Sunday, 10 May 2020

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

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

This LFO 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 print 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. The only thing I had done wrong was the wiring of the potmeters but there's an easy way to invert the potmeter wiring simply by connecting pin 2 of the VCLFO chip to ground, So that's what I did and that's also included in the layout. So the wiring diagram is correct eventhough it seems the wires are on the wrong way around. So in principle you can also view the wiring of the potmeters as 'bottom view' and then not connect pin 2 of the VCLFO chip to ground but leave it unconnected or connect it to +5V. That would work fine, except for the two level potmeters for 'Frequency' and 'Level Amount'. They must be connected as shown in the layout with ground connected to pin 1 (the counter clockwise pin).
The main print 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.
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's a nice project to do if you're stuck at home.

Here is the (verified) layout. Wiring diagram:

Here's the layout for the main print:

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

And here's a close-up of the second print with the extra outputs and rate indicator LED. You can use this print 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 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 change them all for resistors of the same value:

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

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 potmeters 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 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:

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!

Here is a link to the product page of the Electric Druid VCLFO-10 from where you can order the chip:

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 soldering.

As always, if you have any questions please put them in the comments below or contact me on Facebook.

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