This topic is to describe the challenges and solutions to using Zynthian as a guitar processor and to share experiences, hints and tips and of course .
Connecting Electric Guitar
Zynthian has evolved over the years to use various soundcards and there are also many custom builds that use other (non-official) sound cards. Invariably these cards do not have an audio input optimised for direct connection of an electric guitar pickup. An electric guitar pickup is a set of windings of (usually) copper which is connected via potentiometers, resistors and capacitors (volume and tone control) to its outlet jack. This expects to be connected to a very high impedance amplifier, in the order of one mega Ohm. This differs from electronic instruments such as synthesizers which are designed to connect to inputs in the order of a few kilo Ohms.
The science of impedance and its effect on audio is reasonably complex involving the use of imaginary numbers to represent phase shifts, etc. I feel this is beyond the interest of most gigging musicians so let’s not go there but instead make some broad descriptions of behaviour.
Plugging a guitar directly into an audio processor with an impedance below 500 mega Ohms is likely to affect its tone and possibly increase the risk of hum and buzz. The impact on tone is the most significant here though. Most Zynthians will have an input impedance much lower than this and hence will not be optimised for direct guitar connection. You can certainly do it and the V4 series (and possibly earlier models) have a great Hifiberry soundcard with sufficient gain to boost the signal and an analogue front end that saturates (clips / distorts) quite nicely if over-driven but the poor impedance matching can lead to difficulty equalising the frequency response. The guitar can sound muddy, failing to cut through with notes sounding indistinct. This may not be an issue for some types of music but you may prefer to have a cleaner, more accurate reproduction.
You could try to use an LV2 effect to adjust frequency response, like a multi-band equaliser but this is fighting to fix something that you have just broken and is sub-optimal, generally giving poor results and using valuable processing power. A better way to resolve the issue of impedance mis-match is to… well, match impedance. This is done by inserting a device between the guitar and the Zynthian.
You could plug your guitar into a guitar amplifier then use an effects loop or a direct output from the guitar amplifier to the Zynthian.
You could build or buy an impedance matching unit. These are usually called direct injection (DI) boxes but beware that many do not have the very high impedance input we want for a guitar.
You could use a guitar effects pedal. For example, the input of a BOSS pedal is matched well to a guitar as it expects one plugged into it. Its output should behave like a pickup coil (because it expects to be plugged to a high impedance guitar amp) but its electronics actually allow connection to a lower impedance such as our Zynthian. It can be used in bypass (even without a battery!). This may be the simplest method for many who already own a pedal as it allows for practice, rehearsal and performance rigs and decouples the guitar from the Zynthian by having the guitar cable (which may be moved dynamically) going to the pedal with a more static cable going to the Zynthian.
You may find plugging your guitar directly into your Zynthian is fine and I have been doing it for some time but you may, like me, have struggled with tone and want to add that extra device in an attempt to resolve it. (I am not at home for a few weeks so am without guitar and amp to test all this but will be adding my venerable Boss pedal to my rig upon my return.)