Note that if i don’t plug two guitars, I do plug one guitar and one bass guitar simultaneously 
Well, basses don’t count! 
Does a hi-z input make significant difference for bass? I guess there is quite a lot of hf in the harmonics, especially if playing hard, e.g. slap.
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I use a Behringer UMC202HD as preamp only (the Zynthian audio interface is a Hifiberry DAC+ ADC), it also works without a computer (you have to push the Direct monitor button). I’m experimenting with process guitar and vocal simultaneously. One channel act as a cheap microphone preamp with XLR input and gain control, the other act as a Hi-Z guitar input, with gain control and pad, and you have a headphones monitor output with volume control. No need to put it all inside the Zynthian…
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I have not a single clue… my wireless system seem to take car of that, I have a good level of input with my guitar, bass, electroacoustic ukulele bass, ukulele, kazoo
For my microphone with xlr to jack adapter , it work only with jack-wire.
Yes it should be fine with 5V supply. It is designed to connect directly to a guitar which tends to have peak output less that 1V so plenty of headroom.
Running a simple transistor buffer that has been designed for 9V on 5V runs the risk of distorting the signal, either because the signal hits the rails or because of the misbiasing. Moreover, these simple circuits are designed to run on clean power either from a battery or a well filtered and regulated linear power supply. Running them on a switching power supply is likely to introduce noise. Worse, running these analog curcuits from the same power supply as a digital device will likely introduce more issues. Here are a few suggestions ordered from the simplest:
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Use any pedal that has a buffered bypass. All Boss pedals (in)famously feature a buffered bypass. If you don’t want to bust the bank spending 100 EUR for something that you will mostly use as a buffer, you can get one of these Behringer clones. You can find those for as little as 10 EUR second-hand.
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Use any other device that provides buffering for a guitar input, with a direct out, such as a portable recorder, an audio interface, a preamp, etc. Again, Behringer has these little tube preamps that you can find for as little as 25 EUR second hand.
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Build a simple JFET buffer with a 9V battery supply. JFETs are getting more difficult to find these days, but since this is a boost, you can use any JFET that is reasonably priced (e.g. 20c). Alternatively, you can build a buffer using an op amp.
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If you must run the buffer on the same 5V supply as the Zynthian, I would recommend using a charge pump (e.g. ICL7660/MAX1673) to get from 5V to 9V, and add some power filtering and/or regulation.
I personally use 1 and 2 for both guitar and bass, and I have built 3, but I did not find it too practical. 4 could work but I haven’t tried.
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A small mixer with various inputs (Hi-Z, XLR for dynamic mic, stereo/mono line level ) can do the trick too I think.
As @gilrain, I personaly use an Behringer UMC 204 USB audio interface for this and it’s Ok. But yes, I agree with
@ksg: great summery - this should definitely go to the wiki - and special thanks for bringing the charge pump solution to notice.
I realize this may be considered a sacreligeous act for some, but I used a Behringer MA400 headphone amplifier to amplify my lap steel to the Zynthian. I have to turn the volume of my guitar all the way up, but then it sounds quite decent, perhaps a little thin on the low. Really small, sturdy built, additional XLR in and thru for mics. Really works for me.
Buying a Boss BP-1W would be considered to be more sacreligious to this crowd when there’s an envelope of LM741s, a breadboard and assorment of capacitors, resistors, gaffer tape, a used laptop swichmode psu and string right there.
Hm, this is what I use at the moment.
Boss’s JFET buffer works great and spark is fully back in my strat but 12V old boss power supply is overkill to the whole setup.
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Here is the few comparisons:
direct guitar cable into hifiberry DAC2 ADAC PRO
audio in gain set on zynthian to 24db
wireless digital 2.4GHZ https://amzn.eu/d/09HWYlHE
audio in gain set on zynthian to 12db
my preferred setup
cable to boss pedal - pedal is switched off so that only buffer is in use
audio in gain set on zynthian to 12db
wireless is also very usable and no need for additional power. Howevere I can feel latency - maybe more expensive UHF type wireless would be better but good one are few hundred £. Also good one would typically need power on receiver side.
Maybe it is a time for me to build a buffer similar to boss one and put it into mini stomp box.
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Not a guitar player here, though a long-time listener of the likes of Robert Fripp, Robin Guthrie, Michael Brook, Terje Rypdal and Steven Wilson.
To my ears the third example, with the usage of the bypassed Boss pedal for gain level enhancement, is by far the best-sounding. Warmer, smoother, more harmonically rich and nuanced.
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I’ve built numerous JFET buffers similar to some links above, and they solve the impedance problem simply at very low cost if you can DIY; this would be my favoured choice if looking for the tighest integration possible (a small pcb with smd components could be fixed inside the guitar aswell and switched by a push/pull potentiometer. I’ve also seen a design where the jfet buffer was built inside the jack itself !!)
I have ordered few buffers for testing.
One simple JFET buffer J-Buffer - Simple jFet Buffer - Guitar Pedal DIY - Five Cats Pedals | eBay
and another one based on KLON buffer K-Buffer - KLON Style Buffer - Guitar Pedal DIY - Five Cats Pedals | eBay
I am expecting them tomorrow so I will report back with sound comparisons. If they are good I will report back their schematics for other to built.
While I was looking for them I also found old novation foot pedal “rig kontrol 2” for 25£ only that comes with sound card with high z, 7 foot switches and pedal.
Searching the net i found out that foot switches are recognised as numkeys 1-7 and pedal is recognised like joystick over USB. It would be great if sound card is also supported by Linux/zynthian. I will report back on this too.
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Is it not easier to connect a good usb soundcard? I find the buildin soundcard quite useless For electric guitar and mic. So i bought a behringer umc 1820 .works pretty Well.
I don’t find the UMC1820 to be any better than the Zynthian’s own audio interface. They both have input impedance far too low for a guitar and hence load the pickup coils causing hf roll-off and undesirable distortion. The Zynthian audio input has a good input stage that analogue distorts nicely but if you want to connect a guitar to pretty much any audio device you need to match impedance by presenting a very high (>1MΩ) input impedance. There are some USB audio devices that purport to be guitar inputs like the miditech guitarface but they don’t seem (or at least don’t report) to have the appropriate input impedance.
You may be able to use your guitar amp’s preamp for this if you have a direct output from the amp - before the power amp stage but this might seem a nuisance.
I would love for Zynthian to add a (simple jfet) switchable input stage but it isn’t its core function and there is currently little appetite to add such an extra function that adds cost and complexity that few users benefit from. It is more awkward / costly to add this externally but there are solutions.
[Edit] There is the Rockboard Natural Sound Buffer which requires a 9V PSU (is not battery powered) which presents 2.2MΩ input. Or you could spend about twice as much on a Boss SD-1 SUPER OverDrive which can be battery and/or 9V PSU powered and looks like the cheapest Boss pedal. Boss include input buffers on their pedals (1MΩ) that is in circuit when the pedal is in bypass or active. I use my old Boss DF-2 for this purpose.
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It is easy to connect addition audio interfaces but it is not easy to put them on pedal board and probably not something that you would gig with even you squiz it.
I was looking what is the minimum required so that it is elegant and hopefully easy solution to fix in regards of low impedance input problem.
Finally it is not just about guitar audio in interface. For guitarist we need at least few foot switches so that zs3 can be switched while playing. Hence I posted cheap old “guitar rig kontrol 2” foot usb device that could solve both problems.
Yes the kontrol 2 looks good but it was £350 new and is no longer available. You got lucky with grabbing one for £25 but most of us won’t be so lucky, hence various ideas of what can be done with commonly available goods.
As @robert, I find these USB interfaces very handy.
First of all they mostly all work out of the box.
They are convinient: I use mine with its two combo XLR/jack plug, built in preamp whenever I want to plug a guitar, a bass or a dynamic microphone.
[edit] and quiet cheap [/edit]