Fulltone ABY-ST V2

We don’t really understand why the Fulltone ABY-ST is a bit of a niche product.

Guitarists think nothing of spending loads on pedals to shape their tone — drive pedals, modulation, reverb, delay, compressors. The average pedalboard represents a serious financial commitment. And yet switching between amps seems to be a bit overlooked.

The idea is straightforward. A Fender-style amp with chime and clarity. Another for your driven sound with warmth and grit. Switch between them, or combine them, at the press of a footswitch. The result is two completely different tonal characters available instantly, without touching your guitar’s volume or any of the pedals on your board.

The Fulltone ABY Soft Touch v2 was designed specifically to solve all the problems that arise when you try this. It was built after Michael Fuller watched Eric Johnson switch between Fender and Marshall amplifiers at a NAMM show in the mid-1980s — and realised this was a fundamental part of professional two-amp tone.

What is the Fulltone ABY-ST Soft Touch v2?

ABY stands for A, B, or Y (Both). An ABY switcher takes a single input — your guitar — and routes it to two outputs, each going to a separate amplifier.

The CS-ABY-ST v2 does this without switching thump. It uses four custom-developed optocouplers that rapidly pan between the outputs, completely eliminating the thump that every other ABY switcher produces on switching.

Importantly, no audio signal passes through the switching circuitry itself. The opto switching circuit’s only job is to trigger the switching without producing noise. Your guitar signal passes through one resistor, or through a JFET buffer, or through the isolation transformer on the B channel. That’s it. All the other circuitry in the unit is switching logic — not audio path.

The Isolation Transformer

The B-Out has a custom Nickel-core 1:1 transformer permanently in the signal path. This is the component that prevents electric shock when combining two amplifiers on different mains circuits — it physically breaks the electrical connection between the two while allowing audio to pass. Standard transformers boost treble frequencies and make fuzz pedals sound strident. Fulltone’s custom-wound version is designed to be completely transparent.

Each channel also has its own independently switchable JFET buffer — one for A-Out, one for B-Out. These drive long cable runs without treble loss or signal level degradation.

The reason the buffers are independently switchable is fuzz pedals. Fuzz circuits — particularly germanium and vintage silicon designs — need to see a high-impedance signal from your guitar pickups to sound right. A buffer’s low-impedance output can change a fuzz’s character significantly. Turn off the buffer on whichever channel feeds your fuzzes and leave the other on if you need it for a long cable run.

Controls and Connections

• A or B footswitch: Selects between A-Out only (yellow LED) or B-Out only (green LED).
• Both footswitch: Sends the signal to both A-Out and B-Out simultaneously. Can be engaged whether you are currently on channel A or channel B.
• Channel A Buffer switch: Turns the JFET buffer on or off for the A-Out signal path only.
• Channel B Buffer switch: Turns the JFET buffer on or off for the B-Out signal path only.
• Ground Lift for B-Out: Lifts the ground connection on the B-Out. Default is Grounded. Can reduce hum in some setups — consult a qualified technician if you have questions about ground lift in your rig.
• Phase switch for B-Out: Corrects phase cancellation when running both amplifiers simultaneously. When two amps are out of phase, the combined sound loses bass and appears to come entirely from the extreme left and right with no centre content. The Phase switch inverts the B-Out signal to correct this. Always check phase before a gig — the correct setting varies between amp models.
• Input: Standard quarter-inch input from your guitar or the end of your pedalboard signal chain.
• A-Out: Quarter-inch output to Amplifier A. No transformer. Switchable buffer.
• B-Out: Quarter-inch output to Amplifier B. Fulltone custom Nickel-core 1:1 isolation transformer permanently in signal path. Switchable buffer.
• DC Power Port: 9V to 18V DC centre-negative, 2.1mm barrel, minimum 100mA supply. Running at 18V doubles the available headroom on the buffer circuits.

Key Features

• Four-opto silent switching — no thumps, no pops, no clicks
• Custom Fulltone optocouplers — 50 ohms series resistance vs 300–400 ohms on standard units
• No audio signal through switching circuitry
• Custom Nickel-core 1:1 isolation transformer on B-Out
• Independent switchable JFET buffers on A-Out and B-Out
• Phase correction switch for B-Out
• Ground lift switch for B-Out
• Soft-touch silent footswitches — no mechanical click in the audio or ambient sound
• A, B, or Both (Y) switching modes
• Yellow LED for A channel, Green LED for B channel
• 9V to 18V DC centre-negative, 100mA minimum — run at 18V for maximum buffer headroom
• Input impedance: 1.1 megaohms
• Buffer output impedance: 100 ohms
• Transformer primary and secondary impedance: 10K ohms each
• Dimensions: 3.9″ W x 4.0″ D x 1.8″ H — 25% smaller than v1
• 5-year limited warranty (original owner, proof of purchase from authorised dealer required)
• Made in the USA

Power

The ABY Soft Touch v2 runs on 9V to 18V DC centre-negative at a minimum of 100mA from your supply. Running at 18V is Fulltone’s own recommendation when using the channel buffers — it doubles the available headroom and makes a noticeable improvement to the buffer performance. The CIOKS DC7 v2 supports selectable voltages per outlet including 18V, making it the ideal single-supply solution.

Watch The ABY-ST In Action

FAQs