[sdiy] 4069 VCO to get started? My answer

[Scott Gravenhorst]

If "LS" is short for loud speaker, it "kills" the signal because it's
resistance (or impedance) is very low and draws more current than your
output circuit is capable of providing.  Your Oscope has an impedance
(input impedance) of a million ohms as opposed to your LS which is
probably 8 ohms.  A loudspeaker should be connected to a circuit like a
VCO only through an appropriate amplifier.  You can build such an
amplifier, it doesn't have to be commercial, but you do need an amplifier.

It also sounds like you need to take some time to do a bit of reading. 
The 4069UB VCO already IS simple.  And it's about as simple as it gets,
unless you really want something that is crap.  I can make an oscillator
with 3 parts, but it won't be musically useful - so I don't bother.  

The 4069UB VCO is composed of 3 basic circuits blocks that you should
strive to understand, those are an integrator, a schmitt trigger and a
current source.  There are many books, one being "The Art of Electronics"
(which was probably already mentioned).  Building things will give you
only a certain level of understanding, but not full understanding.  If I
were you, I would start reading about things like basic opamp principles,
ohm's law, transistor basics, etc.  Integrators are used in both VCOs and
VCFs, also envelope generators and other things.  Schmitt triggers are
also everywhere in analog music devices.  These things need to be
understood as building blocks before you'll understand how a VCO or other
module works.

And again, a 4069UB inverter gate IS AN ANALOG DEVICE.  ALL DIGITAL GATES
ARE ANALOG DEVICES.  PERIOD.  They operate with high gain and are
nonlinear, but they are still analog.  The 4069UB inverter is just an
amplifier made of 2 transistors.  Don't think of this circuit as a
bastardized approach to using "digital" parts, that won't help you.




Nicolai Czempin <nicolai.czempin at alcatel.de> wrote:
>Scott Gravenhorst wrote:
>
>>Richard Wentk <richard at skydancer.com> wrote:
>>
>>> Laurie Biddulph wrote:
>>
>[..Snip...]
>
>All this info is very interesting, and it sounds really clever to use 
>something meant for digital in an analogue way. However, it does nothing 
>to make it simpler.
>Perhaps it is simpler to *build*, but not simpler to *understand*. I 
>should have been clearer on that point, I guess.
>In the meantime, I have built a basic VCO, with lots of limitations. At 
>this stage there was enough debugging before I got any signal at all, 
>and it's very exciting to finally get something on the scope. I even 
>managed to get something to the loudspeaker, by simply connecting the 
>saw out to a transistor.
>For reasons I haven't fully understood yet, simply putting the LS where 
>the scope used to be "kills the signal". My workaround did produce some 
>sound on the speaker, and I was able to vary its frequency (just using a 
>pot connected to +V and the input line. However, the output signal 
>seemed to die after a while, not sure why. Disconnecting the LS, waiting 
>for a few seconds and then reconnecting got the signal back, only for it 
>to die again. Looking forward to understanding exactly why all this is 
>happening.
>
>In the meantime, I guess I need to finally dig out the soldering iron 
>and build that little (pre-)amplifier kit I have.
>

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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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