An ABY is the generic term for a switcher that enables you to switch between two guitar amplifiers. The A & B designates the two amps while the Y means that both amps can be turned on at the same time. Inputs and outputs employ standard ¼" phone connectors as used with guitar effects pedals and amplifiers.

There are several variations when it comes to ABY switchers, but these are generally grouped into two major categories known as 'true-bypass' and 'buffered'.

Passive 'true-bypass' switchers

These are switchers like the Radial BigShot ABY that do not buffer the circuit in any way. In other words, the signal is driven from the guitar pickup to the amp without any amplification or 'power steering' thus resulting in the pure signal of the guitar going to the amplifier. The 'purist' will often select a true-bypass pedal even though these types of switchers often introduce problems such as switching noise, ground loops and level changes.

Active buffered switchers

These are switchers like the Radial Switchbone and Twin City that employ a buffer or unity gain amplifier to drive the guitar signal while lowering the impedance. This reduces susceptibility to noise from lights, transformers, radio frequencies (RF) and other magnetic fields and enables longer cable runs. More advanced active ABY switchers incorporate extra features such as an isolation transformer to eliminate ground loops. The transformer also enables the circuit designer to change the polarity of the signal so that the two amps play in phase.

Which is better – true-bypass or buffered?

The 'purist' will often shy away from an active switcher due to the tone altering effects of the buffer. This is because buffers differ greatly between brands just as they do with studio preamps and guitar amplifiers. The more discerning the guitarist, the more demanding he will be when it comes to 'messing around' with the original guitar signal. This is also why most guitarists prefer true-bypass effects pedals: when switched off, the active pedal circuit is basically removed from the guitar to amp signal path.

The problems with true-bypass switchers

But true-bypass pedals are fraught with problems: when you connect two amps together, you often create unbearable noise known as a ground loop. Some guitarists will actually snip off the electrical safety ground as a means to eliminate the noise, thus putting them at risk to get a severe electrical shock. This is both extremely dangerous and 'illegal' as it will void any insurance policy. Further, when two different brands of amplifiers are used, the amps will often play out of phase whereby one speaker will be moving forward while the other is moving backward. This makes the amps sound distant and lack punch. When both amps are on at the same time, the guitar signal is split thus reducing the signal going to each amp by half the power. This reduces distortion and bottom end. Finally, when playing with high gain amps - switching between amps can introduce a very loud pop into the signal chain due to stray DC currents that travel between the amps. To eliminate these problems, one has no choice but to buffer the signal.

The problems with buffered switchers

Many guitarists prefer the sound and feel of a tube amplifier versus a solid state one. This is due to both the tone of the amp and how the guitar pickup reacts to a given circuit. So if you put a solid state buffer in between your guitar and your tube amp, you eliminate the pickup-to-tube-circuit relationship. The load changes and the feel and tone of the instrument are no longer the same. The other big problem is cost. As you add circuitry, you increase the cost to manufacturer which ends up affecting the final street price.

Drag Control Load Correction

When you buffer a guitar signal, you are inserting a unity gain amplifier between the guitar pickup and the guitar amplifier. A unity gain amplifier is one that drives the signal without amplifying it. But when you do this, it causes the tone to change because the guitar pickup is no longer part of the guitar to amp circuit. During our research, we also noticed that guitar pickups react differently when connected to tube amps and solid state amps even though they may have the same impedance or load. To compensate for this effect, Radial invented a function called Drag Control that enables the guitarist to 'dial in' the desired amount of loading on the pickup. This lets you replicate the feel and tone as if connected directly to a tube guitar amp.

Eliminating Noise Problems

Once the signal is buffered, it enables you to add other functions to the pedal that can solve more problems. For instance you can eliminate switch popping noise that is common with mechanical switches and relays by electronically controlling the way the amps are turned on or off. Photocells or opto-couplers provide a very good means to do this as they can be made to ramp up or down the audio signal to eliminate the 'spark' of a contact closure. Inserting an isolation transformer in between the ABY switcher and one of the guitar amps helps eliminate the hum and buzz caused by ground loops.

Choosing the right ABY for your needs

There is no right or wrong when it comes to selecting the ideal ABY switcher. As mentioned above, the purist will often lean towards a passive true-bypass ABY based on the belief that this is the only way to retain the pure tone of the instrument. Passive switchers are also less expensive which means they are going to appeal to a wider audience. Today, some of the active buffered ABY switchers are so good at managing the guitar signal that they are now used by the best players on the globe. These are designed to eliminate the problems that encumber passive switchers, but alas do so at a higher cost.