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How to Avoid "Hidden Distortion" in Amp Sims

Not all distortion is created equal...

 

by Craig Anderton

 

 

Amp sims have slowly but surely gained acceptance over the years. Although some guitarists will always prefer using tubes, it’s also true that amp sims can often provide sounds that are difficult, if not impossible, to obtain in the physical world and that has inspired many guitarists to start using amp sims.

 

But another factor is that amp sims are not just “plug and play.” It takes a certain amount of effort to get them to sound good—which can sometimes involve adding EQ before and/or after the sim, avoiding particular amp cabinet combinations (or choosing “golden” combinations), and so on. But one of the most important elements is gain-staging, and here’s why.

 

Many guitarists experience bad amp sim tone because they don’t realize there’s the potential for two types of distortion within modules like amp and cabinet emulators: The “good” amp distortion we know and love, and the “nasty” digital distortion that results from not setting levels correctly inside the sim.

 

 

KNOW YOUR DISTORTION

With analog technology, if you overload an amp input you just get more distortion. Because it’s analog distortion, it sounds fine—just more distorted. But if you overload a digital amp’s input, remember that digital technology has a fixed, and unforgiving, amount of headroom. If you don’t exceed that headroom, the amp sim will sound as the designers intended. But if your signal crosses that threshold, the result is ugly, non-harmonic distortion. Never go “into the red” with digital audio—unless you’re scoring a Mad Max sequel, and want to conjure up visions of a post-apocalyptic society where the music totally sucks.

 

 

SETTING INTERFACE INPUT LEVELS

To avoid digital distortion, it’s important to optimize levels as you work your way from input to output. The most important gain setting is the audio interface’s input gain control, which will often be complemented by a front panel clipping LED. Adjust this so that the guitar isn’t overloading your audio interface, which will likely have a small mixer application with metering so you can verify levels (just note that the application’s fader isn’t what’s controlling the input level—it’s the interface’s hardware level control). If distortion happens this early in the chain, then it will only get worse as it moves downstream.

 

Set the audio interface preamp gain so the guitar never goes into the red (Fig. 1), no matter how hard you hit the strings. Be conservative, as changing pickups or playing with the controls might change levels. You can always increase the gain at the sim’s input.

 

Fig. 1: The metering for TASCAM's US-366 interface shows that the guitar input (Analog 1) level control is set so the input levels are avoiding overload.

 

 

 

AMP SIM INPUT TRIM

Your sim will likely have an input meter and level control; adjust this so that the signal never hits the red. Going one step further, Peavey’s ReValver includes an input “Learn” function (Fig. 2). Click on Learn, then play your guitar with maximum force.

 

   

 

 

 

 

 

 

 

 

 

 

 

Fig. 2: ReValver’s Learn function automatically prevents the input and/or output from being overloaded.

 

Learn analyzes your signal, then automatically sets levels so that the peaks of your playing don’t exceed the available input headroom. Beautiful.

 

 

TRIMMING LEVELS WITHIN THE AMP

Like their real-world equivalents, amp sims can be high gain devices—high enough to overload their headroom internally. This is where many guitarists take the wrong turn toward bad sound by turning up the master volume too high. The cabinets in Native Instruments’ Guitar Rig include a volume control with Learn function (Fig. 3); for sims without a Learn function, like IK’s AmpliTube, you’ll find a meter—adjust the module’s volume control so there’s no overload.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 3: Guitar Rig has a Learn function for optimizing internal amp levels.

 

SETTING OUTPUT STAGE LEVELS

The final stage where level matters is the output. AmpliTube has an additional level control and meter to help you keep things under control, while Guitar Rig has a special “Preset Volume” output module with a Learn function that matches levels among patches, but also prevents distortion. ReValver offers an additional output Learn function.

 

If you set gains properly through the signal chain from interface input to final output, you’ll avoid the kind of bad distortion that ruins what the good distortion brings to the party. - HC- 

 

 

_____________________________________________________

 

 

 Craig Anderton is Editorial Director of Harmony Central. He has played on, mixed, or produced over 20 major label releases (as well as mastered over a hundred tracks for various musicians), and written over a thousand articles for magazines like Guitar Player, Keyboard, Sound on Sound (UK), and Sound + Recording (Germany). He has also lectured on technology and the arts in 38 states, 10 countries, and three languages.

 

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