Complex VCO
The front panel
is divided in three parts: To the left the modulation generator, to
the right
the main oscillator and in the middle, at the
bottom, the modulation routing section.
With this module I wanted to break away from the
"standard" synth oscillator, which tends to produce the
albeit classic, but also clichéed analog synth sound. My
experiences with the previous Bergfotron modules hinted that you
probably need different raw waveforms to get really different, but
still musical sounds. The module does produce the traditional
waveforms, but in addition to this, a selection of more unusual
waveforms. For even more vareity, you can select two waveforms at
once and either add or subtract them in the desired proportion. You
could for instance subtract the sine from another waveform, to remove
the fundamental. There are so many possibilities, that the module is
almost a synth within the synth.
The module is
inspired by the Buchla 259 Complex Waveform Generator. It is not a
clone of the Buchla oscillator however. Only a small part of the
circuitry is similar to Buchla's and my VCO have many features that
you won't find on the Buchla 259.
Just as on the Buchla, my module
consists of two oscillarors, one called modulation oscillator and the
other called main oscillator (Buchla call it "principal
oscillator" for some reason). The modulation oscillator is not
only usable for modulation. It can also be used as an independent VCO
in it's own right.
Both oscillators are equipped with my octave
quantizer, which make it easy to tune the oscillators to eight
different octaves or the corresponding fifths. When the modulation
generator is set to the LFO range, the octave quantizer is
automatically switched off.
Like on the Buchla 259, there are
toggle switches to route the modulation generator to different
destinations. On my VCO, these switches are three way. In the third
position, one of two avaliable modulation inputs is routed to the
actual destination. The destinations are divided in two groups of
three, with one input for each group. So you can have a maximum of
three modulators, the modulation generator and two external. But as
you can only select two waveforms at a time, it wouldn't make sense
to modulate everything at the same time. If I really want three
external modulations, there is a jack that replaces the modulation
generator with an external signal. Useful if you are using the
modulation generator as a second audio VCO.'
The modulation
generator is routed through a VCA, so you can modulate the modulation
index. This VCA is Vactrol-based, to avoid that any offset-problems
could affect the tuning. This can be a problem with OTA-based VCAs.
This Vactrol is the only one in the module, so I also view it as a
tribute to Buchla, who seemingly loved Vactrols. At least that's the
impression you get, as his modules are often sprinkled with Vactrols
(The 259 is no exception).
The waveforms
My main oscillator have no less than nine
different waveforms. Four of these have voltage controlled parameters
for modulating the waveshape, so the range of waveforms the module
can produce is vast.
In addition to that, you can select two
waveforms at a time and crossfade between them. Of course the
crossfade is voltage-controlled. By the way, the waveform selection
can be voltage controlled too.
Sine
This is an ordinary sinewave. It uses my tried and trusted triangle to sinewave converter, that was inspired by the one in the Moog 921B. Because it is fed by a purer triangle wave, the sine is cleaner than on my old VCO, which has a sawtooth core.
Triangle
The triangle is the basic waveform that is produced by the VCO core.
Sawtooth
With the triangle and square wave from the core, a 4066, an op-amp and some resistors form a nice sawtooth wave.
Spike
This is the squarewave, processed by a fixed high pass filter. The circuit is copied from the Buchla 208 Programmable Sound Source (and Music Easel). As the filter is fixed, the waveform, and therefore overtone content, varies with frequency. Note that this waveform does not appear on the Buchla 259.
Odd/even
With this waveform you can crossfade
between a wave that only contains odd harmonics and one that contains
only even. The Buchla 259 has a similar function, but my
implementation is totally different.
The odd harmonic waveform is
simply a square wave. The even harmonic waveform is a sawtooth with
double frequency, mixed with a sinewave in the proportions 1:1,27
(the picture at the top left).
Double pulse
This waveform was pioneered by Ian Fritz. It is derived from the triangle wave. You can modulate the pulse width and in addition to that, you can crossfade between an up-going pulse and a down-going, that is phase-shifted 180 degrees. So this waveform has two voltage controlled parameters. The circuit for generating this waveform is taken from Ian Fritz' website. You can read more, and view the schematics there.
Pulse
This is an ordinary pulse wave, that is derived from the sawtooth. As usual, the pulse width can be modulated. Note that you have both sawtooth- and triangle-derived pulse waves at your disposal.
Cut saw
This waveform is a spin-off effect of the fact that I had one unused 4066-section.With it, three op-amps and a couple of resistors, I generate this wave, that is a sawtooth which have had a variable (voltage controlled) portion in the middle cut out. This is similar to ordinary pulse width modulation, but with narrow pulse width the fundamental of this wave is weaker than the lower harmonics. This is never the case with classic pulse width modulation, where the fundamental is always the strongest, eventhough the lower harmonics are almost as strong at narrow pulse widths.
Double frequency
saw
This is a normal sawtooth that has twice the frequency of the other waveforms. It can be useful when mixing with another waveform. It is really a spin-off, as I originally needed it for the odd harmonic waveform (see above). It is generated from basically the same circuit as the ordinary sawtooth. But with different biasing, it produces two "sawteeth" for each period of the triangle wave.
The waveform selector have the following positions:
wave A
wave B
1: sine
sine
2: triangle
triangle
3: sawtooth
sawtooth
4: spike
spike
5: odd/even
odd/even
6: double pulse double pulse
7:
pulse
2F saw
8: cut saw
modulation generator
The setting "modulation generator"
means that the two generators are connected in parallel, and works
like an ordinary two-oscillator synth. You can of course use the
crossfade CV for crossfading between the two oscillators. A separate
three-position toggle switch selects between sine, triangle or
sawtooth for the modulation generator.
As you can see, you cannot
select 2F saw and modulation generator at the same time. But as these
are different oscillators, you could use normal sawtooth and just
tune up one oscillator one octave.
You also can't use pulse and
cut saw at the same time. Well, you can't get everything, can you?
The cut saw was added as an afterthought so I'm happy that I have it
at all!
Mixed waveforms
Here are some examples of selecting two different waveforms, and mixing them with the help of the built-in crossfade VCA.
To build the Complex VCO module
This is a rather complicated circuit and
it requires that some components are carefully matched. You should
not attempt it, if you don't have access to an oscilloscope and a 4
1/2 digit DMM.
The circuits are built up on three separare printed
circuit boards. Board one contains the oscillator cores and the
octave quantizers for both oscillators. Board two contains all
waveshapers and the waveform selector for the main oscillator. Board
three contains the waveshapers and VCA for the modulation oscillator
and the crossfade VCA.
Board 1, containing two VCO cores and two octave quantizers.
Board 2, containing the complex waveshapers for the main oscillator.
Board 3, containing the simple waveshapers for the modulation generator and the crossfade VCA. Note that these are my prototype baords. Several changes have been made to the final board layouts, to correct errors or improve performance.
Schematics
Circuit board layouts
There were several errors on my prototype boards, which needed to be corrected. Instead I replaced it with the new Advanced VCO, which has mostly the same functions and is much easier to build. Just add the Basic VCO as a modulation generator (or second sound sorce) and the Advanced VCO replaces the Complex VCO.
Matching
In
the VCO core, the two 5.1 kiloohm resistors between the emitters of
the PNP transistor pair and the positive supply must be very
carefully matched. Buy a belt of 100 resistors and find two that give
the exact same reading on a 4 1/2 digit ohmmeter. Any error here will
give a non-symmetrical triangle wave. The same effect will be evident
if the PNP taransistor pair is not matched well enough. You can
adjust small errors with the 20k trimmer.
If the trimmer doesn't
have enough range to get a perfect triangle on the
oscilloscope, either the resistors or the transistor pair must be
replaced with something better matched. You can try to swap the
resistors over. This will either make the error worse or better. If
you are lucky, the matching error in the resistors will cancel that
of the transistor pair.
For PNP transistor pair a MAT03 is
excellent, but unfortunately very expensive. Hand-matching two
plain-vanilla BC560 can work just as well. The only drawback is that
both transistors need to be kept at exactly the same temperature,
otherwise the triangle symmetry will be affected at low
frequencies.
In the octave quantizer there are five 20k resistors
and three 100k resistors that need to be matched. Use the same
procedure as above. Here the requirements for matching aren't quite
as high as for the VCO core. The matching is done to guarantee that
the octave steps are exactly in tune.
Here we see the tempco/dual NPN module (bottom right) and the PNP module with two matched BC560C (top right). Inbetween, there are two BC550C, which are matched also. I'm not sure that is necessary though (but it doesn't hurt either..).
Alignment
These are the alignment steps for the waveshapers:
1. Adjust the supply voltages to exactly 15.0 volts (on your power supply).
2. Adjust the trimmer "saw shape" on the complex waveshaper board, to get a nice and smooth sawtooth, without any step in the middle.
3. Look at the double pulse output with the scope. Dial in a symmetric and very narrow pulse. Adjust "triangle offset" on the complex waveshaper board, until the positive and negative pulse have the exact same width. This will also eliminate DC offset on the triangle wave.
4. Adjust "sine sym" and "sine shape" while listening to the sine output. You will hear when you have adjusted them for the cleanest sine wave. This adjustment is to be carried out on both the complex waveshaper and the modulation waveshaper board.
5. Adjust all other offset trimmers for minimum DC offset on the waveform in question. You can see the offset, if you alternately swith the scope between AC and DC operation. When the trace no more moves vertically while doing so, the adjustment is correct
