Woodwind VCO

The Woodwind VCO is amazingly just a half size AMORE board. This picture shows a prototype.

I call this the Woodwind VCO because it is loosely based on woodwind instrument theory (see below). This doesn't mean it only can produce woodwind-like sounds though. The VCO is based on a Minimoog style sawtooth core. This feeds the woodwind waveshaper of my own design (see below), which produces three different waveforms. In addition, you have the sawtooth wave from the core for those warm and fat synth brass sounds. All four waveforms are selectable under voltage control and available on output 1. The sawtooth also has a separate output – it is always available on output 2. Amazingly, I was able to fit all this circuitry on a half size AMORE board. That of course means the circuit board is quite densely packed.
This VCO has surprisingly stable tuning with low drift, fast swing-in after switch-on and good temperature stability. The voltage controlled oscillator core is similar to the one used in later Minimoogs but utilizes today's op-amps for even better stability. The exponential converter uses a conventional NPN transistor pair and a tempco resistor that are potted together for best temperature stability. The woodwind waveshaper is the same circuit as used on the Advanced VCO but here you will not get all the other waveforms, no voltage controlled divider and no octave selector.

The Bergfotron Woodwind VCO has the following functions:

• Sawtooth wave plus three special waves (see text)

• Sync

• Linear FM

• Voltage controlled waveform selector

• Digitally controlled mute function

Compared to other VCO modules, the Woodwind VCO does NOT offer:

• Sine wave

• Conventional triangle wave

• LFO mode

• Octave selector

The woodwind waveshaper

The Woodwind VCO contains a waveshaper circuit of my own design that is mainly intended to mimic the behaviour of woodwind instruments. It is inspired by the vintage German wind synth Variophon. You can read all about the theory on the Variophon homepage.
According to the theory, the proper way to emulate a reed instrument is with triangular pulses where you can adjust the up and down slopes. For some reason, the Variophon used rectangular pulses instead. Probably because it was easier to implement in electronics back in the 70's. Because of this, I just had to test it the proper way, with triangular pulses. And in my circuit the pulse widths are even voltage controlled. To be able to emulate the Variophon, I added rectangular pulse too. And as an added bonus I even added a double pulse, where you can voltage control the width of the positive and negative pulses separately. 
The circuit basically works like a linear VC-AD envelope shaper. You could actually use it as just that (see my Dual VCAD module). Note that the pulse width is independent of the oscillator frequency, which is according to the reed instrument theory. This is where this circuit differs from ordinary pulse width modulation. If you set the up slope slower than the VCO frequency, the circuit will perform frequency division. This can be utilized to good effect when emulating wind instrument attacks. You can hear it in the last of the following sound clips.

Woodwind triangle Double pulse Single pulse Double pulse LFO modulation Double pulse+VCF resonance Sweeping PW up

Temperature stability

For additional temperature compensation the VCO uses a 1 kohm temperature compensation resistor. The ideal temperature coefficient for this resistor is supposed to be 3300 ppm. I used one with 3000 ppm temperature coefficient. This VCO uses the Bergfotron standard socket for the exponential converter transistor pair and temperature compensation resistor. Here you plug in a component carrier where you solder the transistor pair and tempco resistor of your choice. These should be in thermal contact and preferably potted together for lowest temperature drift. I used surface mount parts (dual transistor + tempco resistor) soldered to a small circuit board on the component carrier which plugs into the 8 pin IC socket. The parts were potted with Araldite. If you don't use surface mount parts, you can solder the dual transistor and tempco resistor directly on the circuit board. For practical reasons I recommend the solution with the component carrier even for these though.

This swing-in graph was measured by letting the VCO warm up for at least 1 hour, tuning it carefully, switching the power off for at least an hour, turning it back on and immediately start to measure the tuning (without re-tuning it). This procedure was repeated with the VCO tuned in four different octaves. As you can see, it starts almost spot on but goes slightly flat for a couple of minutes while the components warm up. This deviation is however very minor, especially at higher frequencies, and already after 10 minutes it is almost perfectly in tune. After warm-up there is essentially no drift. However, I have not been able to measure the stability over varying ambient temperatures yet.
These measurements were made with the AMORE Starter Kit and also include the warm-up and temperature stability of the AMORE power supply (which is very good).

The waveforms

Sawtooth

The sawtooth is the basic waveform that is produced by the VCO core. Measured on the oscilloscope at 1 kHz.

Woodwind triangle

A few of the different waveforms possible by adjusting the PW up and PW down knobs. As you can see, it can produce a normal triangle wave. But note that the waveform is frequency dependent unless you add the note CV to the PW up and PW down CVs.

Pulse

The pulse has a traditional shape but the pulse width has a fixed time, independent of VCO frequency.

Double pulse

The double pulse wave can produce different shapes, depending on the PW up and PW down CVs.

Bill of materials

You should have access to the parts in the general bill of materials.
In addition, you need the following less common parts:

8 pin DIL component carrier
1 kohm 3500 ppm/°C temperature compensation resistor (1)
Dual NPN transistor or a hand-matched transistor pair (1). The 2SC1583 is a good choice, if you can find it. I have used a SMD type transistor pair and SMD type tempco resistor mounted on a small circuit board on a 8 pin component carrier. Even if you use conventional parts, it is a good idea to mount them on a component carrier and cover them with heat insulating material. I used epoxy glue for this.
2N4391 or compatible FET. This must be a fast switching, low Rds-on type.
LF412 dual OP-amp (1). Other decently fast op-amps with low offset and drift can be used.
OP177 single precision op-amp (1). Other ultra-low drift op-amps can be used. This one doesn't have to be fast.

Matching

No hand-matching required if you use a matched transistor pair. You can match the PNP transistors for the expo converter in the woodwind waveshaper if you want. This isn't very important though.

Trimming

These are the alignment steps for the Woodwind VCO:

1. Adjust the supply voltages to exactly 15.0 and 10.0 volts (on your power supply). Note that this module uses the 10 V supply for the tuning-critical parts, so his has to be very stable. Using the AMORE power supply is recommended.

2. Adjust "scale" to get exactly 1 octave per volt on the KOV input.

3. Adjust the HF-trimpot to get exactly 1 octave per volt at higher frequencies (above 2 kHz).

4. Adjust "init freq." to the desired base frequency so the VCO is in tune with other VCOs when fed the same control voltage.

5. Set the PW up and PW down pots fully clockwise (shortest pulse width) and adjust the Offset trimmer so that the pulse width just reach the shortest possible.

6. Adjust the tracking trimpot so that each one volt increase of the pulse width CVs exactly halves the pulse width.

Skill level required: MEDIUM

This board is quite densely populated but does not require matching of components. For best temperature tracking you should glue the tempco resistor and the exponential converter transistors together.

VCO core schematics

Woodwind waveshaper schematics

Voltage controlled waveform selector schematics

Circuit board layout

Component placement

The component placement plan contains pairs of capital letters. These are supposed to be connected with insulated wires.