Post Arzo7WJJAzLSJi2KWW by awai@fosstodon.org
 (DIR) More posts by awai@fosstodon.org
 (DIR) Post #Arzo7TiMpMbiGYjnfM by awai@fosstodon.org
       2025-03-12T16:37:50Z
       
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       Amp switcher: relays array design ๐ŸงตBackground story: https://fosstodon.org/@awai/114093865582292065Project overview: https://fosstodon.org/@awai/114110547162586997As I mentioned previously, I intend to design a single board for both the input and ouput switching sections.I had initially planned to have only the jacks (and associated relays/circuitry) going to/from the amp on the board; then I wanted to add the speaker out jack, and maybe also a buffer, and make it optional, and... Well, you get the idea ๐Ÿ˜‰1/#Guitar #Amplifier #DIY
       
 (DIR) Post #Arzo7URk6eu4XHy2OO by awai@fosstodon.org
       2025-03-12T16:39:25Z
       
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       In the end, the relays array PCB will include the following features:* choice of on-board jack connector or screw terminal block* ability to handle both guitar- and speaker-level signals* optional built-in buffer (for use in the "guitar input" section only)The first listed feature is the easiest one to implement: it only requires to ensure the board has footprints (and holes!) for both types of connectors, and decide which one should be populated at build time ๐Ÿ˜Ž2/
       
 (DIR) Post #Arzo7VkZG9YWZxmaNE by awai@fosstodon.org
       2025-03-12T16:40:22Z
       
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       Handling both types of signal also comes with a few constraints: first, we need relatively wide PCB tracks to deal with the speaker current (nearly 5A for a 100W tube amp cranked up into an 8ฮฉ speaker ๐Ÿ˜ฑ).I went for 2mm as it's likely enough for 20-30W amplifiers; however, more powerful amps will require a bigger copper section.As the available space is limited, my recommendation would be to order the board with 2oz copper weight (instead of the standard 1oz), that should do the trick! ๐Ÿ˜‰3/
       
 (DIR) Post #Arzo7WJJAzLSJi2KWW by awai@fosstodon.org
       2025-03-12T16:42:31Z
       
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       We also need to mute the input of unused amps and ensure a dummy load is connected to their output. This is especially important for tube amps, which NEED a speaker(ish) load.Unused amps' jacks should then be shorted when used as the input section, and 2W-rated resistors (only dissipating low-power noise) be placed across them in the other case.All this requires is adding a solder jumper: one can then either populate the resistor (output), or solder the jumper to close it (input).4/
       
 (DIR) Post #Arzo7XV2kqKI0OXDSC by awai@fosstodon.org
       2025-03-12T16:43:46Z
       
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       Finally, the input buffer: I went for a tried-and-true design using a single OPA134 op-amp, which I have always been happy with.Using similar "tricks" (additional footprints + solder jumper) this section can easily be bypassed and left unpopulated if it's un{need,want}ed and/or the board is used for the output section.REMINDER: this buffer can only be used for the "guitar input" section, as it would be destroyed by a speaker-level signal, possibly damaging the amplifier in the process!5/
       
 (DIR) Post #Arzo7YPPNHPepCuEJE by awai@fosstodon.org
       2025-03-12T16:45:04Z
       
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       The next feature I wanted is the ability to "resize" the board post-manufacturing: I actually plan to build 2 devices, one for each of my amplifier stacks (a 6-way switcher for the "modern" stack, and a 2-way device for the "vintage" one).As I need 2 PCBs per device (4 in total), and minimal order is 5 pcs, I could build both devices just by ordering a single board design made for 6 amplifiers, and "downsize" 2 boards so they only include the circuitry needed for 2 amps.6/
       
 (DIR) Post #Arzo7YtBaZESJYq0iu by awai@fosstodon.org
       2025-03-12T16:47:18Z
       
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       Therefore I designed the PCB as a single block of 2 jacks/relays, repeated multiple times to reach the desired (even) quantity, and simply added the buffer/connectors block to the result.I also added oblong holes separating each block, only leaving the needed space for the PCB tracks to run to the next block: this way, I can easily cut a larger-than-needed board to the desired length without any additional tooling ๐Ÿ˜Ž7/
       
 (DIR) Post #Arzo7ZONiaBZsJQvLc by awai@fosstodon.org
       2025-03-12T16:47:55Z
       
       0 likes, 1 repeats
       
       As initially announced, this project is of course 100% #OpenHardware :openhardware: and is licensed under the terms of the CERN-OHL-S v2 or later.The #KiCAD source files for the "relays array" board are already available from https://github.com/a-wai/amp-switcher , the rest will follow as soon as I implement it ๐Ÿ™‚8/8#OSHW