VCO Analog

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VCO Analog

By reading a news article that covers the SuperBooth Berlin, there was a trend during the edition 2018, the return to physical synthesizers. Hardware, to speak, that is light years away from software that now reign in the study of each manufacturer.

The German festival, now in its third edition, and increasingly focused on tools and strange things related to the world of electronic music and the production in general, He has brought to the fore the added value of real buttons, true knobs, real sliders and, generally, pleasure to touch a synthesizer, ie a device able to shape sounds from scratch.

For those not familiar with anything of the matter, Suffice to say that there is no production Shot, today, where there is not at least one synthetic particle. Whether it's a bass sound, a rug (pad), a lead or simulation of real instruments, in particular guitars, pianos and arches, there is almost always half a synthesizer.

After a few years passed between only bit-made synthesizers, during which hardware models have represented a niche market helped as some control interface, here sanctioned the return to more traditional synth. Or better, traditional in the form, but innovative in substance. And enjoy emphasize how Italy, in this case, face the lion's share.

The interest of foreign press, to say, He went to UNO IK Multimedia.

The company Modena, in collaboration with other Italian Soundmachines, He has developed this very promising synthesizer, that makes the compact and aggressive price its main strengths. But without forgetting of technological equipment at. Let's talk a little box of wonders, just great 26 x 15 x 5 cm, and the weight of 300 grams, equipped with keyboard tuch 27 buttons, arpeggiatore, sequencer, two oscillators, VCO, an LFO, 100 Predefined sounds 80 overwritten and the chance to make excellent analog sounds, especially low and lead. All at 244 EUR including VAT, from July.

By reading the features I was inspired by the publication of an exponential VCO

The exponential VCO

How to create the synthesizer 1V / Eighth old school.

The music of today is mainly made of software to burn traces, add tools, mix songs, manipulating sounds and more, but how can they use to make music?

The music was done entirely with REAL instruments like guitars and pianos, but also synthetic sounds made their mark in the music scene.

These synthesizers (unlike most of the ones that you will find today) were all analog, which means that, instead of relying on software and processors, the sound generation is carried out by manipulating the electrical signals in the time domain.

How does an old synthesizer?

In this project, I will speak in depth of the heart of the synthesizer, the VCO (voltage controlled oscillator), that acquires analog voltages and generate the raw sounds ready to be further processed by filters, modulators, ADSR modules and sequences of steps.

Type of synthesizer and music theory

The synthesizer designed is known as synthesizer 1V / Eighth. This means that for every increase of 1 V input, the output frequency will increase by an octave (that is, by a factor 2).

Now that this module functions properly, It needs an exponential converter on input. This converter takes a linear voltage and produces an exponential voltage that is fed into the VCO. Why do we need an exponential converter? The answer is in the nature of human hearing and music theory!

If you take a plane and you play the note A central (A4), It is emitted a specific tone with a frequency of 440Hz. If now you sound the note A to the right of this (12 note your, A5) the note sounds equal with the exception of a higher tone and has a frequency of 880Hz. (The bottom note is a harmonic of the upper note and that's why they sound good when played together). Now, if you play the next note to the right (A6), the note sounds more acute as the previous note A; It has a frequency of 1760Hz.

Every two identical notes separated by 12 keys is called an octave. For every two distant keys octave, the top button will have a double frequency compared to the first. The reason for this is because the nature of human hearing is logarithmic. This means that if something sounds twice as loud, its amplitude (or frequency in the real field) It has to increase by a factor of two.

HE, eg, Increase the frequency of a waveform from 1 Hz a 2 Hz, this would be considered a higher octave according to the human ear. But the rise of a frequency of a waveform from 440Hz to 441Hz is not determined an octave shift. In fact, the human ear would not be able to distinguish between these two frequencies that the human ear is good at changes in relative rather than absolute changes.

Then, with all this complicated theory, we need to find a method for incorporating a linear voltage source and convert it to a voltage source that produces exponential voltages. To do this we will use a component that has intrinsic qualities exponential, the bipolar junction transistor or BJT.

The exponential converter

So we need a circuit to absorb a linear voltage from the keyboard / controller and produce a voltage that doubles exponential value for each octave.

Since our VCO operates over a single power line 5 V, the output of the converter must be between 0 V e 5 V. Such input range of 5 V, It offers the ability to use a keyboard 5 octaves with a total of 60 keys.

The following table shows the input voltage from the keyboard and the output voltage required by the converter.

 

 

Key

Key #

1v ottava V

Output Expo

Frequency

C0

1

0,0833

0,1655

65,4078

C # 0

2

0,1667

0,1754

69,2971

D0

3

0,2500

0,1858

73,4177

D # 0

4

0,3333

0,1969

77,7834

E0

5

0,4167

0,2086

82,4086

F0

6

0.5000

0,2210

87,3089

F # 0

7

0,5833

0,2341

92,5005

G0

8

0,6667

0,2480

98,0009

G # 0

9

0.7500

0,2628

103.8284

A0

10

0,8333

0,2784

110.0023

A # 0

11

0,9167

0,2950

116.5434

B0

12

1.0000

0,3125

123.4734

C1

13

1,0833

0,3311

130.8155

C # 1

14

1,1667

0,3508

138.5942

Dl

15

1.2500

0,3716

146.8355

D # 1

16

1.3333

0.3937

155.5668

E1

17

1,4167

0,4171

164.8172

F1

18

1.5000

0,4419

174.6178

F # 1

19

1,5833

0,4682

185.0011

G1

20

1,6667

0,4961

196.0018

G # 1

21

1,7500

0,5256

207.6567

A1

22

1,8333

0,5568

220.0046

A # 1

23

1,9167

0,5899

233.0868

B1

24

2.0000

0.6250

246.9468

C2

25

2,0833

0,6622

261.6311

C # 2

26

2,1667

0.7015

277.1885

D2

27

2,2500

0,7433

293.6709

D # 2

28

2,3333

0,7875

311.1335

E2

29

2,4167

0,8343

329.6345

F2

30

2.5000

0,8839

349.2356

F # 2

31

2,5833

0,9364

370.0022

G2

32

2,6667

0,9921

392.0037

G # 2

33

2,7500

1,0511

415.3134

A2

34

2,8333

1,1136

440.0092

A # 2

35

2,9167

1,1798

466.1736

B2

36

3.0000

1.2500

493.8937

C3

37

3,0833

1,3243

523.2621

C # 3

38

3,1667

1.4031

554.3769

D3

39

3,2500

1,4865

587.3419

D # 3

40

3,3333

1,5749

622.2670

E3

41

3,4167

1,6685

659.2690

F3

42

3.5000

1,7678

698.4711

F # 3

43

3,5833

1,8729

740.0044

G3

44

3,6667

1,9843

784.0073

G # 3

45

3.7500

2,1022

830.6268

A3

46

3,8333

2,2272

880.0185

A # 3

47

3,9167

2,3597

932.3471

B3

48

4.0000

2.5000

987.7874

C4

49

4,0833

2,6487

1046.5242

C # 4

50

4,1667

2,8062

1108.7538

D4

51

4,2500

2,9730

1174.6838

D # 4

52

4,3333

3,1498

1244.5341

E4

53

4,4167

3,3371

1318.5379

F4

54

4.5000

3,5355

1396.9423

F # 4

55

4,5833

3,7458

1480.0088

G4

56

4,6667

3,9685

1568.0147

G # 4

57

4,7500

4,2045

1661.2537

A4

58

4,8333

4,4545

1760.0370

A # 4

59

4,9167

4,7194

1864.6942

B4

60

5.0000

5.0000

1975.5747

The component that will be used for its exponential property is a BJT. Most du you know the equation that connects the base current to the collector current, but this relationship is linear.

The equation that connects the base-emitter voltage to collector current is exponential:

Dove

  • Ic – Collector current
  • Is – Current saturation
  • q – Upload electron
  • Vbe – base-emitter voltage
  • k – Boltzmann constant
  • T – Temperature (in kelvin)

Below is the complete diagram of the VCO to convert the linear input voltage in a frequency

in the scheme, shown above, There are three different inputs that feed the U1.2. You can add additional resistors 100 K for more inputs, but generally three should be sufficient.

  • KEY – This is the input from the musical keyboard
  • TUNE – This is connected to a trimmer that can be used to make small adjustments in the output frequency (adding a small amount of voltage)
  • LFO – Low Frequency Oscillator – This can be used to add effects like UFO, police sirens or even arpeggios

Here is a simple explanation of how this circuit works:

  • U1.2 is used to add the individual inputs (KEY, TUNE e LFO) and scale the input voltage so that 1V -18mV produce output (note that it is an inverting configuration).
  • Q1 and Q2 form a differential pair.
  • U2.2 is used to maintain a constant current through Q1. The changes in the base voltage of Q1 lead to corresponding changes in the base-emitter voltage of Q2 and, Consequently, in exponential changes in the collector current of Q2.
  • Q1 and Q2 SHOULD HAVE A VERY SIMILAR HFE!
  • U1.1 is a current-voltage converter (R9 and R10 are chosen in such a way that when the input voltage is 5 V, also the output voltage is 5 V).

The VCO

Now it is time to actually create an oscillator controlled by a voltage source.

This oscillator has four sections:

  • supplements (U5.1)
  • Reversal of the Schmitt trigger (U5.2)
  • reset circuit (Q3)
  • Buffer (U3.1 and U3.2)

supplements (U5.1)

The integrator output will do two things, depending on the state Q3 (and the presence of an input voltage) :

  • If Q3 is off, C12 will charge and then the integrator output will gradually decrease.
  • If Q3 is active, C12 will drain and then the integrator output will gradually increase.
  • The speed with which the output rises or falls is determined by C12, R9, R10, R11, R12 and the input voltage.
  • The greater this voltage, the faster the C2 charges.

Schmitt Trigger (U3B)

The Schmitt trigger will do two things depending on the output of the integrator:

  • If the integrator output exceeds the upper threshold, the Schmitt trigger output will be 0V.
  • If the integrator output exceeds the lower threshold, the Schmitt trigger output will be 5V.

Emergency operation (Q3)

The Q3 reset circuit will do two things depending on the output of the Schmitt trigger:

  • If the trigger output is high (5 V), Q3 will be turned on.
  • If the trigger output is low (0 V), Q3 will be off.

As swings

The circuit oscillates in the model listed below:

  1. Q3 is turned on and then the integrator output increases.
  2. The integrator output finally crosses the upper threshold of the Schmitt trigger.
  3. The output of the Schmitt trigger now passes to 0V.
  4. Q3 is now turned off and then the integrator output begins to decrease.
  5. The integrator output at the end falls below the lower threshold of the Schmitt trigger.
  6. The output of the Schmitt trigger now passes to 5V.
  7. Q3 now lights (then returns to step 1).

Construction

During construction, There are two things to keep in mind:

  • Make sure that Q1 and Q2 have hFE values ​​close (within 10 each other). The best method is to use a multimeter that can measure the hFE.
  • Q1 and Q2 must be thermally bonded with each other so that the thermal change in one reflects the thermal change in the other, in the print you can be seen with the bodies touching.

For those who wish to test the operation below I have prepared an easily replicable scheme to the simulator for each choice to fully understand and because of the value of certain components.

Between the simulator and the final realization I made some changes dictated by the fact that the simulator uses Ideal operational and practice behaviors are slightly different.

Probably setting the simulator with real operational you could do better, me enough already get to this point with the simulator, The rest of the work I have done with a mount oscilloscope completed, but most was done and it was enough to change the value of a few components to arrive at the best result

Amilcare Greetings

VOTE
1 reply
  1. Avatar
    theremino says:

    that longing. Not having dual transistor (for the exponential converter), I am doing everything possible to keep them at the same temperature, uniting them with a copper tube and inserting them in the polystyrene. But it is difficult to keep in tune these gizmos.

    I have seen far more complex schemes, unnecessarily complex, but this is a really well thought out scheme. E’ simple but has everything you need to run well.

    Approvals

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