I (eventually) tested inserting an effects pedal between my guitar and Zynthian with good results. The low-pass filtering and general lack of presence is gone which gives me confidence I can get the sound I want from this setup.
Direct:
Via Boss DF-2 in bypass:
(Just a quick and unrefined recording into the Zynthian audio recorder for comparison. Don’t judge my playing!)
I don’t play a guitar, but I have used a Line6 G10 wireless system with my (now sold) AE-10 Aerophone wind controller. The latency is negligible. They’re not cheap, and the receiver is larger than I’d like, but they perform well (and I assume they provide high impedance). Mine even has a balanced output.
One caution is that they don’t work nicely with a stereo input socket (to the transmitter). That’s not a problem with a guitar, but it was for my AE-10.
Gonzo
I just use a cable.
I know this was 8 months ago, but I’m reading this topic now and just wanted to point out that you can use two 9 volt batteries connected in series and get +9 -9V, and can get whatever voltages you want if you’re willing to use more-different batteries. You connect ground to the point where you connect the batteries together.
Thanks for that, and you are certainly right. Most of my teenage years were spent building 741 based op amp circuits so plus and minus 9 volt supplies from small batteries were pretty much the starting point for everything I did.
Keep in mind that Boss pedals are not really bypassed in the bypass mode. Instead, they have a buffered bypass, essentially acting as a buffer. Using a simple (e.g. DIY) buffer will achieve the same effect of matching the signal impedance.
Yes indeed! I use my Boss pedal as an impedance matching device. Of course I would prefer a high impedance input on the Zynthian but it is challenging and not required by many users. (Guitarist probably account for less than 10% of Zynthian users.) So a simple external buffer is a fair compromise. Maybe someone could design and build an economical device. Most buffers are rather expensive.
I always wanted to include this in my custom build Zynthian:
https://www.jensign.com/mig/
a bit dated but seems to be designed by someone who knows what he does and is well documented. Using an OPA 134 PA it can be powered by 5V. I think I even got the parts already but never came to solder them together, so can’t say anything about feasability/usefullness.
That looks like a very nice design and the article gives a lot of useful info. The idea of putting the actual ‘solder-less breadboard’ in a metal case both intrigues and horrifies me.
Another interesting possibility is the PedalPCB Terrarium:
Although designed to interface a guitar to the ‘line in’ of a Daisy Seed on the board you could either just leave the space for the Seed open, use it for a modification area, or perhaps use a Seed to do some pre-processing before passing the signal to the next device - like a guitar pedal you can program.
The “Build Documentation” includes Schematic and Parts List. The product itself is just the bare board, “No components, hardware, or Daisy Seed is included.”
A very common method of buffering (impedance matching) guitars is to use a JFET preamp. You can create large input impedance to a JFET amp and they can be run from single rail supplies.
This post describes how to build a very simple kit from this supplier / circuit. The circuit is simple but because this designer configures the board to support three different configurations (including NPN/PNP BJT& JFET) it looks more complex. The construction posting describes the JFET build which I would recommend.
Opamps can be a good solution, offering high imput inpedance but they do have complications that can lead to high frequency how-rownd (positive feedback loops). Such issues can result in the thing behaving very oddly as it makes lots of signal beyond the hearing range, i.e. it is hard at work but you can’t hear it!!! Of course these are simple to resolve, as described above but KISS is a good approach.
I think we should start to lobby for a selectable Hi-Z input stage on the V6. It could be on just one of the two inputs because it seems unlikely that many of us want to plug two guitars in at the same time… Adding a JFET input stage (as demonstrated here) is not that difficult or expensive but adding the switching may prove challenging, i.e. we would need to be able to switch the Hi-Z input stage in and out of circuit without adversly affecting the input signal which may be balanced (differential). Of course we could tie this in with the input configuation so that it would automatically configure as unbalanced (single ended) when the Hi-Z input stage is used. Gosh - so much detail too early in our lobbying campaign!!! But it is better to offer solutions and not just problems. 
don’t want to be pedantic - and to protect people from following funny ideas - but Gallant clearly states:
This is useful for experimental work to see the effects of circuit modification. However, for best performance and greatest mechanical stability, the circuit should be finalized to a printed circuit-board.
thanks for the musikding circuit, seems pretty simple (great!) - but would it work also for a single rail 5V supply, resp. what would be the implications?
I would vote against this - as you stated yourself the actual need for this is pretty low and threads like the current one provide plenty of information for those who have an interest in this (and after all this information could go into a wiki article).
But most of all instead of cramming feature after feature into Zynthian imho there should be a much greater focus on usability and workflow - there are several examples of open source projects which benefited very much from restraining feature overflow and concentrated instead more on UX and interface design, e.g. Blender or Musescore as recent examples. /rantOff 
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 bust there is quite a lot of hf in the harmonics, especially if playing hard, e.g. slap.
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…
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:
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.
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.
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.
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.
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.