Re-inventing (?) the VC envelope ?

[jhaible]

Ladies and Gentlemen,

I 'd like to proudly present a circuit for the VCA core of a voltage
controlled
envelope generator.

The problem is all too well known: You can build a decent voltage
controlled
envelope around an OTA like the 3080, plus the usual control logic for a
normal ADSR circuit. But the offset voltage change of the OTA over its 
control current will result in relative large offset voltage errors at the
ADSR
output. This can be several millivolts, and it will change with different
time constants for attack, decay and release.

This problem was solved in the CEM3310 chip long ago by building a rather 
complicated "composite" 2-quadrant exponential multiplier. (I hope this
describes the idea more or less). I have thought about building a discrete
circuit that uses the Curtis method for a long time, but the complementary 
output opamp and the heap of transistors involved in current mirrors have
frightened me away.

Then Joachim Verghese came up with his brilliant idea of splitting the
exponential
multiplier and the positive / negative direction switching of the VCA into 
separate parts. He presented a circuit with a sign / magnitude full wave
rectifier,
an exponential multiplier, and an OTA that only switches the sign of the
output
current (i.e. doesn't contribute to the offset voltage anymore).

Joachim's circuit is, while pleasantly inexpensive, still a rather large
circuit, and
so I tried to simplify it a little. My idea was to build an expo multiplier
that would
perform the sign / magnitude stuff all by itself, in other words
auto-configure
itself to current source or current sink mode, depending on the input
current 
sign. So I came back to the old Curtis idea, but I ended up with a much
smaller
circuit.

First step: Build two separate expo multipliers, one for source, and one
for sink.
Their linear inputs and outputs could be connected together - they would
not
interfere with each other, because one will be passive when the other
conducts.
This idea was taken from the CEM chip, but I used two npn transistors for
the
sink part, and two pnp chips for the source part, just as one would do with
two separate circuits.

Second step: Use the same opamp for the servo loop around both differential
pairs. Unlike the Curtis method, I don't need complementary outputs. A
normal
opamp will feed both halves alternatively with the right sign of current
flow, with 
a little "dead band" near zero, just like a precision full wave rectifier
circuit.

Third step: Both halves of this composite source / sink exponential
multiplier would
normally need complementary control voltages at the npn / pnp transistor
bases.
One could simply add an inverter here, but then again these are two
transistor
*pairs*, so in order to give them complementary steering from one single
CV, I just
grounded the 1st transistor's base of the npn pair, and the 2nd
transistor's
base of the pnp pair, and connected the CV to the two remaining
transistor's bases.
(It looks like a diagonal connection if you draw it as separate exp
converters.)

Fourth step: Redraw the circuit (;->). Well, not such an important step.
But if you
look at the final version of the schematics, the circuit looks so dead
simple that
I had the biggest doubts whether it would work at all. But no need to worry
-
it actually works. I have built it on the breadboard, and I got time
constants from
milliseconds to the minute range, and no more than 3mV offset. With TL071
opamps and unmatched BC550C and 560C transistors.

I have only tested the core of an envelope generator (a lag circuit like
Joachim's),
but I have no doubt that it will work in a full VC ADSR as well.

I hope that the encouraging results of this little test will keep their
promise in
general applications - any hints about possible problems are welcome.

And finally a question (Paul might be able to answer this) : When I look at
the
CEM3310 from this new point of view I wonder if this simple circuit might
have been what Doug Curtis had in mind as well. Could it be that the idea
is
exactly the same, but all this current mirror stuff in the 3310 was just
because
of the lack of "good" pnp transistors in the bipolar process he used ??

Ok, this was a lot of words for those who are interested in the progress of
finding
this circuit. Here comes the schematic diagram. It's a small file, and from
our recent
discussion I get that it's ok to send this to the synth-diy list directly.
So here it
is:

JH.

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