Category Archives: VCO

Waveshaper: Saw to spaced, square and pulse

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For wave shaping from saw to spaced saw I kept the original circuitry from the Elektor Formant. I only have to change some part values due to the different signal level.

 

Waveshaper: Saw to spaced saw

Waveshaper: Saw to spaced saw

New added is the saw to square waveshaper because I found it usefull to have a symmetrical square output.

Waveshaper: Saw to Square

Waveshaper: Saw to Square

The saw to pulse shaping is quite different to the original but quite standard.

Waveshaper: Saw to Pulse

Waveshaper: Saw to Pulse

Oscilloscope screenshots:

Screenshot: Saw to square and spaced saw

Screenshot: Saw to square and spaced saw

Screenshot: Saw to pulse

Screenshot: Saw to pulse

Waveshaper: Saw to triangle to sine

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I kept all the waveforms of the original Elektor Formant, but changed the circuitry significantly. The only thing i added a symmetrical square out, that i found useful for sync. For converting the saw wave to triangle a precision full wave rectifier is used, followed by a level shifter. The originally used matched Germanium diodes are hard to obtain these days. Here you only have to match two resistors. The switching glitch is not audible and can be suppressed with means of a capacitor of some 100pF if wanted. Don’t choose the capacitance to high, it rounds the edge of the triangle.

converting saw to triangle

Converting saw to triangle

For triangle to sine conversion i used a well known circuit that works better then the two diodes approach from the original. This circuitry can be found at various places in Electronotes and others. The circuitry is very sensible to the used values and input voltage. If you use another input voltage you probably have to change type of FET and resistor values.

Convering triangle wave to sine wave

Convering triangle wave to sine wave

Screenshots:

Screenshot: saw to triangle to sine

Screenshot: saw to triangle to sine

Screenshot: saw to triangle to sine

Screenshot: saw to triangle to sine

Screenshot: saw to triangle to sine

Screenshot: saw to triangle to sine

 

Waveshaper: Start

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As said before i loosely follow the Elektor Formant path with my circuitry. So I did with the waveshaper for my Next Generation Formant, NGF. It will have 10Vpp saw input from my NGF VCO and saw, spaced saw, triangle, sine, square and pulse output, all 10Vpp. This follows the original. I only added the square output.

Waveshaper Block Diagram

Waveshaper Block Diagram

I changed some of the circuitry for better performance, particularly the saw to triangle and triangle to sine part. And did a lot of breadboarding.

Waveshaper Breadboarding

Waveshaper Breadboarding

Building and testing the prototype on stripboard.

Waveshaper prototyping

Waveshaper prototyping

Basic Synthesizer

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Now with the VCO, VCA and ADSR i have a basic minimal synthesizer. Every module is self designed or a renewed design of an existent older module. This modules makes a basic patch for playing around.

Basic Synthesizer

Basic Synthesizer

Those Elektor Formant based VCO modules are tracking that well together that i thought it is worth to dig up a rather old project of mine. I have made a working prototype some years ago and then forgot about it. A keyboard scanner. This makes my synthesizer to a poly synthesizer!

Keyboard Scanner

Keyboard Scanner

On the left you can see the “keyboard” and underneath the keyboard electronics build with 4014 and the PCB from the German computer magazine C’T, issue 17/2005. Yes, this prototype is that old. On the stripboard on the most right is the sample and hold part and under that (not visible) the microprocessor, the discrete digital – analog converter (R2R ladder) and some multiplexing and switching circuitry.

Keyboard Scanner Block Diagram

Keyboard Scanner Block Diagram

But before going into building the keyboard scanner I will build a waveshaper and some filters.

NGF VCO Core one (part three)

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The PCB for the NGF VCO Core one (based on the Elektor Formant) with the corrected footprint for the uA726 finally arrived this week. I stuffed the PCB and wired them up to the front panels.

NGF VCO Core one (Elektor Formant based)

NGF VCO Core one (Elektor Formant based)

They worked as expected. No faults. No problems. I only made some minor changes for the next PCB run. I added a trim-pot to make the output voltage adjustable and put a 5p6F capacitor in parallel to the feedback resistor of the output OpAmp.

NGF VCO Core one PCB

NGF VCO Core one PCB

You can see my “wind shield” on the left and the added small capacitor on the low right site. For measuring the temperature for the uA726 it is a must to use heat transfer paste. Otherwise your figures are way of.

NGF VCO Core one PCB temp measuring

NGF VCO Core one PCB temp measuring

I did this to determine what resistor to use for the heater temperature control. In the datasheet 75kOhms with 15V are used. The Mini Moog used 49k9Ohms with 10V. I decided to use 160kOhm (15V) with a “wind shield” used to avoid airflow around the heated uA726. This keeps the temperature well above room temperature and gives lower thermal stress for the device. Which hopefully makes for a longer life.

NGF VCO Core one (Elektor Formant based) Part two

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This week I stuffed the PCB of my Elektor Formant based NGF VCO Core one. Before soldering the precious uA726 in, I checked the footprint on the PCB, because I developed it in Eagle by myself. Lucky me! I screwed it up! I did the connection between pins and pads the other way around. I have to correct this before the next run of boards. To make use of the board anyway for testing I soldered the uA726 from the back and used some wires to connect the pins with the right pads.

20140927_01

I left some resistors out because I wanted to determine their right values while testing. None of them necessary for the basic function. I put the control voltage IC’s REF02 and REF102 in and tested the voltage. Fine. I put the other IC ‘s in, fired it up and the VCO worked as intended. A nice clean saw at the output.

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Octave switch worked, fine adjust worked, sync in worked. FM lin, FM log worked with input resistors yet to be determined. Temperature control resistor is set to120k at the moment but should be checked again later. The Elektor Formant uses 200k here, the example in the datasheet of the uA726 says 75k with 15V. The Mini Moog used 49k9 with 10V on the second version VCO. So everything fine? Nope. I forgot that the output peak voltage should be at 5Vpp. I simply forgot the output amp and the output C to eliminate the d.c. offset as well.

The sync output pulse was derived directly from pin 8 of the 74LS13. But because you can sync the VCO with any input wave form I just eliminated this part completely. This gives me the chance retrofitting the 5Vpp output with the now freed OpAmp.

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This week left me with a quite good working VCO. The PCB board must be redone but that’s not a big thing.

NGF VCO Core one (Elektor Formant based) Part one

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To start with this Blog i want to write about a new PCB project which roots go back to the seventies. At this time a German electronics magazine published an diy analog synthesizer. The Elektor Formant. They sold PCB’s and some third party vendors sold kits as well.

So i bought and build my first few modules of an analog synthesizer. The VCO was build around the Fairchild uA726 temperature controlled differential pair, the same one used in the Mini Moog second oscillator board. Over the years a added some features to the VCO that i found useful. For instance: octave switch, sync in and out, fixed square out, FM lin, voltage reference, output mixer and some more. Getting rid of the 5V PSU was a big step. I made the changes recommended in the second Formant book as well. This leads to some wiring …

IMG_3639and an additional stripboard.

IMG_3638

So i decided to consolidate what i have done and put all those changes on one PCB. This was possible because i have all the used obsolete parts at hand. If you want to follow this path first make shure that you can get hold on those parts (uA726, 74LS13). They are still available at a price but you have to search for them.

I only did the core VCO here. The wave shaper will be tackled later in a separate project. There are two categories of changes i did. Additions i found usefull to have and changes to increase stability. Usefull additions: Octave switch, FM lin, Sync in, Sync (pulse) out. Increased stability: Stabilized control voltages, getting rid off the 5V power suplly. Details and schematic can be found on my website.

The PCB just arrived. Now time to stuff it.

IMG_3679