Friday 23 February 2024

Synthesizer Extra's No.4: QUADRUPLE OFFSET BOARD for 3340 VCO.

 A simple little expansion board that you can use to turn the 0 to +10Vpp signals from the Digisound 80 VCO (Project 18) into -5/+5Vpp signals to make it more compatible with the rest of the modules on this website.

The one thing that always bothered me about the Digisound 80 VCO is that the outputs signals are unipolar. They're 0 to +10 Volt peak-to-peak and that is not very compatible with the rest of the builds on this website. Because this Digisound 80 VCO is the most popular project on this website, I thought I would design a little stripboard that gives you 4 offset options to turn all the signals of that VCO into more useful bi-polar signals at -5/+5Vpp.
There are more elegant ways of doing this perhaps but this project is meant more for people who are beginning in the DIY synth hobby and who are building the Digisound-80 VCO as their first VCO and they want bi-polar signals from that VCO. If you're one of those people you can build this project to solve that problem. It's a very easy to build and cheap project.
Btw, the Digisound 80.6 Lowpass filter works well with the Digisound VCO because it has a 1µF capacitor on the input that shaves off the offset voltage But I don't recommend capacitors on the VCO outputs because they can also act as filters.

You don't have to build this module into the 3340 VCO (project 18) if you don't have room. You can put this stripboard behind a small panel with just 4 input sockets and 4 output sockets and attach that next to the VCO. That way you have a choise of either using the outputs straight from the VCO at 0 to +10V or to patch them through this offset board and get -5/+5V output signals. That way you can also use the offset module for other things like LFO's if you want to. 
You can even 'normal' the VCO outputs to the socket-switch lugs of the offset input sockets and save yourself the trouble of having to use patch cables.
You can even add bi-polar LEDs on the outputs so you have a visual indication of the Voltage they output. As you see, if you want, you can really go mad with this project but I leave that up to you. 
Anyway, we also have the Dual Voltage Processor project to cover that functionality and it has extra options too so maybe it's better to keep this project simple.

I put in four offset stages eventhough three will be enough for the 3340 VCO so you can use the other for something else.  

It's a very simple design. Just 4 dual opamps, in this case TL072's (but you can use other ones if you wish as long as the pinouts are the same) each with an offset trimmer that allows you to give a negative 5V offset to the signals coming from the VCO and so turn them into bi-polar signals.  I choose to give every stage its own offset trimmer so you can set them all differently should you need to, but in principal you could feed all four opamps with the voltage coming from one trimmer and so have them all produce the same offset. That's simpler but not preferable I think so I went with four trimmers.
The schematic below shows two of the four offset circuits that are on the stripboard but they are all the same.

Below is the layout for this project. 

(Layout has been updated on 9th of March 2024. Previous version had 2 little mistakes in it.)

Below is the layout with just the cuts and wirebridges, seen from the component side. As ever, mark the cuts on the component side with a waterproof Sharpie. Then put a pin through the marked holes and mark them again on the copper side. Then cut the strips with a sharp hand held 6 or 7mm drill bit.

If you decide to build this into the Digisound VCO module, then it may be better to do away with the eurorack connector and simply use wire connections for the power. That way you can make the stripboard more compact too. The lower 4 copper strips are not used either so you can cut those off too or use the space to house the standoffs to connect the stripboard to the rest of the VCO.
I did not actually build this project myself but I know it should absolutely work the way it's presented here so that should not be a problem. I built so many offset circuits while I've been doing this hobby that I can dream them.

Here's the Bill of Materials for this project. Also order four 100nF ceramic bypass capacitors. I forgot to put them in this list.

You need an oscilloscope to set the offset trimmers to the right value. Make sure you set the oscilloscope to DC when measuring otherwise the scope won't show offset voltage. Remember offset voltage is a DC voltage.
Connect the signal(s) from the VCO output to the input(s) of this stripboard and then connect the scope probe to the output(s) and set the offset voltage so that the signal displays the same amplitude on the positive side as the negative side of the zero Volt line. In other words, set it so the zero Volt line cuts nicely through the middle of the signal. That's it. 

Okay, that's all I have to say on this little extra project. I thought it might come in handy because the Digisound-80 VCO is really a very cool VCO and now you can make the signals more compatible with the rest of the projects on this website.
I hope it comes in handy.

If you have any questions about this project feel free to comment below or on the special Facebook Group for this website.