Update : dec. 3rd, 2007

VCO
En français
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Description

Should modular DIYing compare to a meal, the VCO is the main course ! It is the heart of the system, the module without which a system cannot be called a Music Synthesizer.

The design of such a module is not easy and building one with good accuracy, low drift and good thermal stability is not simple. I have tried various architectures and eventually came up with quite a classical design.

Herein I propose, three different PCB designs that let the builder choose the solution that matches best his/her needs or budget.
  • The first design uses high grade components such as LM394 or SSM2210 chips for the dual transistor of the exponential converter and an OPA2137 low bias OPA for the voltage summer and servo OPA.
  • The second design uses a Japanese 2SC1583 dual transistor which is much cheaper.
  • The third design uses a pair of hand matched BC547B.

Recently, I have improved a few points in the circuit, thanks to Mariano who pointed at some difficulties with the previous design, namely Hi-Track linearity and PWM. Here is the new schematics and PCBs which integrates the mods proposed by Mariano. The Hi-track mod is based on a circuit by Ian Fritz (see ref. at the end of this page)

Schematics




Printed circuit boards and component layout
LM394 version

PCB design


Component layout




 2SC1583 version

PCB design


Component layout
Matched BC547s

PCB design


Component layout
Download the schematic as a PDF file


Download the PCB as a PDF file (LM394 version )

Download the PCB as a PDF file (2SC1583 version )

Download the PCB as a PDF file (BC547 version )



WARNING ! The document is formatted to be printed directly on a mylar for photo-etching or a "press & peel" paper. Make sure that when the printed face of mylar is in contact with the copper side of the PCB, the lettering can be read normally.

For those who have built the VCO with the old version of the PCB, here is how to modify it in order to integrate the new features
HI-track mod of the old PCB

First change the value of C8 to 220pF 1% (stiroflex or silver mica).
Then install the new 1N4148 diode and the new 1M resistor (R53) as shown below

Track side
Old PCB before mod, under the LM394.
 Drill a hole midway from the pad going to pin 5, and the track going to pins 3 and 6 of the LM394.
 The diode (ring side) must be soldered in the new hole. Bend the leg of the diode and solder it to the track going to pins 3 and 6 of the LM394. Solder the new 1M resistor (R53) in the pad going to pin 4.

Component side
Old PCB before mod, under the LM394. Before installing the tempco resistor.
 The tempco resistor is soldered to pin 4 and 5 of the LM394. The diode and the 1M resistor (R53) are soldered together forming a bridge.
Supression of the old HI-TRACK trimmer
Suppress the HI-Track trimmer (leftmost)
 Connect a jumper in the extreme trimmer holes
The PWM mod of the old PCB


Install the 470K resistor (R52) on the track side, one leg connected the track going to pin 10 of U4, the other leg connected to the ground track.
Remove the resistors R36,R51 and replace them with two 12K resistor. Remove R34 and replace it with a 180K resistor.


Components and building details
reference
 value
 quantity
U1
 78L15 low power positive regulator (15V) 1
U2
 79L15 low power negative regulator (15V) 1
U3
 OPA2137 precision dual OPA (or TL072 for small budget) 1
U4
 LM394 or SSM2210 or MAT02 for PCB 1
2SC1583 for PCB2
2 x hand matched BC547B for PCB3 		1
U5
 LM311N fast comparator 1
U6,U7
 TL074 quad FET OPA
 2
D1,D2,D3,D4,D5*,D6*
 1N4148, * matched
 6
Q1
 J112 or J111
 1
R12
 1K tempco resistor (PTC 3000ppm)
TYCO ELEC. model LT300014T261K0J or LT300016T261K0J at www.farnell.com
 1
R1,R2
 10 5% 1
R39,R40,R42,R47
 1k 5%
4
R45
 1.5k 5%
 1
R38 4.7k 5%
 1
R25
 5.6k 5%
 1
R15,R17,R18,R27,R28,R37
 10k 5% 6
R36,R51 12k 5%
 2
R46
 15k 5%
 1
R11,R48
 22k 5%
 2
R9,R20
 33k 5%
 2
R10
 47k 5%
 1
R8,R32,R49
 50k 1%
 3
R3*,R4*,R5*,R14,R16,R22,R23,R24,R27,
R29*,R30*,R31*,R33,R50
 100k 5%, *1%, note that R23 may need to be replaced by a 82k resistor
 14
R43,R44
 150k
 2
R34
 180k
 1
R41
 200k 1%
 1
R26
 220k 1%
 2
R7
 270k 1%
 1
R52
 470K
 1
R13,R19,R53
 1M
 2
R6
 3.3M
 1
C10
 10p ceramic
 1
C9,C11,C12
 47p ceramic 3
C7
 100p ceramic 1
C8
 220pF or 270pF 1% styroflex or silver mica
 1
C3,C4,C5,C6
 100n yellow LLC
 5
C1,C2
 22µF/35 V polarized
 2
T3
 10k 10 turn trimmer 1
T2
 22k or 25k 10 turn trimmer 1
T4
 47k or 50k 10 turn trimmer 1
T1
 200k or 220k 10 turn trimmer 1
P1...P6
 22K ou 25K lin potentiometer
 6
Jk1...J10
 jack socket
 10
IMPORTANT NOTE : for C8 use a high grade low temperature drift capacitor (styroflex or silver mica)
Thermal coupling
Thermal coupling between the tempco resistor and the transistor pair of the exponential converter is an important condition for insuring a good stability of the VCO. The following pictures show how to install these critical components on the PCB.


Coupling LM394 and R12
The tempco resistor R12 is not installed on the PCB. It is directly soldered to pins 4 and 5 of the LM394. These pins are connected internally and then can be used to connect R12. The tempco resistor is directly glued on top of the LM394. Thermal grease must be used to improve thermal coupling.

Coupling 2SC1583 and R12
Bend the 2SC1583 legs as shown. Install and solder the 2SC1583 first. Then add some thermal grease on the 2SC1583. Then install and solder the tempco resistor in bridge over the 2SC1583 such that there is a close contact between the body of the resistor and the 2SC1583.


(Photo, curtosy of  Csaba Zvekan)
Coupling the BC547s and R12

Glue the transistors together by their flat side. Solder them to the PCB. Bend the legs of the tempco R12 such that the body of R12 can  touch the transistors. Apply thermal grease on the body of the transistors and solder R12. Press the body of R12 to make a tight contact with the transistors.

Proposed modification of the tempco section
Mariano from  Dallas, Texas has moded the VCO in order to use an easy to source NTC resistor as a replacement for the PTC tempco resistor. This mod is described as well as other mods in a PDF file located at the end of this page in the Gallery section. Thanks Mariano !
Wiring



Front panel
Panel design


 Download the silkscreen mask as a PDF file

Download the silkscreen mask as a  JPEG file



Setting and trimming
For setting and trimming of the VCO, you need a good Oscilloscope, a good source of control voltage (calibrated CV/GATE 5 octave keyboard) a good voltmeter (at least 4000 pts), a digital tuner (I am using a Korg CA tuner) and/or a good frequency counter.

WAVESHAPE CONVERTER SETTING

  1. Before powering up, set all the trimmers to their mid-range position, set the FREQUENCY pot to 5 (mid-range position), set the FINE TUNE pot to 0 (mid-range position), set the LIN FM LEVEL, EXP FM LEVEL, PWM potentiometers to min (counter clockwise position), and the PW potentiometer at 0 (mid-range position).
  2. Connect the SAWTOOTH output to the oscilloscope (2V per square). Power-up the circuit. You should observe a sawtooth signal. Adjust the T3 trimmer in order to balance the sawtooth around the 0V level. The sawtooth should ramp down from +5V to -5V.
  3. Connect the TRIANGLE output to the oscilloscope (2V per square). Adjust the T3 trimmer to refine the triangle shape. Then adjust the T4 trimmer in order to balance the triangle around the 0V level. The triangle should ramp down from +5V to -5V.
  4. Connect the SINEWAVE output to the oscilloscope (2V per square). Adjust the T4 trimmer to refine the sinewave shape. Then adjust the T4 trimmer in order to balance the sinewave around the 0V level. The triangle should ramp down from +4V to -4V. If necessary refine the setting of T3 .

V/OCT TRACKING
  1. Connect the calibrated voltage source to one of the 1V/octave input. Set the CV to 0V. Connect the tuner / freq.counter to the SINEWAVE output. Adjust the FREQUENCY potentiometer such that the tuner displays A1 (55 Hz)
  2. Set the CV to 1V, check the pitch/freq, ideally we want it to be A2 or 110Hz. If the pitch is what we expected go to point 4. Otherwise, if the pitch is lower than expected : adjust the T2 trimmer (V/OCT) in order to lower the pitch/frequency  (NOTE this may not seem logical to lower the picth when it's already too low but that's the way it works !).  Conversely if the pitch was higher than expected, adjust the T2 trimmer in order to increase the picth/frequency.
  3. Set the CV to 0V, the picth is either lower or higher than A1/55Hz . Adjust the FREQUENCY potentiometer such that the tuner displays A1/55 Hz. Repeat points 2 and 3 until there is a perfect one octave shift when switching the CV from 0V to 1V.
  4. Repeat the same procedures but with the following CVs 0V and 2V in order to read A1/55Hz for 0V and A3/220Hz for 2V
  5. Apply the same procedure as above for the next voltages/octaves up to A6(1760Hz)/A7(3520Hz). Depending on your component tolerance you may expect also to achieve a good tracking up to A8 (7040Hz).

TUNING
  1. Disconnect the CV source from the V/OCT input.
  2. Turn the FREQUENCY potentiometer to 0 (fully counter-clockwise position)
  3. Turn the FINE TUNE potentiometer to 0 (mid-range position)
  4. Adjust T1 in order to read a frequency of 16.2 Hz
  5. Plug a CV/GATE keyboard (starting at C, and giving 0V for lower C)
  6. Press A3 key, and check that the pitch is A3 / 220Hz. If not  adjust T1. 
Waveshapes - vertical scale : 2V/square, 0V on the middle horizontal line

sawtooth  1000Hz
sawtooth  20kHz

triangle 1000Hz
triangle 20kHz

sinewave 1000Hz
sinewave 20kHz

50% pulse 1000Hz
50% pulse 20kHz



References

Understanding TEMPCO
Ian Fritz Sawtooth VCO



The DIY builders' gallery
Here are the photographs of the yusynth MiniMoog VCF modules built by other synth geeks around the world.
Thank you  guys for sending me these nice photos.
   



In this PDF file Mariano gives some hints on how to add a range switch and moding the VCO in order to use regular NTC instead of the PTC tempco resistor.



Name : Mariano
Pseudo : Dhar2kma
 Modular project :
Location : Dallas, Texas, USA
Website :  		Name : Czaba ZVEKAN
Modular project : 
Location : Basel, Switzerland
Website : 
Name  : Patrick
Pseudo : Baronrouge
Modular project: JHC live lab
Location Toulon, France
Web site : http://myspace.com/patjhc


Name : Julien
Modular project : 
Location :
Website :



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