[sdiy] expo circuit

[JH.]

Q4 is the reference (zener diode), Q1 is the servo amp (where others use an 
opamp), and Q2+Q3 is the differential pair.

JH.


----- Original Message ----- 
From: "Dave Manley" <dlmanley at sonic.net>
To: "sdiy" <synth-diy at dropmix.xs4all.nl>
Sent: Saturday, May 24, 2008 3:35 AM
Subject: [sdiy] expo circuit


I was looking at JH's String Filter schematics 
(http://www.jhaible.heim.at/string_filter/jh_string_filter_sch.pdf)
and the resonance expo circuit with the transistor (Q4) connected as a 
reverse biased diode seemed similar to something I'd seen before (lower 
right on the schematic).  A little digging shows it is very similar to 
the synthi expo (as seen in the schematics on the Hinton site: 
http://www.hinton-instruments.co.uk/ems/vcs3osc1.gif).

So...what exactly is the function of Q1 and Q4? Does it give better 
conformance?

-Dave

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