[sdiy] First DIY synth (555 osc.'s)

[Scott Gravenhorst]

IMHO, an easy and good way to use a 555 as a
musical VCO is the PAiA FatMan way.  Even
better if the 7555 is used instead.  It produces
a saw and a pulse waveform as is, although
the pulse wave is not routed to the mixer
in the FatMan unless modified to do so.

The oscillator in the datasheet which uses
pin 5 as a CV input has very limited pitch
range and may also cause saw amplitude variations
depending on pitch.  This is because the pin 5
input changes the threshold value to control
pitch instead of supplying a controlled current
to charge the cap (leaving the threshold constant).
The latter method (controlled current) is what
is employed in the PAiA circuit.

"Seb Carr" <star at all-carrs.demon.co.uk> wrote:
>I looked at the link to geocities, and the sample of the 555 osc is very 
>cool, but is there a schematic for it, or is it just the osc from the
>datasheet?
>
>Seb Carr
>
>----------
>>From: perpetual <perpetual at uswest.net>
>>To: synth-diy <synth-diy at node12b53.a2000.nl>
>>Subject: Re: [sdiy] First DIY synth (555 osc.'s)
>>Date: Mon, May 7, 2001, 5:37 pm
>>
>
>>
>>> > I wanted to build a bear bones system for more of a learning experience
>then
>>> > anything else.  I planed to build the VCO and ADSR from simple circuts I
>>> > found based around 555 timer chips and I was going to use CEM chips for any
>>> > or all of the VCA,VCF and LFO (if I included one).
>>>
>>> Huh? VCO with 555, but LFO with CEM chip?
>>>
>>> > I would use batterys in lue of a powersupply.
>>>
>>> If you do so, use a VCO that is unsensitive to power supply voltage. Or
>>> you'll have to re-tune all the time when the battery goes weak...
>>
>> so i was playing with a 555 oscillator last week.  more info here:
>> www.geocities.com/aurelialuz/shadow.htm
>>
>> what i've found is that all this talk about instability is true.  this
>> thing is really temperature sensitive.  and we're not talking like a
>> couple cents over an hour, i'm talking like octave flucuations from
>> minute to minute.  this thing is powered off a battery too, so i've got
>> both demons of instability working against me.  if i was really
>> interested in creating some sort of stable oscillator, i'd look
>> elsewhere.
>>
>> but lucky for me, i care not a whim for stability.  and what's really
>> cool about this thing is when you feed a wave output from another
>> oscillator into the CV input.  it becomes something like a sub-osc., but
>> instead of creating a low note, it gives all these weird overtones, sort
>> of like an "effects" oscillator.
>>
>> also, i've always read that 555's put out a pretty weak tone, but i put
>> a pretty big poly cap in it (.18uf) and i was suprised how thick it
>> sounds.  so if you go the 555 route, the bigger the cap the better.
>>
>> best,
>> alex


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