LF411 Op-Amp dilemma

brad sanders radioactive at geocities.com
Wed Jan 22 09:56:20 CET 1997


>>Well, i got some op-amps from Mouser and they said they didn't carry the
>>LF411 but they have an equivalent replacement according to them.  So they
>>sent me CA3140's in an 8pin DIP.

The CA3140 is a very poor sub for an LF411. In many apps, it will NOT,
in any way, perform as a "drop in replacement."

I agree with Jeff R; send Mouser back their parts.

 I wanted to check
>>with everyone here that these were a good substitution.  In the catalog
>>they're listed as BIMOS Op-Amps...

They have a terrifically high input impedance, yes - but they also
have an incredibly poor output stage. It's asymmetrical, AND can only
sink or source a few mA. In most cases this MAY not be a problem - but
if you're driving even a moderate load (like a patchcord connected to
another piece of gear) it CAN be a problem.

Add to this the CA3140 is a known lousy "audio" performer, and has a
slew rate at least half that of the already antiquated LF411 you're
replacing - AND has almost TWICE the noise to boot!

Don't use the 3140. You should have ZERO problem finding TL081s.
(TI also has a "new and improved" version, called a TL7081- or
something like this; consult their web page for details.) A TL081 has
lower noise than an LF411, equally good input characteristics, unity
gain stable, decent output drive, blah blah blah. An AD711 would be
even better - but really only in the S/H and directly in the audio
path.

DON'T use the 3140. 

I can't think of an application for the 3140 that can't be done better
by another device. Frankly, I wouldn't use a 3140 in a piece of gear
even if someone gave me a tube of'em.

Oh - and Mark Pulver: the 5534 is a great GP opamp - but in any S/H or
filters, the TL081 would, again, be a FAR better "cheap replacement."
Input impedance, input current, and offset stability are terrifically
important in these apps. This is where the 5534 fails as a "cross" -
it's not an FET input. The 5534 has input impedance several orders of
maginitude lower than a FET input device - and input bias currents
many orders of magnitude HIGHER than FET input devices. 

Other than this, 'tho, the 5534 is a great GP device. I'd imagine an
old synth with these *properly applied* throughout the audio path
would sound dynamite!


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