Matching transistors

[David Hopper]

Mark Smart wrote:
> 
> Since I have been working on ARP Omni's recently, I need to ask a question.
> The Omni's filter module contains discrete matched transistor pairs.
> According to Joachim's information (thanks Joachim) these transistors are
> Motorola 2N5087 PNP transistors, or at least they were on later models.
> So far I haven't seen any of these blown out on the Omni's I've worked on,
> but I would like to know how to match them if I have to change them.

> Joachim's schematic specifies "transistors matched for delta Vbe <= 0.5
> mV". What I want to know is what circuit I need to build on a breadboard
> to stick the transistors into whle measuring Vbe.

OK, here's the text from my Moog Modular manual, on matching
transistors:

Matching Procedure

1.  Setupp +- 10V supplies to +- 10.00 V or +- 15.00 V.

2.  Take transistors (approximately 20) and place them in styrofoam to
stabilize at room temperature.

3.  Place Transistors into socket, one at a time, and measure base to
emitter voltage.  DO NOT use your fingers.  Use gloves or pliers with
insulating jaws.  Your finger heat will cause the readings to vary.

4.  Mark down the Vbe from the DVM and find two transitors that the Vbe
matches to +-2 mv.

The document also has two circuits drawn out, one for NPN testing, and
one for PNP testing.  I have no scanner here at the apartment, but I'll
try to scan the document in at work and put it up on the cheezy web site
(http://www.seanet.com/~costello - Check it out, it only has one thing
on it!  Almost worthless!!).

The 4075 seems like an awful lot of work to make properly, if you wanted
to make one from scratch.  There are something like five matched
transistor pairs, each wrapped in copper - and matched to a high degree
of precision to boot.  The 4035 seems like it might be an easier project
to undertake - all of the transistors are in 3086 form, except for the
two FET buffers (thanks to Joachim for his incredible schematics, BTW). 
Even this seems far more complex than the good old Moog filters, which
appear (from my untrained non-EE eye) to be far more "sloppy" in design
and construction than the ARP gear (only a few matched transistors, and
not that precisely matched at that).  Are the ARP filters really better
sounding than the Moog filters?  I mean, you don't need to have too much
precision in the filter for an Omni, for pete's sake - it doesn't even
track the keyboard.

OK, I'm ranting...I'm on sort of a Moog kick lately.

Sean Costello

P.S.  As I type this, my roomate is in the other room, sqeezing tortured
sounds out of my Multimoog.  It's always nice to induct a new recruit
into the cult of analog.