Synthesizer Build part-69: FEATURE RICH SUB-OSCILLATOR.

A feature rich sub-oscillator taken from the Thomas Henry VCO Maximus. It has -1,-1.5, -2,-3 octaves, octaves or fiths (interval), polarity setting and a blend option with the original pulse wave. The suboscillator will react to the pulse width of the input wave too.

Finally a new stripboard project and this is a good one too. I never featured a sub-oscillator design on my website before but I was looking for a good design which I wanted to combine with the X4046 VCO to build my version of the ultimate analog VCO for Techno Music in Eurorack format on PCB.

So I found this design on the Birth of a Synth website, a website that provided many excellent projects already, and I needed to test it before I could add it to the 4046 VCO I'm designing in KiCad, so I made a stripboard layout for it and it worked great. 

This sub oscillator comes from the VCO Maximus design which is a 3340 VCO like the Digisound-80 from project 18 and the squarewave output is at 0V to +10V instead of +/-5V like the X4046 VCO. The sub-oscillator does not work with a bi-polar signal on the input so I added an offset feature so you can trim the input wave to be unipolar and therefore this suboscillator as presented here will work with both uni-polar and bi-polar signals on the input as long as you trim the input signal to be 0V to +8 to +10V. The output wave is bi-polar +/-5V.

SCHEMATIC:

Here's the schematic I used for this project:

In this schematic I drew in the offset feature and the IC numbering is adjusted to match the layout. I also changed the voltage from +15V to +12V and adapted the output resistors to this value.

Here's how it works: a bi-polar pulsewave, coming from the X4046 VCO (in my case) goes into the inverting input of the opamp and gets pulled down to become a negative unipolar signal by the trimmer voltage. So the input becomes 0V to -10V which is inverted to 0V to +10V by the opamp's inverting action. Exactly what we need.  After the signal has been processed by the logic chips it gets turned back into a bi-polar signal by the output opamp which has half of the positive powerrail voltage on its inverting input by means of the resistor voltage divider (the two 10K resistors) which will push the signal down again to become +/-10V. By using the input trimmer you can set the output signal to be symmetrical. The output signal amplitude then gets cut in half by the resistor voltage divider R15 and R8 which, for use with 12V, need to be 2K2 each. I altered the schematic accordingly. 

Capacitor C3 (47pF) is very necessary. It ensures the incoming pulse signal doesn't overwhelm the flip-flops which would result in tiny spikes in the signal which would disrupt the working of the sub oscillator.

This suboscillator consists of a number of exclusive OR gates in the CD4070 and two flip-flops in the CD4013 for octave switching. You can set the octaves between -1 and -2, you can set the polarity of the signal between positive and negative. It allows you to add or subtract the subdivided pulse from the original. The different sub division options through the switch settings can give a divide by 1.5, 2, 3 or even 6. Any changes in pulse-width made to the input wave will be carried through into the sub-oscillator further expanding its sonic diversity.

I urge you to read the original Birth of a Synth article I linked to above for more information on that. It's very informative and explains it better than I ever could.

LAYOUTS:

Here are the layouts I made for this project. As always they are verified, I used them for my tests.

I alterred the layouts in so far that I changed the two output resistor values from 3K3 and 1K8 to 2K2 for both resistors. This will increase the output amplitude of the pulse wave for use with +/-12V powersupplies (Eurorack/Kosmo).

Wiring diagram:

Stripboard only:

Cuts and wirebridges:

There are 31 wirebridges to put in and that's why it takes up a lot of space dispite this being a project with very few components. However, I managed to keep the board to 41 holes width with room to spare so it will fit behind a Eurorack faceplate if necessary.

 

Bill of Materials:

Make sure you get your logic chips from a reputable source. There are a lot of fakes on the market and this design might not work well when you use those. Never buy IC's from AliExpress.

OSCILLOSCOPE SCREENSHOTS:

Here are some of the test results from the oscilloscope. They look amazing and show that this is not your average sub oscillator but quite a cool addition to any VCO. It's easy enough to make this as a stand alone module so you can plug any VCO into it. The Blue wave is the input wave.

Divided by 3:

Blend potmeter half open:

Polarity lets you add or subtrackt the subdivided signal from the original pulse:

In the next 3 pictures you see what happens when the blend potmeter is used. The middle bits of the waves move past eachother. The top one moves down and the bottom one moves up until they form one squarewave again. That sounds really cool. Unfortunately there's no CV control for the Blend feature but it wouldn't be too difficult to put a Vactrol over it so it's not impossible.

With the switches at -2 oct and oct/5th at oct I measure for instance 40Hz

If I switch to 5th that goes up to 53.4Hz and switching to -1 octaves it goes to 106.6Hz

Below a picture of my stripboard build. Because I only built this to test how it works and see if it would be compatible with other VCO's, I did not make a panel for it so this is as far as I got with this project. Well worth the effort. It's a great suboscillator and would make a great stand-alone module.

I've used this design in a recently finished project, the 4046 SUPER VCO as I call it. It works like a charm thanks to the input offset feature. It really is an awesome addition to any VCO and in this one you get that really low bass in your stomach combined with the awesome hard sync and FM features. It's a killer combination and I'm really proud of this one and dispite the reputation that this VCO has with bad tracking when using a Texas Instruments CD4046, I had no such problems. But anyway, this article isn't about this VCO so I'll leave it there :) 

And that's really all there is to this article. It's short but sweet. 

VIDEO DEMO:

Instead of making a demo myself I though I might aswell post Thomas Henry's own demo video of this suboscillator here. It shows all the options including the influence of pulse width modulation.

I hope you enjoy building this project. If you have any questions please leave them in the comments below or on the special Facebook Group for this website.

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