One of the most important variable that affects the sound quality and playability of a mandolin is how well it has been “set up.” How an instrument is set up is a combination of a number of factors, which are detailed below. An important thing to keep in mind is that a well set up $300 mandolin can sound much better and be much easier to play than a $10,000 mandolin that has been poorly set up.
While setting up a mandolin is not rocket science, I recommend that people new to the instrument take their mandolin to a well respected instrument shop or repairman instead of attempting to do a setup on their own. Unless you are a tinkerer at heart or have an interest in fixing things yourself, your time, money, and patience can be better spent paying $50 to $100 and getting this done by a professional.
If you tune your mandolin up to pitch and it sounds good when played without fretting any notes, but sounds out of tune when you start fretting notes, that means your intonation is off. Fixing this can be as simple as moving the bridge or as complex as taking the neck off the instrument and reattaching it. Other things that affect intonation are improperly made nuts, excessively high action, warped fretboards, or fretboards with frets in the wrong place (more common on vintage instruments).
The height of the strings off the fretboard. The higher the action is, the harder you have to push to get a clean note out of the instrument. If your action is too low, the instrument will buzz, but if it is too high it will be too difficult to play comfortably. Some people lower their action slightly to improve playability at the expense of volume and tone, and others raise their action slightly to improve volume and tone at the expense of playability. Action is adjusted by raising or lowering the bridge height. Sometimes, the action can be too high or two low at the nut as well, which may require having a new nut made or the current nut adjusted.
If the instrument has strings that buzz instead of sounding true, this can be due to a number of factors. Poor nut construction, high or irregular frets, warped fretboards or necks, excessively low action, or excessive fret wear can all lead to buzzing.