Author Archives: Harald

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.

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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.

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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.

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