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 need to be corrected. On one board there need to be ten additional diodes. Making room for them is goig to be difficult, as the board was quite packed to begin with. I'll try to update the board layouts, and publish them here, but they're not done yet. If you want to build this module, you're advised to check back here later, for the availability of the board designs.
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
This is the front panel with just the jacks and two thumbwheels mounted.
The subpanel for mounting the pots and switches is made from a blank fibreglass circuit board.
This is the subpanel seen from the other side.
Here the subpanel is in place.
And seen from the other side.
The wiring that is local to the panel has been completed.
All boards "folded out" on the finished module. The wires must be long enough to allow the boards to fold out like this. Otherwise it will be impossible to service the module.
The completed module seen from the top.
The completed module seen from behind.
The completed module seen from the side.
