[sdiy] Favorite VCO options

[cheater cheater]

David,

On Tue, Apr 13, 2010 at 18:14, David G. Dixon <dixon at interchange.ubc.ca> wrote:
>> Well done! :-) Now it's time to think about features on the PCB.
>
> What sort of features?

Ain't that a question :)

Here are some basic features:
1. DC- and AC-coupled expo FM
(AC coupled for feedback)
2. DC- and AC-coupled linear FM
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.
4. a document that explains how to change the core to get a certain
frequency at 0V. Better yet, a formula on the silkscreen :-)
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!
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)
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.
8. sync in, sync out
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.
10. two-wire signaling. This thing will be quite a bit complex, no
reason not to carry ground around with the signal.
11. specify the power consumption after you build it
12. Make sure to mention me on the PCB ;-) very important ;-)


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.
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.

So something like this:

(ref voltage source)
|
o--o (terminals)
|
| (cut here if needed)
|
o--o (terminals)
|
(ref voltage input into comparator)

An extra hole doesn't cost a lot..


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...

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

4. start phase (for sync)

5. soft sync

6. linear FM offset potentiometer

7. exponential FM offset potentiometer

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

9. glide; you can put as a slew limiter *after* the expo, that's why i
put it here.
10. your favorite thing here :)

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

I wonder what favorite features other people have :-)

Cheers
D.

On Tue, Apr 13, 2010 at 18:14, David G. Dixon <dixon at interchange.ubc.ca> wrote:
>> David,
>> that's great news!
>
> Thanks!
>
>> so what's the reason it worked on the breadboard?
>
> No idea!
>
>> Where is that filter located?
>
> It's just a 100pF cap from the comparator output/JFET gate (same trace) to
> ground.  It is essential.  100pF may be slightly too large, but 47pF is too
> small and 120pF is definitely too large.  I'm going to get some inbetween
> values today to try, but 100pF works.
>
>> 18 octaves is more than you'd ever want, I think. I've never composed
>> a synth part which spanned 18 octaves.
>
> I agree.  However, the tradeoff is in good audio tracking up to or slightly
> beyond the top of the audio range vs. the option for long LFO periods at the
> bottom.
>
>> Well done! :-) Now it's time to think about features on the PCB.
>
> What sort of features?
>
>