YuSynth VCO Module Bare LM394 PCB

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YuSynth LM394 VCO VCF


A bare double sided through hole plated PCB (no need for the wire jumper links shown on the PCB layout) 100x75mm for the Yusynth Steiner VCO synth module uisng the LM394. These PCBs are manufactured by Soundtronics with a percentage of the sale going to the creator of the YuSynth - Yves Usson. These are early days for the YuSynth Modular Synth at Soundtronics, our plan is to stock PCBs for all of the projects as well as components, kits and front panels. This is going to take time but will eventually be as comprehensive as our MFOS range.

No components are included with the PCB but check out our Synth Components section where you should find what you need. We do suggest visiting the YuSynth Steiner VCO project page for detailed information including Yves panel layouts but a summary is shown below.


The PCB is designed to use 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.

This VCO provides also an input for SYNCing, on the PCB Yves provided two different inputs, one for SOFT SYNCH mode where the slave VCO is allowed to lock on the any harmonics of the master VCO, and the HARD SYNCH mode where the slave VCO is forced to lock on the fondamental of the master VCO.

The PCB can accept a choice of three types of power connector, it can be either a Synthesizers.com 6 pins MTA connector, a MOTM 4 pins MTA96 connector or a 10-pin Eurorack connector.
The module was designed to work with a +15V/0V/-15V PSU but can be used with different components on a +12V/0V/-12V PSU. The parts list identifies the component changes requires plus there is different PCB layout for the 12v & 15V versions but both use the same PCB. See thumbnail images above.


Click on the schematic thumbnail above for the circuit diagram but note the component values are for the 15V version only, see the BOM below for the 12V values..


The wiring schematic image in the above thumbnails show the panel wiring to the pots and sockets etc.

Wiring option : the SYNC IN jack may be either connected to the HARD SYNCH or the SOFT SYNCH connector depending on your taste. You may even add a switch on the front panel in order to select between the two hard synch modes.

Design Note:

Yves received a lot of emails concerning the voltage regulators used in the circuit. Here is an explaination for the reason why they are there in relation to the +/-15V version but the explanation also applies to the 12V version which uses the 78L12 & 79L12.

FAQs :

  • Are these two regulators useful here considering I am already using a regulated +/-15V ?
  • Considering that the 78L15 and 79L15 require an overhead of 3V to give the right voltage why are these for in this circuit ? Could not one just suppress them ?

The answer to these two questions is :
The aim of using these regulators is not to provide +15V and -15V as one would think first. As a matter of fact, because of the lack of voltage overhead and the low current draw they will rather deliver something like 14.2V and -14.2V. This is not a design error but was done on purpose : they are here to isolate the power supply circuit of the VCO core from the general power supply while providing a reference voltage of +/-14.2V. This helps very much in increasing the stability of the VCO.
As such, all the circuit of the VCO core was designed considering the voltage drop through the regulators, and it is a very important point. As such, if one would like to remove/bypass these regulators, one would need to recalculate many resistor values of the VCO core to have it working as expected.

In conclusion, they are there for a good reason!

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.

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.


  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 .


  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).


  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. 

Parts List

The parts list below is direct from the YuSynth website. Please note that Yves has identifed some components for use with the 12V version and for the 15V version. Our kits include components for both the 12v and 15V versions as standard. Simply do not use the parts for the version you do not want.

The parts list excludes knobs although we have standardised on the Cliff KM20B but it does include 1/4" jack sockets. All parts are available individually (use the part number in the search box above) or as a components kit that excludes sockets and knobs.


  value quantity
U1 78L15 low power positive regulator (please read the DESIGN NOTE above)
78L12 for 12V version
U2 79L15 low power negative regulator (please read the DESIGN NOTE above)
79L12 for 12V version
U3 OPA2137 precision dual OPA (or TL072 for small budget) 1
U4 LM394 or SSM2210 or MAT02 or MAT12 for PCB 1
A modern alternative is the LS312
2SC1583 for PCB2 - not yet available from Soundtronics
2 x hand matched BC547B for PCB3 - not yet available from Soundtronics
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 or equivalent
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%    (R15 must be changed to 100K when using a SSM2210 instead of a LM394) 6
R36,R51 12k 5% for 15V version
or 7.5K for 12V version
R46 15k 5% 1
R11,R48 22k 5% 2
R9,R20 33k 5% 2
R10 47k 5% 1
R32,R49 50k 1% 2
100k 5%, *1%, note that R23 may need to be replaced by a 82k resistor
** R3, R4 and R5 are 1% but must be matched to 0.1% for best tracking
***R50 100k for 15V version, 70k for 12V version
R43,R44 150k 2
R34 180k 1
R41 200k 1% 1
R26 220k 1% 2
R7 270k 1% 1
R8,R52 470K 2
R13,R19,R53 1M 3
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 polyester film 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)
If you choose to use a SSM2210 instead of a LM394, the value of R15 (10K) must be increased to 100K.

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