[sdiy] Effect of unconnected linearisation diodes in LM13600/13700

ASSI Stromeko at nexgo.de
Mon Jan 27 21:36:47 CET 2014

On Sunday 26 January 2014, 17:52:36, Dave Kendall wrote:
> > But the 3080 and the 13600 are based on totally different technologies
> Apart from the different sized packages, do you have a link to more
> info on this?

The packages have nothing to do with it.  That one uses an RCA technology 
and the other a National Semiconductor one ~ ten years later does.

> I notice that the datasheet schematic for the 13600 looks almost the
> same as the 3080, but with the addition of an NPN transistor with
> collector connected to the amp bias input, emitter to -V, and base
> connected to the base of a second additional NPN which goes to the
> darlington buffer. Could that possibly affect the behaviour of the
> 13600 VS. the 3080? (assuming the buffers are not used)

That's a Wilson current mirror in contrast to the somewhat less precise 
Widlar current mirror on the CA3080.  The additional diode voltage needs to 
be accounted for in the biasing of course and really that's the sticky point 
when considering a replacement.  But often the biasing is via a current 
source anyway, so it doesn't really matter.  You can ignore the additional 
transistor that mirrors the current again for the Darlington buffer bias if 
that buffer isn't used.

> What I'd really like to find out, is if there is any truth in claims
> that the 3080 "sounds different" to the 13600. One way to do this would
> be to build a test circuit that would take either chip, after matching
> the two chips' performances as nearly as possible, although I doubt I
> have the right gear (or expertise) to match them in a meaningful way at
> the moment.

Start with answering the question if one CA3080 sounds differently from 
another CA3080.  I can't say for sure, but I don't believe that the 
Harris/Intersil parts were made in the same process than the original 
RCA ones.  Even with the technology differences outlined above to the 
LM13600, the characteristics of the circuit are that these will become 
almost invisible.  That's not to say these are drop-in compatible in any 
circuit, but you could in general expect nearly identical performance from 
the same circuit topology with minimal changes in biasing.

> The differences may not be so important now that Rochester electronics
> are producing the 3080 again. According to one source, they have the
> masks as well as stocks of wafers and dies, so things look hopeful for
> the 3080s (relative) availability. I'm still interested in working out
> if there is a significant difference though. Many claims of different
> "sounds" flying around the internet seem to be the result of not
> comparing like with like, but maybe that's just my perception of it.

Even if Rochester has the masks, they are mostly useless since the reason 
for EOLing the CA3080 was that the line that produced them is no longer 
available.  It appears that plenty of wafers were pre-produced however, so 
they can probably work off that stockpile for quite some time.

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