AW: Formant (and jbv!)

[Haible Juergen]

	>Some time ago I mailed about the dual transistors in the
formant's exp
	>converters and it looked like there was no simple (which means
cheap and
	>easy) way to replace them. The VCO is desinged to use the uA726
(dead and
	>buried) - some nice replacements would be MAT02 (damn
expensive) or LM394
	>(dead), and some of the modules use PNPs that can be
substituted with
	>MAT03 (even more expensive), by matching a pair of single
transistors or
	>by using 2 PNPs of a bigger transistor matrix (CA3096...). The
last two
	>ways will lead to a loose match or to poor heat conductanse.
	>
	>But now it seems like I have found the ultimate solution:
>  >the SSM2210 (dual NPN) and the SSM2220 (dual PNP) by Analog Devices.
	<They are both matched tight and nice, they are still in
production (as far
	>as I know) and they are not so expensive. And the best part is
than some
	>of the specs on these chips are even better than in the MATs!
According
	>to the data sheets, a typical offset voltage drift of the
SSM2210 is
	>0.03uV/C which makes the MAT02 look pretty lame with its
0.1uV/C (if I
	>remember correctly)!

Forget the matching, offset voltage and its drift - this all is most
unimportant in expo converter applications.

What you need is thermal coupling, plus regulated heating,  or 
3300ppm temperature compensation.

Sorry for the bad news. But buying expensive dual transistors
is completely in vain without temperature compensation. (You need
it even with perfectly ideal transistors).

JH.