[sdiy] Fwd: SDIY list troubles

[Paul Schreiber]

I'm going to make some GWACs (General Wild-Assed Comments):

a) never believe National Semi app notes written before 2000. Most of them
were authored by co-op students, especially the 1980s ones. Linear Tech app 
notes
are wonderful as are Maxim Semis. But a lot of National's are a bit....odd.

b) Is the substrate of the array tied through a 10K to -15V? National likes 
to leave this
little tidbit off most app notes. It's the 'Q5' of the *array*, not in the 
schematic
(notice how 'Q5' is skipped in the LM3046, but there is no reference to 
connecting
the substrate to anything.). If this pin is let floating (which I'm betting 
it is) then you
can get all sorts of interesting things, especially if you use old-fashioned 
ProtoBoards
(the white things you can stick stuff in and not use solder).

c) If you see *any* circuit using a LF412 *RUN LIKE HELL*. Easily the worse 
op amp ever
made (second to the National LM833 total POS audio part...ack!). I worked 
for a major
central office telecom supplier, and we almost went bankrupt over LF412s in 
circuits doing
all sorts of interesting things like oscillating at 10Mhz as the *humidity* 
changed, Q of filters
moving depending on how much *light* the IC package was exposed to. I 
designed fancy
multi-band filter (a delay equalizer for 2400 baud modems over leased-lines) 
using NE5532s
and the factory "upgraded" to LF412s and I spent many hours sitting in 
telecom closets in elevator
shafts getting it to work, we finally had VP of Engineering literally throw 
out every LF412 in the system,
the factory was solder-sucking parts off finished goods for weeks.

It would not surprise me if this particualr circuit used a 'quirk' of the 
LF412 to "start up", or worse
the designer 'just got lucky'. This is like Buchla paralleling CA3160 op amp 
*outputs* to exploit
a 'quirk' in the output stage (plugging in any other op amp causes bad 
things to happen).

d) LF412s are *VERY VERY VERY* sensitive to power supply noise and 
decoupling.
Silly observation: is the op amp powered off true +-15V? You need to be sure 
pins 4 and 8
ahave at lrast a 0.1mf ceramic bypass, perferred with a 10uf electrolytic in 
parallel. Even if using a
TL072 here, is it bypassed?

Paul S.