Theo Verelst Original Synth page
Original because is the synth I learned to play chords and scales
and not too complex songs on, which I built somewhere halfway
highschool (15 years or so ago). Still some decently valid
thinking in there. In fact it still works, crancking it up with
a stabelized 12V supply is all that is needed for most keys to
work (two or three have probably gathered up too much dust).
I want to remark here that until some time ago I didn't think
much about the machine, and not too many people from university
or music scene have seen it at all, let alone in action,
except for some people in the time I made it, simply because
starting from my purchase of a Korg Poly 800 around the
end of highschool, I gathered up a professional array of synth
equipment like a DX7, a DW8000, Rev-7, Rx-15, bit-one, tg500,
tx802, and some more, so I simply didn't feel much need for it.
Only lately I've been somewhat forced by the arising notion
that people in the graphics project I was in had such a high mind
about their own computer engineering capabilities, and
apearently a fairly low of mine to generate some serious
correction. And what is better than circuitry that actually
Apart from that I rediscovered the joy of electronics design,
and the fact that it is one of those areas where even today,
and that is what I doubted before, without much of an updated
view on part availability, it may pay extremely well to
construct one's own devices and circuitry.
The keyboard had a mixed analog digital disign, with parts associated
with on the one hand electronic organs, and on the other hand
with analog synthesizers. A top octave (2pow(1/12 spaced) divider
ic with high frequency clock generator and followed by
4 stage dividers per half note generate 5 octaves of fully
polyphonic square waves. A simple mixing rail gathers the
singals of the keys being pressed trough coupling resistors,
feeds them into an opamp based mixing amplifier annex
vca annex filter, which also receives an envelope triggered by
a mixing rail envelope follower , and which is fed through
another (adjustable but fixed) LP filter to the audio output.
Add an adjustable sine wave lfo, small range frequency
modulation input to the top oscilator and a link to the
vca voltage input, and voila, a 'hybrid' synth as they called
some quite expensive korg synth in the days following.
Would have loved to be able to design prophets 24bD/oct VCF's.
The mechanincs, as always in my home projects at the time where
based on wood, because I could process it, with a plastic
plated hardboard tilted front plate holding a set of pots
and switches I'd gotten from everything between old tv's
and dump stores, and a bar graph led display, one of the other
projects I could at some point afford the chip for.
I bought a keyboard for te incredible amount of Fl80,= (about
$40) when I could afford it, and was never sorry: they should
fit metal framed light but solid and non sticky keys like
that on modern synths. The DW8000 comes closest.
Unfortunately, the keyboard had to be fitted with contacts,
costing another guilder a piece, which for the 4 octave + 1 =
49 keys was too steep. I though a lot about it, and came
up with the following. Each key has a piece of fairly strong
and bendable hard-cored flatcable bent to form a long legged U
lying under it, with legs of about 5 cm, suspended from to rods
containing all these tricked springs kept at the right distance
and in place during construction by thick cellotape.
The springy ends have a little piece of silver wire soldered
unto it (keeping the remaining silver part hidden from tin
additives during the soldering), which rest against a global
mixing rail perpendicular to it, running over the length of
the keyboard, suspended by simple nails (...). The key
ends, after the turning point, in other words they go up
when a key is pressed, are fitted with a small piece of rubber
which presses the piece of silver wire down (away) from the
mixing rail in rest position. When a key is pressed, the
flatcable bends outward, the silver ended last part hits the
mixing rail with fixed strenght, and a note sounds. The nice
part being that no matter how hard a key is pressed, the
contacts never get bent, it took me quite some thinking and
experiments, but appearently I did my job well, most keys
still recently worked even without undusting. It has a funny
sound to it, a nice 'clang' when the mixing rail is hit.
Since the action is only a few millimeters, and the contact
force limited, the spring function is (as yet) not temporal.
The top clock generator is a standard 2 inverting cmos
(4032? or so) gate generator, with as extension a diode and
resistor in series couples with the 'hot' end of the capacitor,
to make it modulable in frequency, as ling as the oscilator
isn't forced out of operation, that is.
The top divider is a chip (undoubtedly Radio Shack, since
they had component characteristics and application notes on the
back of the package) with 12 dividers in the range of about
200 to 400 which frequency divide the top frequency into
the semitones of one octave.
The per semitone octave dividers are simply 4020 dual
4 stage asynchronous binary dividers.
The mix amplifier/vca/filter unit
I could add distortion unit, because the vca function is
achieved by using a FET in non-saturated mode, more or less
like this, from memory:
| | |
| | | | -------
Notes keys | |\ | | ----- | |
------ / | | \ | | ----- \ / |
>--| |-/ o| Mix | .5Vs|--|+ \ | | ------ |
------ | | ----- | >--------------| |------->
------ / | -------| |----|- / ------ | Audio
>--| |-/ o| | ----- | | / 741 | out
------ |------ ----| | |/ -----
------ / | | | | | -----
>--| |-/ o| ----- | | | | |
------ | | | | | | |
. | | | | | ---
... . | | |
in (appr 0-.5Vs)
Probably I'm forgetting some voltage converting R's and some
additional C's, and there may have been another buffer amp
as well. I'm sure there are more pots in the fet circuit as
well, to adjust non-linear distortion figures, or even make
the opamp clamp (nice on fifths), this is a long time ago,
and the time left playing I spent on stuff like this.
Unfortunately, I never realized I could make steeper filters
simply by cascading, and that resonance can be achived
by feeding back two or more sections.
envelope detector, adsr generator (d and r common rate)
low frequencey generator
Seriously thought through and feasible extensions