I’d be using mine as part of a filterbank and not restricted to the SID, so I like the separate boards thing.
Well, maybe some form factor discussion is in order. This will be a long one, but hopefully will spark some discussion on where we want to go…
I am definitely a fan of doing a two-board system. Board A = Dual 3372 VCF/VCA; Board B = Dual Multimode Switch Board. I think I may opt to put the cell buffers for the Xpander mode on the 3372 board, to be jumpered in and out, that way we have a buffered signal before it travels along any wires to the switch board.
Let’s discuss Board A first (I think Board B will come naturally, as really its a direct copy of the 4051/4053 circuit from the Xpander service manual).
The 3372 is equipped with the following:
- 2 Separate Pre-VCF Audio Inputs w. CV Control of Input Volume
- VCF w. VC Resonance (Resonance is not exponential, so no linearization required!)
- Final VCA w. CV Level Control
For switching the audio routing (ie. SID thru vs. 3372), a similar relay circuit to that seppoman used on the 2044 board would suffice.
I also think that it would be beneficial (and easier on the design side!) to, as seppo did with the 2044 board, make use of the scaling options on the AOUT NG, which this will probably link up to, for fine tuning and offset adjustments. This will avoid the necessity of trimmers for each of the CV functions on board the 3372 PCB.
MIXER INPUTS
Even though I don’t have any particular intention of using more than one input or the volume control, it makes sense that we provide facility to use the other input and volume controls. For the audio inputs, perhaps a jumper to ground where you’re not going to use the input. For the CV Mixer controls, perhaps a jumper which can be installed either to ground (to silence the input), or to app. 4.88v to use full volume (makes sense if you’re strapping on to the SID for example), and if the jumper is not installed, the middle pin is your connection point for a variable CV?
Standard setup for the SID setup would be, for example, SID output (via switch or relay) to Input A. Input B jumpered to ground. Mix CV A jumpered to 4.88v (essentially full-on), Mix CV B jumpered to ground.
VCF
I have already discussed the bizarre “biasing” thing going on with the CV input. If anyone sees anything wrong with the following let me know.
It appears that all that is happening at the front of the Xpander circuit is a slight biasing of the input voltage by -.0253807… volts by way of a resistive divider (196k and 1k for -5V). To keep this same figure with a -12V feed, we’d need to up the 196k to a 475k (if my maths are correct?) to give us something close (exact value is 471.8k).
Other than this, there is a simple divider on the front end (38.3k, 1%), using the same 1k resistor to ground. This maxes a +5V input out at 4.872V. We can keep the same value, on the presumption that the Xpander feeds a 0-5V control signal, and that this is what we will be adjusting our AOUTs to.
Resonance seems to be a standard 0-5V input, and seems wired direct to the DAC on the Xpander.
VCA
Although the VCA is independently accessible (it’s own I/O), I don’t see much point to adding a separate relay for accessing the VCA alone… Personally, I don’t see that it’s worth the effort.
In any event, there will need to be (as with the cells - jumper = connection to the on-board filter caps, no jumper = pin to the requisite buffer and on to the switch board) a jumper here to either route the VCF in, or the Switch Matrix output…
There is a little bypass network on the Xpander circuit for the VCA level CV, so for continuity I say we follow that.
Did I miss anything??? 
Gav.
