[sdiy] Buchla 266 fluctuating voltages

[Scott Stites]

Hi Peter,

> My good synth chum Cynthia was kind enough to donate a 5837 she had to a
> worthy cause (me), and I went ahead and built an 'inspired by page three
of
> the 266' module (uh-hum).  It works like a charm, but...

I didn't use the 5837 - it was a tad too unobtainium for me, and I was
a-feared it would come out sounding cyclical.  I've been dying to find out -
can you discern a 'heartbeat' using the chip?

I opted instead for a negative feedback transistor noise circuit driving a
comparator 0 to +15V, which I figure was about the output of the 5837 -
whether it is or not, the results have been excellent.

> Any of you thinking of making one might want to jot this down:
>
> You'll be much more pleased with the results if you replace the documented
> 100K pulldown at the output (after the diode) with a 220k to -15 instead.
> It will increase your voltage range by about 4+ volts, which tends to vary
> randomly only between 5 to about 7 without this.   And this range gets
even
> crummier (narrower) when you speed it up. This resistor mod will give you
> true random between about .5 to 7+ regardless of the speed and it rocks.

Other than the 5837 and the choice of counter, I've stuck to the original
Buchla component values, and I have had a totally random swing from 0 to
around +8V on the smooth output.  By coincidink, Jeff Pontius is coming over
tomorrow and we're going to dive into it - I'll double-check that figure,
but it's definitely enough to drive a VCO subsonic to ear-piercingly high
whether the 'Probability' control is full CW or CCW.

> I opted to not only bring the -3db, flat and -3 dB digital noise out to
the
> faceplate, but the noisey triangle wave as well, which is the heart of the
> fluctuating voltages and really really fun as a source of stepped random
via
> a sample and hold.

I've thrown the kitchen sink at mine - the noisy triangle was a thought - I
discounted it, but after your post, I'll re-think that.  I've actually
buffered the stepped output right after the 4016, and it is an output well
worth considering - I find myself using it all the time, actually.  I also
buffered the trigger firing the 4016 and again....use it all the time.
Another thing I've done is buffered the ramp voltage off the timing cap.
It's fairly handy to have around.  All of these outputs lend a certain
synchronicity to some Buchla madness.  There are other bell and whistle
modifications I'm considering.....

> I have one small snaafu yet to fix - tit seems the vactrol controlling the
> 556 r/c is such that the thing will crap out (stop oscillating) if you
bring
> the pot lower than about 2K in its range.   Simple matter of pulling out
my
> dog eared 555 cookbook to see what I've exceeded and I'll get to this
during
> the week sometime.

I deviated slightly from Buchla on this - I used a 7556 (easier on the
rails), but I've kept the same component values.  It runs from verrrryyyyyy
slow to pretty dang quick (50 Hz or so?).  I've never made it just stop.

> Cost of parts (sans the 5837) - 22 bucks and change at Mouser, including
> pots, knobs and jacks.  Quite a bargain.  I'll see if I can post a
soundclip
> in a couple of days.

On that, I'm always afraid to look.  If I want to know, I can always ask my
wife (she keeps a running count on what I spend on all this folly).  I've
pleaded with her not to tell me.........

Cheers,
Scott