Table of contents:
The Amp Switcher is a device for easily switching between (guitar) amplifiers when a single speaker cabinet is used and/or available.
It is designed so that, at any given moment:
- the guitar input is routed to the selected amp (and only this one!)
- the selected amp's output (and only this one!) is routed to the cab
- all unused amps have their input muted ("shorted" in electrical terms)
- all unused amps have a dummy (resistive) load connected to their speaker output
Note: this repository only includes the electronics schematics and PCB designs. Mechanical design and building is left to the reader's skills and imagination.
This is the part responsible for the actual switching. In order to minimize the number of different parts needed, this PCB is meant to be "universal", in the sense that it can be used both for the "Guitar -> Amp Inputs" section (with an optional built-in buffer) and for the "Amp Outputs -> Speaker Cab" one.
The board should be populated differently depending on the intended function:
- for a speaker out board, the 2W resistors (R1-R8) should be populated, especially when used with tube amplifiers
- guitar in boards shouldn't include those resistors, but instead the corresponding solder jumpers should be bridged by applying solder across them
The input/output section (part of the PCB on the left of J1 & K1) can also be used in 4 different ways:
- external connector, direct routing: do not populate any component except J10 (+12V) and J15 (to/from external connector)
- on-board jack connector, direct routing (recommended for speaker out board): populate only J14 and J10, and (generously) apply solder across JPX1
- external connector, input buffer (only suitable for "guitar input" board): do not populate any component except J10 (+12V) and J16 (to/from external connector)
- on-board jack connector, input buffer (only suitable for "guitar input" board): populate all components except J15 and J16
Finally, the PCB is split in blocks of 2 relays, with oblong holes in the board so it can easily be broken to only keep the needed number of I/Os. This allows the same design to be used in projects of different sizes. Of course, you can avoid wasting material by using the properly-sized PCB: we provide design and Gerber files for variants with 2, 4, 6 or 8 relays.
WARNING: although using relatively wide (2mm) PCB tracks, we recommend the boards to be manufactured with 2oz copper weight if they are to be used with amplifiers above 20W RMS of nominal output power.
The control board is kept as simple as possible: it is made of a single rotary switch, with as many LEDs as needed to indicate the currently selected amp. The first switch position (fully counter-clockwise) is the OFF position where no amplifier is selected; turning the switch clockwise will select amplifiers one after another, lighting the corresponding LED on (the first being D1 -- the leftmost one -- then D2, D3 and so on...).
This PCB is meant to be mounted directly on the front panel, with screw terminals on the opposite side of the board for connecting to the corresponding terminals of the "Relays Array" boards. Like the latter, it also includes oblong holes for easily resizing the PCB to your needs (watch out for #1 though), and we still provide design and Gerber files for 2/4/6/8-way variants.
Finally, a simpler (and more compact) version is also provided for 2-way devices, using a smaller ON-OFF-ON toggle switch instead of the rotary one.
Note: those familiar with matrices will likely notice the "Control Matrix" naming isn't really accurate, as a 1xN matrix is basically an array, just like the "Relays Array" board is; however I didn't want to use "array" in both boards' names, and I think it sounds better that way anyway 😉
This project was kindly sponsored by PCBWay who provided PCBs for the prototype builds free of charge, many thanks to them! Moreover, working with this company has been a really nice experience:
- the ordering process was as easy as it gets (upload Gerber files, select features, place order, and that's it!)
- build time was 3-4 days as announced at order time (they also offer "express" options for an additional fee)
- quality and finish are both excellent! 👍
When populating the boards, one should place and solder the components in the following order:
- Jumpers as needed (R1-R8 on the input board, JPX1 on the output one -- and on the input board when not wiring the optional buffer)
- Diodes & low-wattage resistors
- Input board w/ buffer only: DIP-8 socket & 100 nF capacitors
- Screw terminals
- Input board w/ buffer only: electrolytic capacitors
- Relays & jack connectors
- Output board only: 2W resistors; keeping the resistors' bodies away from the board is a good idea in case things go wrong: only the resistor will burn, not the PCB!
For the control matrix board, resistors should be mounted first, then the rotary switch. The LEDs are first mounted on the front panel, then soldered after putting the board in place (this can be tricky, but is feasible).
A few additional notes:
- R1-R8 values should be tweaked depending on the LEDs used, especially when using various colors (e.g. 10k could work fine for blue LEDs, 5.1k for orange ones, and 47k for "warm white" LEDs)
- the GND post of the output board should remain unconnected
- on this same board, the jumper connecting the ground plane to the GND signal should be cut (this will probably be converted to an open solder jumper soon)
Once all the boards are in place (but not fully secured yet), one just has to run the wires to/from the appropriate solder terminals and the DC input connector. Here, using wires of various different colors is an efficient way not to get lost and ensure the wiring is correct.
Copyright 2025 Arnaud Ferraris.
3D models for the Neutrik jacks are © Inventor eFX Technology and have been downloaded from https://www.inventfx.com/_ifxkicad/_ifxkicadxtra.htm
3D models for the rotary and toggle switches aren't included due to licensing restrictions, but they can be downloaded from https://componentsearchengine.com
- CK1049 rotary switch
- A105SYZQ04 toggle switch
This source describes Open Hardware and is licensed under the CERN-OHL-S v2.
You may redistribute and modify this source and make products using it under the terms of the CERN-OHL-S v2 (https://ohwr.org/cern_ohl_s_v2.txt).
This source is distributed WITHOUT ANY EXPRESS OR IMPLIED WARRANTY, INCLUDING OF MERCHANTABILITY, SATISFACTORY QUALITY AND FITNESS FOR A PARTICULAR PURPOSE. Please see the CERN-OHL-S v2 for applicable conditions.
Source location: https://github.com/a-wai/amp-switcher
As per CERN-OHL-S v2 section 4, should You produce hardware based on this source, You must where practicable maintain the Source Location visible on the PCBs and user documentation of the Amp Switcher or other products you make using this source.