[sdiy] On-chip transistor pairs

[Ben Bradley]

The talk of exponential converters on VCAs brings to mind the use of
transistors for expo conversion, specifically on-chip transistor
pairs. Was there any use of National's (and later TI until they
stopped production) LM324 precision dual transistor in a synth expo
converter? The only product I know of it being used in was the Jensen
990 discrete op-amp. This tells Jensen's story of going through
replacements for the LM324:

http://www.technicalaudio.com/pdf/Jensen_Transformers/Jensen_OpAmps_990_and_related/Jensen_JE-990_opamp_JAES_reprint_1980.pdf

On the first couple pages of text here is Deane Jensen's descriotion
of the LM324, how it's made with many on-chip transistors wired
together as two:
http://www.johnhardyco.com/pdf/990.pdf

I noticed our friends from Latvia are now making an LM324 replacement:
http://www.alfarzpp.lv/eng/sc/transistors.php

There was also the ua726 self-heating temperature-regulated pair used
in classic synths that is basically unobtainable now. It would be good
to have that remade, or perhaps even an LM324 version with an on-chip
heater/temp regulator.

The newly-remade module-on-a-chip designs are wonderful in having
everything built in, but I still want to make some "discrete" designs,
and having these transistor-pair-on-a-chip products available helps a
lot.