X-4046 VCO

Hmmm another VCO as well! Have tweeked in a couple of other oddities...

Mutant VCF

This is the almost stuffed Serge Mutant VCF -- it is an attempt to be a Serge VCF with voltage controlled Frequency, Resonance & Slope. Not really sure that having all three parameters Voltage tweekable will work but it is easy enough to drop back to just two, as in the originals... I have used That Coporation quad transistors for the gain cells in a hope that it will be both quieter and more stable than the original. I have also used silver mica capacitors in the analog path. Looks like and intersting filter, from reading a few reviews on the net. It's not like I really need another filter...

XFade Vactrol

Not the typical Buchla Vactrol but still seems to work OK...

XFade

Part of the Buchla Music Easel wavefolder clone. This is a VCA/Mixer that originally mixed between two wave shapes in the VCO. Here it has been just used as a VCA for a sinewave. The sawtooth used to modulate the signal gives a bit of an idea about the response but there is an enormous lag in the response time.

That's Spoilt it's Looks...

Found the problem with the Sync Circuit... the Transistor Array got some of it's pins changed along the way and whilst they are correct there is a special pin that is tied to the substrate... This is supposed to be at the lowest potential of all the transistors on the device... in theory... So we end up with this to get around some of those problems! We do now get s signal through to the output of the OTA and the transistor beyond... seems to get a bit lost after that though....

VCO Basket weaving...

The Sync circuitry on this thing has got knobs on it...

Sinewave Fun...

Well slowly more and more of the 921B VCO comes to life. I am having fun with the Sinewave shaper today and getting a reasonable shape coming out but there are still quite a lot of harmonics... It seems like the amplitiude of the Triangle wave is just too low to get a good Sinewave. The non-linearities of a transistor curve are exploited to get the shape and there is an input amplitude adjustment but I have it hard at maximum to get the best output. The other two adjustments in the circuit balance the two sides of the sinewave and both mimize the harmonics within their travel... I am also getting a fair amount of high frequency "hash" on the waveform from a lack of decoupling capacitors! Will try and clean that up tomorrow then on to the really intersting part -- the Sync section... Not like any sync circuit in any other music synth that I have seen before! As I progress through this I am really tempted to make a clone of this which is more 21st century, use better matched pairs and slightly less obscure parts and newer multipler etc. I suppose then it isn't really a clone... Other intersting things are the output levels. Most modern VCO's are adjusted for 10V peak-to-peak on everything where as this is a lot lower and adjusted for more of an equal audible loudness. By that I mean that the sinewave is 1.8V p-p, the triangle is 1.4Vand the pulse 2.2V -- not sure if anybody else does that? Also seems odd that the pulse is so much more...