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

[>>>marjan<<<]

> hmmm, a couple people wrote saying this.  it's definitely possible that
> it's the battery slowly dying.  i should note i probably overstated the
> stability. i went away for 15-20 minutes came back and it was
> signifigantly differing in pitch; octaves in minutes was probably what
> you might call a fish story.  the battery was dead the next day.  in any
> case, what do i know about stability anyway?

My first synth was based around 555s, and in the start I had them
powered
from 9V batt (or two large 4.5V ones) but 555 has large current peak
while changing it's state, wich strucks battery very hard, and I think
no matter how new batt is it'll drop the voltage during transistion
and batt will recover from it in some finite time, so rail transients
affect the stability in large scale. You can make it better by
decouplong
rails with 10-100n ceramic cap near the chip plus to coonect pin 8 thru
100ohm res and 100uF cap to gnd, while using regulated power (at least
78xx or zener+transistor), battery is inconvinient here.
Chip itself shouldn't be worse performer than resistor used. Second
source of unstability is charging cap. First I used any I had, most
of the time ceramic (disc). Using poly showed to work better in overall
range (audio), but then I had no idea why...

I used to prefilter square out (both audio and lfo) with small
RC constant to tame a little those squares as they are wery sharp
and made all sorts of ringings and bleedthrus around in my system.
Use some opamp or transistor buffer if you use "triangle" out
from the cap (it's rather sort of expo triangle).


marjan