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 …
and an additional stripboard.
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.
Hi Harald,
First of all you did a good job! I have several questions: do you sell PCBs? I am interested in the VCO. Another: because the 7413 (or LS13) contains two Schmitt triggers, do you plan to build a dual VCO board with one 7413 and two 726? And finally: did you try the latest (LF356) VCO version published in the Elektor 1980 February issue? I have experiences with only the original Formant VCO, but because I have several 726s, I thought it would be fun to build a more modern VCO version.
Thanks!
Hallo Attila,
thanks, currently i have no PCB to sell. But if there is enough demand i am willing to order a run. Using both Schmitt triggers might cause coupling problems on the chip. I am not going to try it. I have updated all of my original VCO with the LF356H metall can version as described in the appendix in book one for the Elektor Formant. Works quite well.
Hi Harald,
Thanks for the info, I will try the LF356. But I have only the LF356N version from NS, not the H. What is your opinion? Worth a try?
Hallo Attila,
please use my eMail address as posted on my website for further discussion. The H-Type (Metall can) is easier to put in the original PCB. There should be no difference in performance to the DIP Version. But if going with DIP you might want to use another better OpAmp. Look it up on my website. And yes it is worth a try.