[sdiy] Favorite VCO options

[David G. Dixon]

(Very long post)

Interesting list of features, Damian:

> Here are some basic features:
> 1. DC- and AC-coupled expo FM
> (AC coupled for feedback)

I normally consign AC coupling caps to switches on the panel, because it's
so easy to do.  This is probably the one concession I make to using panel
components.  Hence, the board would just take whichever FM signal would be
coming in, and the option for having this would be completely up to the
builder.

> 2. DC- and AC-coupled linear FM

Ditto.

> 3. a simple scheme how to change the range. It's best if it can be
> done on a daughter-board of some sort, so that it's easy to set up
> your own amount of ranges, which ever ones you need.

The coarse tuning ranges are determined by resistors from +15V and -15V
going out to the legs of the coarse tuning pot.  I typically take the lower
leg directly to -15V to get the minimum frequency for LFO purposes.  I've
been putting 33k to 39k between the upper leg and +15V to limit the upper
frequency to somewhere between 30 and 40kHz.  I don't like a bunch of
inaudible headroom on my coarse tuning pot.  The input resistor is 100k
(standard for CV summing).

The fine tuning pot is wired directly to the rails, and the range is
determined by the input resistor.  I use 3.3M to get about one octave
overall, although it is nice to be able to get tighter control with 10M.
Some would choose a vernier for this function, but I'm not that finicky.

> 4. a document that explains how to change the core to get a certain
> frequency at 0V. Better yet, a formula on the silkscreen :-)

The reference frequency is determined by a single resistor: the one which
limits current into the 2164.  That one's easy!

> 5. terminals with direct core output, to use your own shapers. Shaper
> on daughterboard for modularity: let's be hones with eachother,
> finding the right shaper will be the next step and it won't be as
> easy!

I've already got all the shapers on the board, and I'm very happy with them.
The feed to all of them is the output saw, not the raw integrator output.
The key is the 5V reference from REF02, which keeps all the levels very
tight and ensures nice waveshapes.

> 6. I know you like small pcbs, but don't make the design too cramped.
> I'm good with the soldering iron, but beginners will really appreciate
> it. On the other hand please allow components (physically) bigger than
> you use. This also means chip sockets, big capacitors, and thick
> (coaxial) cables (please include ground at the same spot the cable
> would be going out)

I'll probably have to get someone else to do a proper layout.  I haven't
bothered to learn how.  My PCBs are very easy to stuff and solder, as
everything is laid out on a strict 0.1" grid with no angled traces, but they
are probably too idiosyncratic for most people.

> 7. Mounting areas on the PCB for brackets/screws on all corners, with
> extra space on the 'front' side to make the area of contact between
> the bracket and the pcb a bit larger than necessary. For people who
> don't want to use potentiometers as a way of mechanically mounting a
> pcb.

That wouldn't be an option on my current board.  All board-to-panel
connections are done via 0.1" MTA connectors.  This makes panel wiring a
breeze.  I always mount my boards at 90 degrees with stooge brackets.

> 8. sync in, sync out

The circuit has variable sync in.  Sync out is the saw output (?).

> 9. rather than having terminals that you'll solder cables to all over
> the place, try to have them clumped up so that it's easier to route
> the cables and maybe even make a pin connector to simply disconnect
> all cables at once. Yes, that can take some jumpers, but it's worth
> it, and if someone doesn't want to use jumpers, they can just use the
> hole where the jumper would start.

I love connectors.

> 10. two-wire signaling. This thing will be quite a bit complex, no
> reason not to carry ground around with the signal.

Not sure what you mean here.  It would actually be a snap to bring the
signals out with a ground wire next to them, but I don't really think it is
necessary.  

> 11. specify the power consumption after you build it

Probably won't bother...  If it's more than 20mA, I'd be very surprised.

> 12. Make sure to mention me on the PCB ;-) very important ;-)

Is that "cheater" or "Mr. Cheater"?

> Here are some of my specific requests which I think would/could be useful:
> 
> 1. the ability to correct frequency via diode corrections on multiple
> frequencies. Maybe you could put in space for 4 stages; whether
> they'll be used or not doesn't hurt, they're usually not that big as I
> understand.

Not gonna happen...  If tracking is within +/-0.05% in the musical range,
why would you need to fix it?

> 2. This one is a very big one for me. Direct input into the comparator
> reference voltage. To connect it to your usual reference use a jumper,
> or a trace that can be cut with *two* through-holes on each side of
> the place you'd cut at. You could use one pair of terminals and take
> it through a normalized jack; this jack could be very very interesting
> I think :-) Better yet if we can switch between 'normal reset
> voltage'/'sum of normal voltage +
> jack_input*amount'/'jack_input*amount' (actually the middle one is the
> most interesting, but the last one gives the most control). For
> example using a DC voltage from somewhere else far away on the modular
> could result in usual 'vintage' drifty behavior.
> Why two holes on each side? This way if someone cuts the trace to
> experiment they can later put in a jumper without having to cleanly
> desolder the holes already used for the wires.

On my current PCB, there's a 390R resistor between the reference and the
comparator, for reasons having to do with the sync circuit.  One could jack
in here.  However, I'd rather recommend feeding noise to Linear FM for this
purpose, which would have exactly the same effect and is already accomodated
in the existing circuit.

> 3. core on daughterboard. Terminals only carry current input (from
> expo and mixing stage for linear FM) and vco output (to route to
> shapers etc)
> 
> This would open a lot of possibilities:
> a) using different cores. You can use one 'main board' and then if you
> want to try out a different core you just desolder the old core and
> add the new one. You could experiment with saw, triangle, trapezoid,
> multicore, with soft sync, without soft sync, hard sync or not, etc...

Why bother?  You can easily shape the saw into all those other forms (and
the VCO already does all except trapezoid, but this is easy to do with a
very small off-board variable clipping circuit for the triangle output), and
the VCO already has variable sync with a panel pot.

> b) using multiple cores. Instead of using a core in the spot the
> daughterboard goes, the user could use a 'splitter board'. That
> splitter board would have multiple spaces for cores, as well as
> current mirrors for each core, and a mixer for each of the outputs.
> I'm not saying make that board, but make it possible to use one
> cleanly.
> 
> c) modularity in design makes it easier for people who know nothing
> about electronics, like me, to understand what's going on, and it
> makes it easier to debug (oscillator doesn't work? try swapping the
> daughterboards around)
> 
> d) smaller pcbs could be easier/cheaper to produce, I don't know.
> 
> e) you could use the same expo for multiple oscillators at different
> ranges, a sort of 'driver/slave' design like in the moog modular

Well, that's a completely different VCO you're talking about now.  The
problem is that the expo part of the circuit contains most of the panel
connections, so the limitation is really in panel real estate.  The entire
expo + core circuit could be done on a PCB about 2" square!

I always assumed that the Moog slaves were just synced.  I didn't realize
they used common current sources.

> 4. start phase (for sync)

Not sure what you mean.  You can control the starting point of the wave by
syncing to a gate signal.

> 5. soft sync

Already there.

> 6. linear FM offset potentiometer
> 7. exponential FM offset potentiometer

Not sure what you mean.  The 1V/oct input provides offset for Expo FM (via
the keyboard, typically), and Linear FM causes fluctuations in the reference
current, so offsetting this would be the same as shifting the reference
current up or down, which might be useful in its own right, but probably as
an on-board trimmer rather than a panel pot.

> 8. CV scaling (it is useful to have this separate for each VCO) - to
> make octaves larger or smaller than 1V

Why?

> 9. glide; you can put as a slew limiter *after* the expo, that's why i
> put it here.

All you need to do is glide the CV.  There's no need to mess with the actual
current (?).

> 10. your favorite thing here :)

Well, two switches I always add to my VCO panels are "centre/egde PWM" and
"sine/sigmoid".  Both switches simply send either triangle or saw to the
pulse and sine shapers, respectively.  It's dead easy, and actually adds a
lot of functionality, particularly the sigmoid, which has very interesting
properties when fed to waveshapers.

> I didn't include any requests for the shaper, because that's a
> completely different pair of boots altogether :)

Well, beyond the normal "pure" waveshapes, I think that outboard shaping
modules are the way to go.

> I wonder what favorite features other people have :-)

Gawd, I'm almost afraid to find out now!  :)