[sdiy] VCLFO core circuit..

[Harry Bissell]

LOL. I once discussed this with Bob Pease (in specific about an ARP design)

The transistors should be matched for equal base-emitter voltage drop at some reasonable
current (maybe in the middle of the expo operating range, certainly in some 10's to 100's
of uA.

See the trouble... you might try MANY different types of NPN and PNP without finding a really good match,
and that would be different with each production run. That is why the manufacturers using this expo had such
hard times getting matched NPN/PNP pairs.

Much easier to match all NPN from the same batch, or use a monolythic dual. You are virtually assured of a match
that way. Its more expensive (opamps) but those are dirt cheap now, not like in the ARP days.

BTW BOB mentioned that Alan R. Pearlman was a simultaneous inventor of the "Darlinglton" trnasistor configuration, but he
didn't publish at the time so he was not recognised for having it first. It might have been called an ARPlington   :^)

H^) harry

----- Original Message -----
From: Jean-Pierre Desrochers <jpdesroc at oricom.ca>
To: synth-diy at dropmix.xs4all.nl
Sent: Thu, 08 Mar 2012 08:44:58 -0500 (EST)
Subject: [sdiy] VCLFO core circuit..


I'm about to start building a set of  VCLFO modules
for my synth so I browsed  a lot around
to check for existing designs..
I need it to track with 1v/oct input,
(exponential response) but it would have lin inputs too.
Most of the designs I saw use matched pairs of transistors
for their expo converter part and sometimes
hard to find parts.
Looking around I thought it would be so easy to
use the ARP Odyssey 2800 VCO core design..
http://www.arcenson.com/public/Odyssey_2800_VCO_detail.JPG
Frequency/temperature stability is not a big deal here but
I'd use a TEMPCO anyway (R32).
My question is:
Are the two transistors Q4, Q5 matched or only
stuck together for thermal transfer and stability ?
and if they are matched how are they ?
Thanks
J-Pierre


 
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-- 
Harry Bissell & Nora Abdullah 4eva