Шумы и фильтрация в примочках на КМОП ИС
------------------------------------
Вот как ответил один из американских гуру - Mark Hammer - на вопрос о применении CD4000-ных микросхем в примочках. ИМХО максимально толково плюс великолепная рекомендация по фильтрации в каскадах (касается вообще всех транзючных искажалок, кстати):
-----------------------------------------------
They are known to vary with chip.
When synth-builders are putting together a noise generator circuit, they are always advised to test out a bunch of transistors and select the one the produces themost noise. These are transistors of the exact same part-type (e.g., twenty-five 2N3904s). So, when you look at what is inside a 4049 chip and realize it really is only a couple of transistors per invertor, it should not surprise you that they too can vary in noise produced, even though they may be produced by the same company and have the same part number. So, try a few out and select the one that yields best sound with lowest noise.
Beyond that particular aspect, you can always tame hiss by adding lowpass filtering on . Personally, I prefer to add more on at the beginning and then ease up a bit toward the last stage/s. There are two reasons why. First, gain is multiplicative, so that if there was any hiss in the source signal, boosting it, and then boosting it again, is going to give you a hissy output. Second, when you apply lots of gain to a signal in a stage that is intended to add harmonic content, you end up with harmonics of everything, including noise in the upper mid band. If you boost and clip that again....well, you get the idea.
So, I like to impose a rolloff around 3.5khz or less at the beginning (sometimes lower) and then ease up to a rolloff in the 4-5khz range near the end. I won't claim that this necessarily gives you a dead quiet effect, but it does give a VERY quiet overdrive, with a very pleasing tone. Many pedals, in an effort to be flexible, provide more bandwidth than is actually needed. One needs to pick the bandwidth that you absolutely can't live without, and design around that. You may find that very often you have more bandwidth at either the top or bottom than you really need, and a little more bass or treble-trimming nets you less audible hum or hiss.
It pays to have a lot of small-value caps (33-470pf) handy.
