[sdiy] Integral scope on VCO

[Thomas Dunker]

 Hi,

 I meant to send a message to the list last night on the subject of X-Y
scope fun and some ideas of mine, but I seem to have sent it to some
listmember only. Anyway, there was some embarrassingly faulty reasoning in
there so I might as well give it another go.

 I am still working on my first modular synth, using mainly RJ Wilson's
designs, but I find myself coming up with some ideas and mods to give it a
more presonal touch. Just before Easter I had completed a dual VCO, so now
there's a triple LFO, a 24dB/oct. lowpass VCF, a state variable filter and
a dual LFO to play around with - plus a lot of old signal generators, the
Juno-1, the Korg Polysix and other audio sources I have in the
workshop/lab. It sure is fun to see (hear) the number of possible sounds
multiply as more modules are added.

 A few days before the scope art / spiral waveform discussion began to
happen here I decided to probe into my signals with a scope in X-Y mode,
and ended up staring at swirling spirals for at least an hour.
 I soon decided that I'd go for the ultimate power-on indicator for my
synth: A miniature scope integrated with the VCO.

 I have these super cute miniature (28mm (or about 1.1") screen diameter)
scope CRTs (Philips DH3-91) in my tube stash, and figured that THIS is
definitely the place to use them. Not only as an X-Y scope, but I'm
scheming a way to sync the X (sweep) to the VCO frequency so that the VCO
waveform can be displayed as a stable image without the need for big time scope
circuitry.
 I'll have to experiment a bit with this, maybe implement a simple trigger
circuit and beam blanking... It gets a bit interesting, since the CRT
needs a 500V focus voltage and the deflection amp outputs must be
elevated to 500V. The PSU for the whole CRT and deflection amps has to be
at least 800V, or more like 1000V, with multiple 'taps'. It'll get tight
in there, with transformers and HV capacitors etc.

 The VCO I'll be using has a sync output from which I hope to generate sub
octave squares (for audio output) and also further subharmonics from which
to generate a sweep saw for the scope.

 To make this really interesting I'm thinking about including a mixer on
the VCO module so that waveforms (tri/saw/square/pulse/sine + sub octaves)
can be composed (and studied on the scope in real time, complete with PWM
etc.) before being passed on to other modules for further processing.

 The tube fanatic in me also thinks it would be cool to add a touch of
additive synthesis by adding a couple of triode stages (with typical
gain/drive controls) that can add "tube harmonics" to the sine coming from
the VCO. By overdriving only one triode, one gets the characteristic
asymmetric triode distortion: Dominating even order harmonics, evenly
falling spectrum with increasing order. (Aka. "tube sound") The second
triode would get a phase inverted signal so if it is similarly overdriven it
would be able to approach "squarish" (symmetric) clipping, but much softer
than a real square wave. Or anything in between "clean" or "tube clipped"
 This would give lots of control over the harmonics, and
along with the waveform mixer a virtually infinite variety of waveforms
will be available from the VCO module. Ideally I want the mixer to be able
to split two waveform mixes into separate channels (pan pot on each
waveform) so that these could be routed to separate VCFs, VCAs or whatever,
to get the maximum richness from a one-VCO synth. But I am running out of front
panel space... (Excited about those concentric dual pots...)

 For the tube gain/distortion I will use either some subminiature military
low voltage UHF triodes I have or some of the car radio line of
low voltage tubes so I can run them off the +/- 12V supply with no hassle.

 The tube stages would have to be modularized so that the tube harmonics
can optionally be added after VCFs etc.
 I plan a dedicated panning mixer module, which I could just as well put some
overdriveable triodes into. Hmm...

 But I quickly run out of front panel space, so it's a major puzzle. Wish
me good luck!

Thomas Dunker

Trondheim, Norway