[sdiy] THE LM13600/LM13700 STORY

[JH.]

>Ah, I didn't know that.  I think most people find it superior to the 13600,
>not only in linearity, but in S/N and -- especialy -- in CV rejection.

Do you know what exactly they made wrong that leads to increased CV 
feedthru?

I'm asking because _theoretically_ the 13600's topology should have an 
advantage:
The common mode signal at the input transistors is smaller than in a 3280.
And normally a change of Vce will also cause a little change in offset 
voltage ot the
differential pair. (according to the data sheet of really good monolithic 
matched pairs.)

That's why I've chosen a similar topology in my discrete OTAs for this QVCO:
http://jhaible.heim.at/matrix_fx/jh_matrix_fx_osc1_core_board_sch.pdf
The rather small resistors R108 and R109 keeps the voltage swing at the 
bases of U6
small (and J5 helps to to keep the two CE voltages similar).

If I had chosen the CA3280's topology, the differential pair would have been 
shifted
up and down in a common mode signal that's derived from the (differential) 
input
signal, for which the MAT-02 specs would predict more CV feedthru.

So (asuming I haven't drawn wrong conclusions, which is entirely possible), 
the 3280
must contain much better transistors than the 13600, on order to compensate 
that effect
that would give th e13600 a topology advantage, and be even better.

Does this make sense?

JH.