[sdiy] SSM2164 Balanced Modulator

[David G. Dixon]

> If one wanted to build a balanced modulator using an SSM2164, would I be
> correct in assuming that you'd use two linearised VCA circuits, one of
> which has an op amp inverter added to the front of it. The outputs of
> the two VCA's would be mixed using a virtual earth summer. The X input
> would go to both VCA audio inputs (one via the inverter). The Y input
> would go to two precision half wave rectifiers, one negative and one
> positive. The output of the negative half wave rectifier would be
> inverted so that it outputs a positive voltage and sent to the control
> input of the VCA that has the inverter on the audio input. The output of
> the positive rectifier will go to the control input of the other VCA.

You don't need the rectifiers on the Y signal, just unity-gain inverters.
Otherwise, you're spot on.  It's even better if you convert the Y signal to
two 0-5V signals 180 degrees out of phase with each other and use these to
control the linear VCA gains.  I do this by summing the -5V reference
(required for the linearized VCAs) into both Y inverters, the first of which
is at 50% gain and the second of which is at 100% gain.  The beauty of this
is that I can put a pot with a grounded wiper between the two X inputs and
(with some careful resistor selection) obtain + and - two-quadrant
multiplication at either extreme of the pot, balanced four-quadrant
multiplication in the middle, and various degrees of unbalanced
four-quadrant multiplication in between.  I've also added rectifier networks
to both X and Y signals.  All this creates a lot of sonic possibilities in
audio mode, and some very interesting CV processing possibilities in
subaudio mode.

> When the Y input becomes positive it turns on the VCA to pass the non
> inverted signal. When Y becomes negative it turns on the VCA to pass the
> inverted signal.
> It seems to me that this would achieve the desired function. Any thoughts?

Just this one:  Build my 2164 Multiplying Modulator, or, better yet
(warning: shameless plug), if you use Euro format, buy the new "uMod" from
Intellijel, which should be available from Analogue Haven by New Year's.