Denis,

you find my comments written inline.

Best wishes
Dieter Doepfer

> having another look at the quadrature LFO and the cool looking
> companion quad VCA i stumbled upon one feature that i miss in the
> current design of the quadrature LFO: a reset/sync input for the
> quadrature LFO. I believe that this is a simple yet very
> important feature for such a module. I'll want to use the module
> for creating sweeping FX sounds, and these require a defined
> starting point of the quadrature modulation. Also when using the
> module in the audio range a sync in would greatly expand possible results.

We tried this but it did not work so far. The problem is caused by the main
circuit which is kind of filter in self resonance. If one resets the circuit
(simply by shortening to GND with a electronic switch or transistor) the
oscillation stops but comes little by little until the maximum level is
reached. This is not what you'd expect from a reset/sync function. I don't
know an analog circuit the generates sine/cosine ("quadrature") and can be
reset in the usual way (except digital solutions). Maybe somebody has an
idea.

> Another idea would be adding four switches to the quad VCA that
> switch an inverter into the control signal path of the the
> respective VCA; possibly three-way switches that switch between
> normal, "x-> x*(-1)" and "x-> 5V-x" modes (where the first
> inverter mode basically "mirrors" the control signal around zero
> which would be most interesting for processing control signals,
> and the second mode would be more along the logic of a
> "crossfader" and mainly interesting for processing audio signals).

In principle this would be possible but the existing quad VCA prototype has
only one common current source for all four VCAs (= four OTAs). Your
suggestion would require a separate current source for each VCA which comes
close to 4 separate VCAs (e.g. four A-130). The costly part of a VCA is just
the current source (not the OTA which is about 30-40% only). Consequently a
quad VCA with four completely separate VCAs (and separate controls) would
not be much cheaper than four separate VCAs.
An intermediate solution would be only two (opposite) current sources which
can be assigned to each VCA by switches (e.g. on - off - inverted).

And one has to distinguish between inverting the signal and the control.
These are two different things. Inverting the signal does not make much
sense from my point of view as the inverted signals are already available
(e.g. 180 degrees is nothinf but the inverted sine).