The Apocalypse Audio 4 Horsemen is an overdrive pedal built around four different types of transistors: a silicon BJT, a MOSFET, a germanium transistor and a JFET. The designer, RnFR, said each was used "to their optimum potent[ial]," which is where the name comes from: "hence the name, the Four Horsemen Overdrive."1
The circuit came out of the DIY community rather than a factory production line. Guitar FX Layouts, a blog that publishes "a collection of vero (stripboard) and tagboard guitar and bass effect layouts," posted a stripboard layout for the pedal on 14 February 2016, with build notes from RnFR himself.1 Apocalypse Audio also makes the germanium-based Apocalypse Audio Dirty Boots Version 5, which appeared on the same blog the same month.1
How the circuit works
The signal path runs through each transistor type in turn, starting at the guitar input1:
- A JFET buffer at the beginning of the circuit, which "helps to increase the current, and gives a nice high input impedance," so the pedal presents a friendly load to the guitar's pickups.1
- The first gain stage, a silicon BJT, "used to get a decent push going and a very slight amount of drive."1
- Further stages built on the MOSFET and germanium transistors, completing the set of four types.1
RnFR described the resulting sound as "a very clear, yet complex crunch, that has a very nice character to it."1 As with any designer's description of their own circuit, that is the maker's account rather than an independent verdict.
The tone control
The tone stage is "a variation on the SWTC, but it has an added pole for a steeper rolloff."1 In practical terms, RnFR said this makes it "acting more like a presence control" than a conventional tone knob, trimming the top end more sharply as it is turned down.1
Building it from the layout
The stripboard layout was still marked "Still unverified" as of 17 July 2017, and a later comment dated 29 April 2019 reported that "the 18K Resistor for Q2 is missing" from the layout. Anyone building the circuit should read the full comment thread on the layout page before soldering.1
For players who would rather buy than build, the 4 Horsemen sits in the wider field of overdrive pedals, alongside circuits that lean on a single transistor type rather than four.
Videos
AJ RNFR
Questions
Who designed the Apocalypse Audio 4 Horsemen?
The circuit was designed by RnFR, who described the build and the thinking behind it in the notes that accompanied the stripboard layout published by Guitar FX Layouts in February 2016. RnFR described the result as a very clear, yet complex crunch, with a very nice character to it.
What four transistors does the 4 Horsemen use?
The circuit uses four different types of transistors: a silicon BJT, a MOSFET, a germanium transistor and a JFET. RnFR said each was used to its optimum potential, which is where the name the Four Horsemen Overdrive comes from.
What is an SWTC tone control?
SWTC stands for a tone control design that the 4 Horsemen uses as the basis for its tone stage. In this pedal it is a variation on the SWTC with an added pole for a steeper rolloff, so it acts more like a presence control than a conventional guitar tone knob.
What does the JFET buffer do in the 4 Horsemen?
A JFET buffer sits at the beginning of the circuit. According to RnFR, it helps to increase the current and gives a nice high input impedance, which helps the pedal interact with the guitar's pickups before the gain stages.
Is the 4 Horsemen Vero layout verified?
No. The stripboard layout published by Guitar FX Layouts was still marked unverified as of July 2017. A correction was also posted in April 2019 noting that an 18K resistor for Q2 was missing from the layout, so anyone building it should read the full comment thread.
Sources (1)
- Guitar FX Layouts: Apocalypse Audio 4 Horsemen, Guitar FX Layouts (tagboardeffects.blogspot.com), 2016-02-14
