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

Dave Kendall davekendall at ntlworld.com
Sun Jan 26 18:52:36 CET 2014

Achim, Ian - thanks for the replies.

On 20 Jan 2014, at 19:00, ASSI 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?

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)

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.

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.

Hope this isn't a dumb question, and thanks for the replies so far.


On 21 Jan 2014, at 00:55, Ian Fritz wrote:

> If the diodes are not connected they are back to back across the input 
> terminals, right?  The inputs are typically fed from a low impedance 
> source (typically 220 Ohm), so the RC time constant from any tiny 
> diode capacitance would be miniscule, as far as I can see.  You could 
> always try connecting the diode bias point to the negative rail, 
> reverse biasing them both, and see if there is any change in what you 
> hear.  The poster on the other thread doesn't show any significant 
> technical knowledge.  And never forget that small apparent differences 
> in how equivalent circuits sound are usually due to the amplitudes not 
> being exactly matched (better than 0.3 dB is necessary).
> Ian
> At 11:32 AM 1/20/2014, you wrote:
>> Hi all.
>> If the linearisation diodes (and the darlington buffers) are left 
>> unconnected in a circuit using 13600/13700, will there be any effect 
>> on the operation of the chip? Does their mere presence change the 
>> behaviour of  a given circuit, compared to the same circuit using a 
>> CA3080? (assuming the layout is as similar as can be, given the extra 
>> OTA and the two different DIL package sizes)
>> In a forum thread I am following, it's been suggested that perhaps 
>> the capacitance of the diodes *might* affect the circuit, even if not 
>> connected, making the behaviour of a circuit different to the same 
>> one using 3080s.
>> Has anyone has done any tests, or can otherwise comment?
>> cheers,
>> Dave
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