Compare Devices with the BabyA/B

So you compare stuff.

Let's say you have two preamplifiers and you want to compare them. You have your IPod or some other signal source and your headphones.  You hook up rig A and then listen. You judge. You try to remember what it sounds like. Then you unhook rig A and hook up rig B. And listen. And given a short enough period between connections, you can do a fairly decent job of comparing A to B.

But all that cable swapping is a pain in the ass. And it makes it hard to do A/B testing in the middle of a song. Say you have a particular 30 second passage of Miles Davis and you want to compare it with two different components quickly and easily. Cable swapping makes this pretty tough.

Or what if you have two amplifiers and sets of speakers set up. Wouldn't it be nice to just flick a switch and instantly hear the difference?

That's where the A/B box comes in!

What if you could hook up both devices at the same time, and then with a simple flick of a switch, go back and forth between A and B? In this project, we'll build the BabyA/B switch box: a compact easy to build unit that makes such switching possible.

General design of the A/B Box

An A/B box is a simple passive device that uses a mechanical switch to switch between two or more sets of input/output paths. For comparing stereo audio devices, we want a box that:

• Accepts a stereo input signal at the input jack
• Routes the output to path A or path B
• Recombines the signal at a stereo output jack

The general block diagram of such a device looks like this:

The input is your Ipod or similar audio source. Send A goes to the input of device A, receive A gets the return signal from device A. Similarly, device B gets its signal from Send B and returns its signal through receive B. The output, i.e. your headphones or stereo device is connected to the output.

Here's what a sample test setup would look like:

In schematic form the device looks like this:

Let's Build One!

Now that we have the general idea of what it is, let's build one. First off, the parts list.

Preparing the Enclosure

For the box, I chose a Hammond 125B aluminum enclosure. It is metal so it offers a decent amount of shielding against RFI, and since it is aluminum, it is fairly easy to drill. You can choose any type of enclosure that offers a good fit for the parts.

Using Microsoft Visio, I laid out the following top design. The first picture shows what the top of the unit will look like, the second picture shows the inside view of the lid with the components mounted.

Using a ruler, a square and a sharpie I laid out the drill pattern. Then over to the drill press to drill the actual holes.

Installing the Parts

Now that the box is drilled, let's install the parts. The 3.5mm jacks are easy enough--unscrew the nut, stick 'em in and tighten the not on the other side. Same thing with the switch, but take extra care to align it nicely.

Doing the Wiring

For wiring, use any standard hook-up wire. I like to use 24AWG pre-bond wire because it is a delight to work with. To make the wiring easy, here are a set of step-by-step illustrations.

First wire all the ground connections. (I had originally had all grounds connected together, but eagle-eyed reader John D pointed out that such a design could cause problems with devices that use a virtual ground topology. Below are the updated ground wiring diagrams. April 27, 2009):

Next, wire up the Input Jack and the Sends:

Then the Output Jack and Receives:

And finally, check all your wiring against the following complete wiring diagram:

Pictures of the building of the Glorious Device of Enlightenment

Measuring the Box. I used a 1290NS die cast aluminum enclosure.

Drilled and painted using only the finest Brand-X spray paint.

Primary lego colors FTW.

More wires than an illegal NSA wiretap

Ready to perform valuable compariosnation of audio devices
of fine Baluchistan!