Opinions: On op amp replacement

[jhaible]

> I was recently given the advice to replace all the 741 and 748 op amps
(on
> a paia modular)
> with LM301AN's, and LF351N's--------
> 
> 2 questions.
> 
> 1--Anyone have opinions on *how much* difference this might make? They're
> relatively cheap (1.10 usd or so) and direct pin-compatible, I've been
> told, but I'll need about 20, so thought I'd ask.
> 2--Any good projects with which to use the 741/748s that will be left
over,
> or are they that useless...?
> 
> 
> Thanks gurus, for the thoughts...
> 
> charles morpheus
> icumedia
> 
> >I also found that if you change all the 748 op amps to LM301AN's, and
741
> >op amps to LF351N's that the system becomes much quieter and more
stable.
> >Apparently the 741's and 748's, are the worst op amps you can get, and
buy
> >changing them to newer, higher quality op amps, improves performance
> >dramatically (I got this info from a former ARP tech).

There is no general answer unless somebody is familiar with the whole
circuit in question.
As a matter of fact, you can replace these opamps in *many* cases, but
there are
exceptions. There are circuits that rely on the low slew rate of an opamp
to smooth
glitches, there might be stability problems with a faster opamp in general,
and
the input common mode range of a 741 goes down to lower voltages than a
JFET
input opamp would. I have even seen pin 3 of a 741 being tied to the
negative rail
(not gnd !), and the whole thing still working as linear inverting
amplifier with a
feedback loop comprising an external transistor. (That's the MK 2 VCS3,
btw.)

If you keep these exceptions in mind (are there more ?), you can probably
decide
which opamp can savely be replaced and where to be careful. I'd suggest
exchanging
them one by one, and not all at one time, if you are not sure. I'd leave
opamps
that are part of the power supply alone. (There are PSUs with a 723 for
positive, and
an opamp for negative rail.)

JH.