Ceramic capacitors have received a bad reputation, some of which is not deserved. There is nothing wrong with using ceramic capacitors in guitar pedals, if you pick the proper type. You will see a variety of different ceramic compositions in the capacitor section of the big parts catalogs, including NPO, COG, X7R and Z5U.
X7R is based an barium titanate, which is prone to developing microscopic inclusions or defects within its structure. Both X7R and Z5U have certain non-linear characteristics that are unwanted in audio devices.
Jung and Marsh, in their series entitled “Picking Capacitors”, which was originally published in AUDIO magazine in 1980, tested the X7R ceramics. This was one place that ceramics began to have a bad reputation for audio use.
However, COG and NPO have not only much better temperature stability but also more linear audio response. Distortion from these type devices will be an order of magnitude better than the X7R or Z5U based parts.
Ceramics at very small values of capacitance (typically is the pF range) are used to prevent RF interference and will have no impact on the sound at audio frequencies. The 10pF on the input of the AMZ Mini-Booster is an example of this use. It shunts the radio frequencies but is effectively out of the signal path for audio signals.
It is a good idea to steer away from using ceramics for audio coupling (dc blocking) but many of the latest tiny portable mp3 players will have numerous surface-mount ceramics in them and not many people complain about it. I would not put ceramic capacitors in an expensive high-end audio system but they are perfectly acceptable for guitar pedals where the ultimate frequency response of the amp and speaker are usually limited to 5k Hz or less!
Here are my general rules:
- Use only CGO or NPO type ceramic capacitors.
Use ceramics only for the pF range where alternatives are not available.
Do not use ceramics for dc blocking and coupling.
Stick with those rules and no one will be able to hear any influence the ceramic composition may (or may not) have on the sound of your pedal.