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

Before I made Bi-Polar versions of the 3340VCO using 2 capacitors and an offset trimmer, I had the idea of using offset voltage on all outputs with an extra stripboard. In this article I explain how that would work. You can use this as a stand alone module of course and use panel potmeters instead of trimmers to set the offset voltage. It's up to you :)

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.

OPTIONS:

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.  

SCHEMATIC:

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.

If the output voltages of the VCO waveshapes are 8.2V instead of 10V and you want to crank it up to produce 10Vpp (-5/+5V) waveforms then change the 100K feedback resistor over pins 6 and 7 to a 130K. That will give just enough gain to get the right output voltage. 

LAYOUT:

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.

TUNING THE CIRCUIT:

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.

Synthesizer Build part-32: ELECTRIC DRUID VCLFO-10 with extras.

A truly awesome LFO with 16 waveforms and 8 different controls. I added 2 extra outputs for 10Vpp and a frequency indicator LED.

This Low Frequency Oscillator was one I had on my wish list for a long time. Last month I decided to buy the chip, it was only 5 Pounds anyway, and it arrived precisely a week later from Tom Whiltshire of Electric Druid in Portugal.
I downloaded the Datasheet PDF with all the schematics etc on it but I found parts of the schematic a bit confusing. The Frequency and the Level controls have their own opamps and they both have two potmeters connected which looked a bit weird to me. So I asked on the Synth DIY Facebook Group what the deal was with those two potmeters. It turns out both the Level and the Frequency controls can be connected to an external control voltage so one potmeter is connected to an input jack and serves as the input level control or attenuator and the other potmeter is for manual setting of the Frequency and the same for Level. So after I had that straight I set about making a stripboard layout. Now, I noticed there was no rate indication LED in the schematic. I always find it handy to have a frequency indicator LED on the panel, so I designed a second board and included a rate indicator LED on it, together with two extra outputs that have a DC offset of +5V so the outputs are 0V to +10V. That is the level I use most on my synthesizer so I needed to have that included. This does mean that the Noise level through these outputs has a +5V DC offset so use the +/-5V output for noise.
I made a second stripboard layout and made a mounting hole in it, on the upper right corner, so the second print can be stacked on top of the main circuit board, using a 3 cm M-3 bolt  and a bit of plastic tubing as a spacer to keep the two prints from touching eachother. The layouts worked like a charm and everything worked fine when I tested it.

 

If you find that you wired up the potmeters the wrong way around, you can easily correct that by connecting pin 2 of the VCLFO chip to ground. That reverses the working of the potmeters. Make sure the two level potmeters are wired like on the diagram though. They are not influenced by pin 2 of the chip.
The main stripboard has its own plus and minus 5V power supply included, so everything can be powered from a single dual 12 Volt power supply. I used the big 7805 and 7905 in TO-220 package because that's the only ones I had available but you can use the smaller L versions. This circuit hardly draws any current at all so they won't run warm and don't need heatsinks. I did not include any de-coupling capacitors or electrolytic caps on the power rails (except for the -5V because that was indicated in the schematic). You can put those in though, if you think you need it. Use two 100nF ceramic caps, one from +12V to ground and one from ground to -12V.

SYNCHRONIZATION POTMETER:

There is an extra 100K trimmer in the layout with which you can set the Synchronization mode between Sync Off, sync-ing the LFO, the Sample and Hold or both. Instead of a trimmer, as seen on the layout,  you can also make this a feature on the front panel and connect a potmeter to the same points as where the trimmer now sits, and of course you then leave out the trimmer. That way you can change the sync setting on the panel itself. This is what I later did. Beware these extra's are not listed in the Bill of Materials. I used a 100K potmeter but you can use any value potmeter or trimmer for this function because it is just a voltage divider connected between +5V and ground. (Use a value of 10K or higher.)

Looking at the panel-potmeter front-on with pins pointing downwards, connect the left pin to ground, the middle pin (wiper) to pin 8 of the chip and the right pin to +5 Volt.

It took me 5 and a half hours to solder the stripboard components in place and to wire it all up. The whole proces of designing the layout, designing and making a panel etc. took a whole weekend so it was a nice project to do because at the time I, and everyone else, was stuck at home in Covid Lockdown anyway.

Here is the (verified) layout. Wiring diagram:

The +5V and -5V points on the left side of the stripboard are simply indicators, so you know that voltage is available at that point. You could say they are test points you can check if you are troubleshooting this board (which of course I hope won't be necessary. ;) But nothing needs to be connected to those points. Sorry if that's a bit confusing.

This circuit will run fine on +/-15V too. The voltage only feeds the IC's and the regulators and they can all take it without problem or without any changes needed.

Here's the layout for the main stripboard. Beware that some stripboards are sold with 56 instead of 55 holes horizontally. The layout is 55 holes wide!!

In this layout the synchronization mode is set with a 100K trimmer at the top of the stripboard. As I mentioned before I myself exchanged that for a panel mounted potmeter later. The sync mode depends on the voltage on pin 8 and the trimmer or potmeter sets that voltage.

There is no capacitor on the input of the 7905 voltage regulator, it doesn't need one to work but if you want you can put a 1µF electrolytic cap over the input to ground. Easiest way to do that is to connect it to holes B-11 and C-11 with the negative pole in position C-11 (-12V)

The Zero Adjust 100K trimpot is used to set the symmetry of the output signal. You must use an oscilloscope to adjust this. Set it so the LFO output signal's positive amplitude is the same as the negative amplitude.

Make sure you get the cuts right in the stripboard. Especially those in the power rails at the top otherwise you'll have a direct short circuit between ground and -12V. Always measure continuïty over the power connections to rule out short circuits before you connect it to power for testing.

Here's the Bill of Materials for the main stripboard. Note: component numbering does NOT follow the numbering in the datasheet schematic. 

OFFSET AND LED INDICATOR BOARD:

Here's a close-up of the second board with the extra outputs and rate indicator LED. You can use this board for other projects too, if you need to add a DC offset voltage to a certain output. I chose 150K resistors for around the opamps (R3,4,5 and 6) because I have a lot of them but you can use any value from 47K to 500K instead of the 150K's as long as you use the same value for all four resistors:

The DC Offset trimpot must be set to a +5VDC Offset to get a 0V to 10Vpp output signal. Use an oscilloscope connected to the output socket to set this. I don't have a schematic drawing for this part but it's a really straightforward opamp offset circuit with the offset trimpot connected to the non-inverting (positive) input of the opamp. The signal goes in at the inverting input and then it goes through a second opamp stage to invert it back to normal again. Then the second output (at the bottom) is simply fed the signal of the first output via an opamp buffer. The second opamp of the chip on the right is not used and properly connected to ground to 'park' it so to speak.

Here's the Bill of Materials for the extra board:

You don't have to use this extra board for 0 to 10 Volts outputs of course. If you find you need more Bi-Polar outputs of +/-5V then set the trimmer to 0V offset and it's done. 

WAVEFORMS:

This VCLFO produces 16 waveforms in 2 sets of 8. I have made a little compilation image of oscilloscope images I took of the waveforms and some sample and hold results. The blue line is the original +/- 5V output and the purple line is the one I put in myself with 0 to 10Vpp. You can see that the noise has a +5V DC Offset on the purple line. When you start testing this circuit after completing the build, it's possible you don't see a waveform but just a flat line. That means your offset voltage is too high or too low, so all you need to do then is set the offset voltage with the trimmer on the main stripboard. Then check the 10V outputs and set that offset with the trimmer on the small print. I advise to use multiturn trimmers for those, but you don't have to. Make sure your oscilloscope is set to DC mode for measuring these waveforms.

Each of the waveforms produced can be sent through a sample and hold unit which is built into the chip and as the chip can also produce noise you can also get random tones produced by this LFO if you connect it to the CV-2 input of one of your VCO's. The sample rate of the S&H can be set with a 10K panel potmeter and if you turn it to zero the S&H switches off automatically.
The VCLFO has a synchronization input and it can be frequency modulated by means of a Frequency CV input with attenuation potmeter. There's even a separate input for the Level control which is a volume control changing the amplitude of the waves.
There's also a control on the panel for 'Distortion' which bends the bottom or top part of the wave with the middle setting being the clean, undistorted wave.
The LFO has 4 frequency ranges and they are:
8 seconds per wave to 12,5 Hz
4,6 sec/wave to 25 Hz
2,6 sec/wave to 50 Hz
1,2 sec/wave to 100 Hz
You set the frequency range with the LFO Range potmeter and then you can set the Frequency within that range with the Frequency potmeter. There's a smoothing switch included in the circuit which rounds off the corners of the waves and makes them smoother (obviously, LOL). This is to prevent the sharp edges of some waveforms from causing clicking sounds when you're using the LFO as a Tremolo.
The possibilities are endless with this LFO and with the chip only costing 5 UK Pounds, like I mentioned, you should really get this one.

Here are some pictures of the finished panel and of the stripboard and wiring. I admit the panel is a mess but it works for me:

In the picture below you can see I made a change by adding an extra potmeter (the one with the yellow knob) with which you can set the Synchronization mode between synchronizing the LFO, the Sample and Hold or both.

And here's a little video I shot using the LFO in a reasonably complicated patch. I've got 3 VCO's feeding squarewaves into 3 filters and a triangle wave into the wave folder. Each filter receives an LFO signal from a different LFO. The Electric Druid VCLFO-10 is feeding a quad pulse into the Steiner Parker filter. All LFO's are synced from the main LFO which is the Music From Outer Space LFO. You can also see the Mixer/Passive Attenuator in action with the bright blue clipping LED coming on occasionally and the Digisound 80.6 LPF sounding really good!

If you're interested in recreating this patch then here is the basic set-up I made. The eventual sound is, of course, dependant on the settings of all the potmeters and little changes can make a big difference but this at least is the foundation of this patch:

ORDER THE CHIP:

Here is a link to the product page of the Electric Druid VCLFO-10 from where you can order the chip:  https://electricdruid.net/product/vclfo-10/

If you have any questions about the chip or simply want to say thanks to Tom Wiltshire, drop him a line on his website. He's a really nice guy and he'll appreciate your feedback.

Okay, that's it for now. I have now finished the second stage of my synthesizer and so I have no more room to put new modules unless I build a third case. That will no doubt happen but not right away, what with summer coming it's going to be too hot in the attic to spend all day in there wood-working or soldering. I also built up a Eurorack system in early 2022 which took a big chunk out of my budget, which wasn't/isn't too big anyway, but that's all in the game right?

As always, if you have any questions please post them on the EB Projects Discussion and Help Facebook group, or in the comments below or contact me directly via Facebook.