note the 500Hz peak.

 Fig. 3

Inserting the EQ low-pass filter curve (Fig. 3) after the Dusk Tiger output emulates the effect of rolling the tone control down. Fig. 4 shows the analysis of the guitar’s output without the guitar’s tone control rolled down, but going through the EQ. Compared to Fig. 2, the response is virtually identical.

 Fig. 4

That’s all interesting, but here’s the payoff: Changing the low-pass filter frequency lets you emulate the peak caused by just about any capacitor value. So now you have a virtually unrestricted choice of tone control capacitors—and you didn’t even have to open up your guitar and do any rewiring.

Craig Anderton has played on or produced more than 20 major label releases, mastered hundreds of tracks, and written dozens of books. Check out some of his latest music at youtube.com/thecraiganderton.

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