Guitars produce extremely high initial transients when you first pick a string, which can be an issue with digital recording. If you set the input of your audio interface high enough to accommodate the peaks, the guitar’s average level remains low. This can result in more noise, less apparent sustain and the guitar’s level not hitting the “sweet spot” for processors like compressors and amp sims. You could place a studio-quality limiter between the guitar and the interface, but few limiters have instrument-level inputs, and the transients can cause pumping and other undesirable audio artifacts.
So here’s the solution: a passive limiter that requires no power, costs under $2 in parts, and is small enough to fit inside a small box, guitar cable or even an audio interface input. Because it’s passive, there are none of the drawbacks traditionally associated with limiters. Sounds simple, right?
Shown in Fig. 1, the circuit places red LEDs back-to-back across the output’s hot and ground connections. (The bypass switch is optional; I always leave the circuit active.) The electrical characteristics of red LEDs are the “secret sauce” here. Each LED clips positive and negative peaks that exceed about 2.2 volts. Clipping transients greater than 2.2 volts peak-to-peak doesn’t cause audible distortion because LEDs inherently soft-clip. (For the techs in the crowd, it’s because they have junction capacitance that rounds off super-fast attacks.) Besides, the transients are too short for our brains to process, even though digital gear can “hear” them.
Fig. 2 shows the difference this makes in a guitar’s waveform. The upper row displays waveforms for the bridge, middle and neck pickups through the Transient Tamer, while the lower row shows the same waveforms without it. All waveforms have been normalized to the same peak level. Clearly, the upper row has a higher average level.
Fig. 3 compares the pickup outputs’ average levels with normalized peaks. With the bridge pickup, the average level is -13.4 dB. Adding the Transient Tamer brings this up to -8.2 dB — an average level increase of over 5 dB! The middle and neck pickup positions have about a 2 dB increase in average level.
The photo at bottom right shows a completed Transient Tamer within a guitar cable. Note that red LEDs are best for the circuit because they have tend to have lower forward voltages (the level at which clipping occurs) than other colors, in the range of 1.8 to 2.6 volts. For passive pickups, 2.2 volts is a good compromise value, but if you use lower-output pickups or have them further away from the strings than usual, you’ll want the lowest possible forward voltage.
Don’t dismiss this circuit just because it’s so simple. I don’t plug into audio interfaces any more without my Transient Tamer.