Two alternative spices (spicii?):
TI makes you go through registration, but I have occasionally found it worthwhile:
I canāt believe no-one has suggested shoving a valve into the zynth case yet.
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They have! You should have been at zynthclub last night.
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MIG: Mobile Input for Guitar - this was already posted by @mbvs in October 2023 and the web page is dated 2012-01-22. I thought it was worth mentioning again because it has a lot of good explanations, spice output and scope photos, and a paragraph where the author considers the pros and cons of various op-amps,
I am still planning to use some of my analogue pedals in front of zynthian as well as my diy BAJA Real Tube Driver that needs 16V AC supply. That will never be considered as reasonable addition inside zynthian box. 
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Yes, the MIG article is an interesting read. It is basically the same design and we have been discussing here with a non-inverting opamp biased at mid-range of a low-value single rail supply. The MIG differs in that it adds gain (we are looking at unity gain if possible) and is battery powered which removes the need for supply decoupling. I do like the idea of putting the hp & lp filtering in the opamp (which can do a more tranparent job than passive components) but this may require adding some gain (or using the opamp in inverting mode). We can pad things but every resistor you run your signal through adds to the noise. (Better to do it on the hot / amplified side where the noise may be less intrusive - but we are aiming for unity gain so any noise will be subsequently amplified by the Zynthianās soundcard / processing.) I quite like inverting the audio, especially microphones which can reduce risk of howl-round but positive feedback is a desired feature for many guitarists! (Opamps configured to invert the signal can be simpler to configure and allow any gain whereas non-inverting configuration only allows gains from unity upwards.)
I may need to dig out another laptop. This Chromebook is refusing to run much of my design software that was previously working fine!
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Why do we want unity gain? Iām not promoting that we want gain, I would just like to understand why weāre avoiding it if it causes a stability issue. I didnāt find any earlier mention of it in this context with a search on unity gain. I think I understand it in a mixing context.
The idea of the ābufferā is to provide appropriate input impedance but not to change the signal (much). It makes sense for it to have unity gain when the input stage of the Zynthian has so much gain available (at least the official Zynthian soundcard). It would be ideal if the input stage could be left in circuit with just impedance switching. That is what I would really like to see so that the impedance selection does not colour the audio in a significant way. That may be a challenge but we will seeā¦
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This is a skill Iāve been hoping to build actually, but I canāt volunteer for this, yet.
Can you or anyone direct me to some good instructional materials (paid is okay, I just want to learn) on getting into designing your own circuit boards in the modern world?
I am from the ātrace it with a marker then dip the board in acidā days.
You are entering a world that exists between the purity of simple circuit design and the histrionics of the over enthused audiophileā¦
The original amplifiers that accompanied the iconic electric guitars were valve based and had characteristics all of their own be it form warming up to physical modulation when hitā¦
Also the guitars involved had simple coil based pickups with simple volume controls and single capacitor tone controls so their electrical characteristics were variable at best. The high impedance required meant that long guitar leads would often pick up the local taxi company so stubber filters were often involved to address this issue, but it did leave a lead that was frequently microphonic in that you could produce unpleasant names by simply stamping on the cable.
If your guitar is active then most of these issues are pretty irrelevant as the actve element in the guitar will provide a solid line level signal with a low output impedance that will drive into a zynthian input just fine and if it can be balanced so much the better.
As has been mentioned the J-FET ( a transistor) is probably the best match for a valve amp but they are becoming increasingly rare and also have differing characteristics.
The ultimate arbiter is your concerns and your ears. It works both ways. āitās good but I can surely make it betterā which tend to end up with multiple controls and a circuit that Spice ( a VERY useful tool in this regard). and an inability to see or hear failings in the design cos its MY designā¦
Apologies if this doesnāt directly address the issue but in the pre amp world itās often better to buy something designed to do the job rather than the complexisites of home research, development and implementationā¦
@riban will be along shortly to disagree with thisā¦
I donāt know whether your looking for general electronics info or specifically pcb layout.
General:
The Art of Electronics - Horowitz and Hill
There Are No Electrons - Amdahl
Electronic Principles - Malvino
PCB Layout:
Search Amazon for KiCad
Agreed KiCAD is good.
I own a copy of The Art Of and Iāve slogged through a number of other ābasicsā things; Iām clear enough on quite a few things.
Itās more that I have become aware that there are new tools to go with the new SMD stuff, such that someone who knows these tools can order custom PCBs for fairly reasonable rates, instead of hacking point to point on breadboard-style PCBs for days at a time to get a single device built. But I am a 50 year old dude in the middle of nowhere, by which I mean, Canada, no hackerspaces and no apparent mentors in sight.
I am good at reading and comprehending manuals, if I can be directed to the right manual for the right tool. The Internet has a lot of opinions. A LOT of opinions. :>
True, but the docs page on KiCad is quite manageable, and their getting started manual is good and they have a very active forum.
Also remember that breadboard then veroboard prototyping is still the best way to get something working before you commit to building a PCB and populating it. There will be errors in every iteration!
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Never quite made it past the diy Musikding paint by numbers level, but I did come across this interesting link:
https://www.experimentalistsanonymous.com/diy/index.php?dir=Schematics
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Buffer 3 looks like what I am planning to build.
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!
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A couple of years ago, Iāve found this project on a french guitarists forum
PĆ©dale Vite
Itās a guitar effect processing unit entirely build and coded from scratch based on Raspi 3 then 4 to 5 in the latest iteration, the v3.
The audio input stage
NE5332 for a multi stage guitar/bass preamp
INA103 for dynamic microphone input via XLR connector.
This will feed a Cirrus Logic CS4272 codec.
Hardware side, itās really impressive. Everything is Open source and hosted here (with an extensive build guide):
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Hi Gezynthiansā¦
When I use MOD the input is connected via the plugins with MOD-Monitor BUT also there is a direct connection from the inpu to MOD-Monitor. This results in ALWAYS hearing the dry signal in the audio outputā¦ (even when I disconnect the input in MOD-UI from any plugin)
In patchage I can manually disconnect the dry input from MOD-monitor. Unfortunately this is not saved in the snapshot. 
Is this a bug or my nescience? Anybody suggestions how to at least store the patchage settings together with the Snapshot?
Cheers,
Maarten