[sdiy] very simple vco

[René Schmitz]

Hi Scott and Mike,

>>>But as pointed out in another posting...it is the expo converter that
>>>makes the VCO's complicated.

No, merely makes understanding its function more difficult. The work for 
soldering it together is about the same.

This seems to be based on the thinking that once you eliminate the expo 
converter the temperature problems would be gone. This isn't necessarily 
true! Linear current sources can have similar problems, and the CCO 
cores can exhibit drift on their own. Oscillator design is always a 
delicate matter.

>>Keep in mind, Luca asked about "a simple, almost primordial VCO", not a
>>"musically useful" VCO (or what the more advanced members of this community
>>would consider "useful").

I think 9+ octaves qualifies as being "musically useful". :-P

Ok, its not the most stable VCO with regard to drift. But not really bad 
either.

>>I was in the same situation when I set out to build my suitcase synth
>>project.  I had wanted to get started with DIY for years, but was daunted by
>>the apparent complexity of most of the circuits.  (I later found that they
>>weren't as complicated as I thought ; )

Exactly.

>>I would highly recommend to someone in Luca's position to go ahead & build a
>>simple linear VCO to play around with... I've had great fun with mine!  just
>>the experience of building a circuit that made noise has spurred me on to
>>want to build more advanced circuits!  while there are certainly limitations
>>to the linear vs. exponential design, none of that really matters until you
>>build more modules anyway... ; )

That of course depends on what you want to do. IMHO going for expo 
control for a beginner makes sense. You can transpose using voltages. 
Something that is difficult with v/Hz. You will find more designs (i.e. 
without haveing to modify them), you can build a v/oct keyboard that you 
can later use with other stuff that is exponential. Most MIDI2CV 
interfaces are v/oct as well. So you protect your investments if you 
later go for a modular. (Keep your MIDI2CV, Keyboard...)

> True enough and in most cases, if not all, an expo converter can be
> retrofitted to any linear VCO and vice versa.  I use linear anyway, and many
> circuits I've seen have an expo converter...  I needed to replace the expo
> converter with a linear V-I circuit.

Anyway, we are talking about two transistors a bunch of resistors and 3 
NTCs here, that do make the whole expo convertor. Including temperature 
compensation. In comparision a linear current sink is more complicated.

I know that you were just doing what I said above about protecting your 
investments, the sensible thing for you to do is to replace the expo 
convertor because of the V/Hz nature of your FatMan.

> Personally, I think René Schmitz's VCO4069 is about as simple as it gets. 
> And very inexpensive! (Luca said he's a student, so this may matter).  It's a
> VCO which is quite musically accurate and has a very wide range, I got 9
> octaves and stopped testing, others have seen it do several more than 9.

No objection. :-)

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159