When you want to really evaluate one thing as it relates to another, in close proximity, you call it an A/B comparison. For example, if you had two solidbody humbucker-equipped guitars on hand and you wanted to compare the sound of their bridge pickups to each other, you would “A/B them,” switching as quickly as you could between the two.
The flip side of the two-guitar example would be having two amps and one guitar. Let’s say you’ve already have decided on the guitar, but now you want to quickly switch out between two amps to see which one yields the better sound.
The key in both of the above cases is to make the swap quickly, because you might be trying to gauge something very subtle and you don’t want too much time to elapse in between—like, not more that one second from one sound to the other and back again, if you can help it.
But whether you’re swapping a guitar or swapping an amp, there’s always going to be a lag between the end of Guitar/Amp A and the beginning of Guitar/Amp B. And of course, you'll be focusing on not dropping something in the process. This makes the swapping part of the whole exercise a hassle.
Unless you use an A/B box, such as DOD’s VAC270 A/B Box. Then you don't have to plug or unplug anything. Every connection is made in advance, and you just step on the footswitch to change the signal routing.
The great thing about the DOD VAC270 pedal is its simplicity: it doesn’t require a battery, there’s no complicated internal circuitry (other than the switch mechanism under the footswitch), and it’s non-directional: you can have a guitar as the source going to two amps as destinations, or you have two guitars feeding one amp.
It should be very obvious why both above scenarios would be convenient—especially on stage—but there’s another advantage that comes up when testing gear. The faster you can make switch, the more vivid your “tonal memory” will be between test subjects.
A speedy swap is especially crucial if the differences are subtle, such as when you’re listening for warmth, response, or even slight shifts in volume. For someone on stage, an A/B box is like having a pit crew. For someone testing gear, it’s a tonal-memory-extender secret weapon!
Jon Chappell is a guitarist and the Senior Editor of Harmony Central. He has contributed numerous musical pieces to film and TV, including Northern Exposure, Walker, Texas Ranger, All My Children, and the feature film Bleeding Hearts, directed by actor-dancer Gregory Hines. He is the author of The Recording Guitarist: A Guide for Home and Studio (Hal Leonard), Essential Scales & Modes (Backbeat Books), and Build Your Own PC Recording Studio (McGraw-Hill), and has written six books in the popular Dummies series (Wiley Publishing).