Yep, this is an updated version of the simple Model A VCA that includes some tricks from its bigger sibling, the A+. You have two VCAs in one module, a fancy one with bipolar offset, bipolar CV input and input volume boost knob, and a second, simple in/out VCA underneath. Things get interesting though internally connected signal paths though (more on that below).
Here are some simple demos of the boost function:
So, let’s talk details. Both VCAs are DC coupled, i.e. you can use them for processing audio and CV signals. Input 1 is normalled to input 2, mod in 1 is also normalled to mod in 2 as long as you do not plug a dedicated in/mod source into the second inputs.
The VCA outputs are also coupled, though a bit differently. As long as OUT 1 is not plugged, the output signal from VCA 1 is internally mixed to the signal output of VCA 2, which saves you an extra 2:1 mixer. A cool side effect of this arrangement is that you can easily run both VCAs in parallel. Plug in1 and mod1 but only out2 and you get double the oopmph up to the point where your vca output clips the output buffer amp (see the boost demos above). If you want normal cascaded VCAs (one affected by an envelope, the second by velocity), simply plug VCA 2 out to VCA2 in as this breaks the internal mixing connection)
Note that VCA 2 output also flips the polarity of the input signal while VCA 1 does keep the signal polarity as is. Why is that useful? – Feed an LFO signal into both VCAs and simply pull up the offset knob and OUT 1 will give you positive modulation while OUT 2 sends negative modulation – this is not only cool for modulating stereo filters.
With offset pulled up, you open both VCAs (VCA2 remains unaffected if you plug a dummy cable into MOD2). Since MOD is also bipolar, you can make ducking effects by pulling offset all the way up and sending an envelope to Mod IN and turning the mod knob to the left. More advanced stuff like autopanning or xfading remains a privilege of the Model A+ version.
Gerbers and BOM
Build-wise, this is quite easy; no special parts needed except the Alfa Par AS662D. For offset and Mod I highly recommend using centre detent pots (Thonk has some, for instance).