[sdiy] top octave gneration

[Scott Gravenhorst]

When the 555 is operated using a simple resistor to charge
the capacitor, it will not be linear because the charge
curve is exponential.  So for a musical VCO, a current 
source is used to charge it instead, forcing the ramp to 
be linear.  This makes the oscillator's response to an 
input CV linear as well.  In the fatman, this current 
source (a voltage to current converter) is a circuit 
composed of an opamp and a darlington  pair.  Each 
oscillator has it's own current source driven by a 
common control voltage.  The summing node of the opamps
then receive different currents depending on the setting
of the offset control, but they stay proportional, allowing
the VCOs to track.

If I put that incorrectly, someone please correct me.
 
Not being an EE, I don't know if a more simple setup 
could be used, say a transistor who's bias is fixed by a 
resistor which could be variable (i.e. the tuning pot).  
That would bring it down to an extra transistor per timer.  

EE heavies?

adam at hoodmusic.net wrote:
>didn't think about that, did all this thinking at 4:30am so i was not the most
>awake, maybe try doing it like the fatman VCO, i seem to rememer it going about
>voltage control through some other setup other then pin 5. I do not remember exactly
>what that was though, nor if it would actualy solve this problem, but they have 2
>gang tunned so why not 12 :)
>
>adam
>
>Scott Gravenhorst wrote:
>
>> adam at hoodmusic.net wrote:
>> >Sure you could tune on the fly, use the CV input on the 555s, it would give you
>> >a tune control for each note of the top octave and one for the whole top octave.
>> >it would double the parts count, but be well worth the added control.
>>
>> That thought crossed my mind as well, but has anyone actually
>> tried 'gang tuning' 555 timers this way?  Do they track?  I
>> assume you mean to use pin 5 (as it is on the single timer).
>> This doesn't have a very large range and because it changes
>> the threshold value, it will also change the amplitude of the
>> sawtooth output.
>>
>> =========================================================
>> - Government: The other religion.
>> - The media's credibility should always be questioned.
>> - Lambs who lie down with lions are lunch.
>>
>> -- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
>> -- Linux Rex         | RedWebMail by RedStarWare
>> -- FatMan: home1.GTE.NET/res0658s/TekLab_FatMan_WebSite/index.html
>

=========================================================  
- Government: The other religion.
- The media's credibility should always be questioned.
- Lambs who lie down with lions are lunch.

-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
-- Linux Rex         | RedWebMail by RedStarWare
-- FatMan: home1.GTE.NET/res0658s/TekLab_FatMan_WebSite/index.html