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FM-100 Page 9
Part of the output of U11:A is observed for the level indicators. IC U11:B, D4,
R54 and C60 comprise a peak hold detector. Part U11:B and D4 make up a
real diode, meaning there is no .7 volt drop normally associated with a diode
because it is accounted for in the feedback of the opamp. The “real diode” will
charge C60 quickly on positive going signals, but it doesn’t discharge it. The
discharge cycle is left up to R54. The larger the value, the longer in time the
peak hold function is. The voltage on the peak hold is then observed by using a
LM3915 bargraph display driver U12 and a ten segment bargraph. This part is
pretty self explanatory, it’s really just a voltage meter with a log scale instead of
a linear one.
The whole process of filtering, mixing and level detecting is repeated in the left
channel as well. This completes the basic analog circuit description of the FM-
100, but you may be interested in more detail of how it works. If so, there are
many good books and magazines which deal with circuitry of this sort in smaller
manageable circuits which can help you delve further in what is going on.
Содержание FM-100
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