[sdiy] improved dynamic range diffpair-vca

[np]

Hi list, and hello to some old-timers who remember me.. I've been away 
from the list for several years....

Is anyone familiar with this circuit here:

http://www5b.biglobe.ne.jp/~houshu/synth/Vca0302.gif

It's a long tailed pair with two extra transistors working as diodes. 
Not the same configuration as the linearization diodes we have in some 
OTA's. I gave it a try using a CA3046 transistor array, and it just 
magically works. I can drive the input up to 50mVpp with low distortion. 
Nearly twice as much as without the extra trannies.

To me that looks almost like a free meal. Where's the drawback?

And could someone please shed some light on why these "diodes" work and 
what they do? They somehow linearize the input for sure, but I have no 
clue what exactly is going on here.

Thanks in advance,
  Nils