UralTone MiniSynth - Build Guide

UralTone MiniSynth is a compact analog synthesizer. At the core of the design of the building kit has been the idea of designing a good-sounding and relatively versatile instrument that also gives the builder joy and opportunities to get to know analog synthesizer technology.

The MiniSynth has elements from various inspiring synths: The one-oscillator synthesizer in a compact size is borrowed from the Roland MC202. The normalized signal path with the possibility to patch the signal and modulation routing to completely different is found on ARP 2600, while the modulation outputs selected in advance with switches are familiar from the Odyssey. The MiniSynth circuit uses new production versions of Curtis Electromusic's CEM circuits used in the 80s made by Alfa Rpar. The core circuit of the synthesizer is the AS3394, which original version has been used in e.g. Sequential Six Trak and Multitrak. This chip is a synthesizer in a chip consisting VCO-VCF-VCA. The envelope generator is 3310 and the LFO uses a 3360.

As a "home field" addition MiniSynth has overdrive circuit, which is similar to UralTone's Du-o-Fuzz. There are two parallel connected overdrives, one after the oscillator, the other at the output. This adds even more character and edge to the synthesizer.

UralTone MiniSynth rakennussarja - BOM

First, always check that all parts are included in the delivery. If you find any deficiencies, please get in touch with us by e-mail or via the "contact us" form on the pages. Please note that the components may look slightly different from the photos. Also, note that the voltages of the parts may be higher than what is written in the list. We try to keep the models of the components the same, but sometimes we have to change the manufacturer or model of a part due to poor availability. So, if a part is a slightly different color, don't panic; each component has the necessary values ​​marked on the side of the component. Read the notes on the part list carefully.

MiniSynth is built on single PCB which is attached to front panel and the panel is mounted to enclosure. The front panel is used as template for several times during the build as jig to ensure the panel mounted components are fitted straight. We recommend that you familiarize yourself with the construction instructions before assembling. This way, you will have idea of how it will be done.

MiniSynth pcb. The part designators and values of the parts are printed on the pcb.

The assembly instructions follows the low parts first -order, familiar from most UralTone kits. This makes work bit easier, as the parts are pressed against the surface of the pcb when the board is turned over for soldering.

Assembly starts with resistors. The series resistors are sorted into four different bags by size. In the picture above, resistors in the range 10ohm - 7.5kohm are installed. The resistors do not have a fixed installation direction, but according to good practice, they can be installed in the direction of reading, so that the color codes are in the same direction as the printing on the board. This makes it easier to read the part values.

Solder the values between 100k up to 4.7Mohm.

Finally solder the 0.1% tolerance resistors, These are marked with small circle symbol inside the resistor symbol on PCB. It's important these resistors are on tight tolerances for right tuning.

Solder the the diodes. These do have polarity: align the end of the case with stripe with markings on pcb. Note the D15 is Zener used on noise circuit and it looks similar to 1N4148 diodes. Don't mix this to others.

After the diodes solder the IC sockets. These have half round piece cut of on the other end. This marks the end for the pin 1 for the IC to be fitted. Align the socket according the pcb silkscreen.

Solder the ceramic and polyester capacitors. These doesn't have polarity.

Note: The C73 is missing on the picture. Solder this as well.

Solder the electrolytic capacitors on the pcb. These are polarised so right orientation needs to take care of. The stipe printed on the side of the capacitor marks the negative pin. The corresponding pin is as well shorted. On the pcb the negative terminal is marked with white half circle.

Note: C71 and C72 are not used.

Solder all three pin TO92 case components, transistors and regulators. The shape of the case (round with beveled side) is printed on pcb. Insert the parts relatively deep. The free height between pcb and panel is only 12mm.

Solder the coil L1 and trimmers RV25 and RV27.

Bend the pins of the DC-DC converter like on the picture on 90° angle.

Solder the module on the pcb.

Note: The left side module is not used.

Now all the components which are soldered only to pcb are done. On following steps the panel mount components are assembled. This is done with using the front panel as mounting guide to ensure the potentiometers and others are soldered straight.

Press the potentiometers and toggle switches in the pads on the pcb. Do not solder these yet.

Note: Toggle switches SW5 and SW12 has two throws / positions, the rest havee three. Do not mix these.

Press the front panel on its place and fasten the nuts on potentiometers and switches installed on previous stage. Hand tightening is enough to make parts to be fish with the panel.

Finally solder all of these.

Remove the font panel and solder the slide potentiometers. You can use the panel to check the slider is centered before soldering the potentiometer on it's all pins.

Push the rotary switches on the pads. Do not solder yet. Bend the small guide tab down on and place the plastic washer on the axle.

Press the panel and fasten the nuts of the rotary switches to set the mounting height right.

Insert the 3.5mm jacks and DC jack. There's total of 20pcs of mono jacks and two stereo ones. Again, do not solder these yet. Place the panel and mount it by using few nuts on upper row of potentiometers / rotary switches. Turn the board upside down and let the jacks to drop down so they are facing the panel. It's good idea to press these down one by one when soldering the pins.

Note: Stereo jacks are somewhat shorter so it' important these are secured properly to panel.

Solder the ground pin of the stereojacks by using the cut legs of the resistors.

Note: DC jack and stereo jacks do have relatively tall solder lugs. Cut these after soldering the parts.

Insert the leds on the pads. Note the orientation: the negative pin is marked by cut side of the package and the pin is shorted. On pcb the cut side is marked as well as the pad is square.

Press the front panel back and let the leds drop through the holes on the panel. Solder the pins.

As final task of soldering the parts on front panel are the multi turn trimmers. Before soldering these can be preadjusted to closer to right values than factory set value. This is not necessary, but this may help a bit with calibration, when the initial setting are more closer to right ones.

The values for these are marked on schematics and the given value is either between the pins 1-2 or 2-3.

Note: The value of the trimmer is given as format xxy where the xx states the two first numbers of the value and the y amount of zeros following. For example 503 is 50 000 = 50kohm.

Soldering the trimmers follows the same idea as previously. Insert the trimmers on the pads and install the front panel. Turn the synth upside down and guide the trimmer set screws through the holes on front panel. When all are set use few nuts to secure the panel. Solder the trimmers.

Note: This is bit tricky part so it may be easier to do this in few trimmers at the time instead of all.

The synth is ready for testing. Before connecting the power, check all the components are neatly installed and there's no uncut or long pins on solder side.

When all is fine, plug in the power supply. Measure the voltage between U3 pin 11 and ground. The voltage should be approx -12V. Any jack except the midi in is usable for ground connection for the meter. If the voltage is missing, switch off the power immediately and start fault finding.

Confirm approx 12V between U3 pin 7 and ground.

Confirm 5V on pin 24 of U24.

Adjust the voltage on pin 3 of U4 to -6.5V with the trimmer RV29.

Insert all ICs on their sockets. Switch on the the power. Confirm the -6.5V set on previous stage and readjust if needed. LFO-leds should start to flicker by the set speed of the rate potentiometers. Connect the CV-Gate source (Key CV in, Gate In) and headphones or other audio source for monitoring. The tuning is off now, but it's possible to test the features through. If some part of the synth is not working, switch off the power and check the circuit.

For calibration CV source with good accuracy is needed. Measure the output of the keyboard or whatever from different pitches. If the volts / octave ratio is off, the calibration will be off too when the values will compensate the error. For using the particular CV source this doesn't matter, but the compatibility with other CV sources won't work right.

Initial settings:

  • Dial all potentiometers to zero, except CUT off full open. On mixer the BALANCE full counter clockwise. VOLUME as desired.
  • Turn the OCTAVE to lowest setting, full counter clockwise.
  • Set the TUNING exactly to middle.
  • Set WAVEFORM to saw or square.
  • Set all modulation toggle switches to mid position.
  • Set the VCA MODE middle to DRONE.

Calibration:

  1. Make sure the CV voltage fed in is 0V. Turn the OSC RANGE trimmer as far as the pitch is most low but in audio range. The keyboard used is assumed to have 0V at low C (later C0). If the source is 0V at low F, use F0 - F2 etc instead of C0 - F2.
  2. Play the note at C2. Adjust the OSC RANGE until the tune is at C. If you don't have tuning meter use a instrument of know to have proper tuning as reference.
  3. Press the note at C0 and adjust the TUNE SCALE for tuning the pitch to C.
  4. Return to line 2 (play C2 and adjust OSC RANGE). Repeat steps 2 and 3 until the tuning is spot on on both C0 and C2.
    Note: if the C0 pitch is difficult to hear properly, use C1 - C3. The important thing is use two or more octaves. Wider scale makes scale setting more easy.
  5. Turn the OCTAVE switch to clockwise to highest octave and adjust the OCT TUNE.
  6. Select the WAVEFORM as square wave. Adjust the PW OFFSET until the square wave is 50% wide. This can be looked with oscilloscope or just by ear. The 50% square wave sounds "hollow". Turn the PW potetiometer to get the idea or turn the trimmer. The mid point is relatively easy to hear.
  7. Insert a patch cable to AUDIO IN connector and adjust the BALANCE to full clockwise. Leave the other end of the patch cable unconnected. This cuts the audio signal feed to VCF. Turn the middle MOD CV toggle switch to KEY CV position and MOD AMT fully clockwise. Turn RESONANCE until the filter starts to oscillate. Turn the CUT OFF on suitable pitch. Unconnect the CV source or play C0.
  8. Turn the FILT RANGE trimmer to position where the CUT OFF turned to CCW sets the frequency heard so low it's almost impossible to hear as well as SUT OFF set on clockwise the pitch is above heard audio range. This sets the range of the CUT OFF to cover the audio bandwidth.
  9. Connect the CV source back and play C3, Set the CUT OFF on middle or where the corresponding heard frequency is C.
  10. Press C1 and turn the FILT SCALE until C two octaves down is in right tune.
  11. Press C3 and turn the FILT RANGE until C is in right tune.
  12. Repeat steps 10-11 as needed. This procedure is same as tuning the VCO setting the filter tracking 1V/oct.
  13. As final step the amount of distortion is adjusted. This is mostly subjective matter and these may be set later through the front panel holes. When trimmer pointer is set to eight o clock (These are turned on pcb, so the actual value would be 2 o clock) the amount of distortion is usable maximum. Anything more will saturate the circuit.

The calibration can be done with front panel installed. It's however recommended to do this initially when the circuit is exposed so in case of need of fixing the circuit, the task of removing the panel can be avoided.

Calibration can be done with different tools. Oscilloscope can be used to monitor the waveform at output. Scaling the bottom tone C0 as one cycle per screen width is four cycles on two octaves up at C2. With frequency counter the frequencies are multiples per octave. Tuning with ear is also valid option. Using another synth or known to be properly tuned source, the task is relatively easy specially if using synth as both source for audio / pitch reference and CV source.

When the MiniSynth is tuned, install the front panel and the knobs and insert the synth into metal chassis. Fasten the screws on corners and fit the feet on the bottom. Synth is ready for usage!

We hope fun times with the new made synth!


About the MIDI

MiniSynth PCB has facilities for on board midi to cv conversion. Pinout on PCB is same as Midi-Hardware MiniCV adapter. It's possible to use other converters too by using the same pads. The circuit has separate 5V feed for converter. The CV in resistance is 100k with 1V/oct scale.