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

Dave Kendall davekendall at ntlworld.com
Tue Jan 28 01:52:51 CET 2014

Achim - thanks for the detailed and informative reply. Questions pretty 
much answered!  :-)


On 27 Jan 2014, at 20:36, ASSI wrote:

> 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.
> Regards,
> Achim.
> -- 
> +<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+
> Waldorf MIDI Implementation & additional documentation:
> http://Synth.Stromeko.net/Downloads.html#WaldorfDocs
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