The A/B/X-Box challenges your assumptions about amplifiers


I’ll admit, I am biased!

Given a choice between two amplifiers, I am likely to believe that the more expensive, more powerful, and more beautiful amplifier will actually sound better and perform better.

But, this could not always be true! In real life, some moderately priced, ordinary-looking products actually outperform some so-called “high-end” products. (A term that makes me cringe a bit.)

So, how do we get at the truth? How do you compare the sound of two amplifiers?

Well, typically, you’d just put on some well-produced music—Steely Dan or something—and listen to “Amplifier A” for a while… Sounds nice, doesn’t it? Detailed! Powerful! Nice imaging. Maybe just a little harsh in the midrange… Maybe you heard some detail you never noticed before, like brushes on drum heads, or a singer’s breath. Spine-tingling!

Now, for Amplifier B. Stop the music! Swap the cables! Play the music again!

How exactly are you going to listen to the same thing on Amplifier B? The moment is gone! You will not be the same person 60 seconds from now, and you cannot experience the same moment twice. You will notice other details when you listen to the music again. All you can do is try to remember the way the other amp sounded.

This would be easier if the difference between the sound of amplifiers were as obvious as the differences between speakers, or phono cartridges.

A way to switch quickly is needed.

So, I built this A/B/X-Box. This is not my invention by the way, just my version of it.

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Amplifiers A and B are connected to the binding posts, and speakers are connected to “Common”.

There are relays inside the terminal box to switch between one or the other set of inputs. (Amplifiers A and B.) The remote control switches between amplifiers A and B with almost no audible glitch.


Simple Boolean logic with batteries and switches.
Simple Boolean logic with batteries and switches.

So that’s great! Now, we can listen to two different amps and switch instantly.

And now, science.

The “X” is what makes this box special. Differences in sound between good amplifiers are often very, very subtle. Even an experienced listener will not be able to differentiate between them consistently. A statistical method is needed to tease out the difference!

We can perform a double-blind listening test and we’ll need a friend to act as the experimenter.

CAVEAT: I am well aware that AB/X testing does have limitations. In particular, it cannot prove a negative. However, I believe it is a valid and useful method, and just one of many ways to evaluate a system. The AB/X box is also useful as a simple A/B switch-box to compare two amplifiers or speaker systems.

Ten listening trials are conducted. The experimenter takes notes and flips a coin ten times to determine which amp is “X” for each trial. “X” is hidden beneath the brass cover. Music is played, and the listener is given the remote control. The listener is free to switch between amplifiers A and B, which are known to them, and “X” which is not. The listener’s task is to match X to either A or B. Getting 50% correct would indicate no audible difference, the same as guessing or flipping a coin, and about 70% is statistically significant to suggest a difference.


Already, this device has relieved me from some nagging doubts. I recently refurbished an Adcom GFA-555 II, and I was feeling quite proud of how it turned out, and I wanted to hear what it could do! So, I hooked it to my subwoofer, in place of the Crown K2 I normally use. Listening to the Adcom, I thought, WOW! This sounds great! Maybe it sounds better than the Crown? Suspecting that my affection for this amp that I had so lovingly refurbished was clouding my judgement, I hooked up the A/B/X box and listened…. and listened and listened. Nope, no difference. I even did some room measurements and found no difference. Both amps are fully capable of driving the sub, no sweat. So, as it turns out, the Crown is a great choice in the first place. No need for a new amp!


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