Sunday, 29 December 2019

Synthesizer Build part-13: THE LFO (Low Freq. Oscillator).

A very useful, good working and simple to build LFO for squarewaves plus a stepless transition between ramp- triangle- and sawtooth waves.

This is the LFO from MusicFromOuterSpace. It doesn't have a sync option but nevertheless it's a very useful LFO and it is in fact now the main LFO in my synthesizer.
This LFO has the following features: Stepless transition between Sawtooth to Triangle to Rampwave with one potentiometer. Sinewave and squarewave with changeable pulsewidth. Frequency control and a switch to go from High to Low frequency setting. The overall range is from one wave per minute to 500Hz (roughly).
The higher frequency possibilities are very useful to drive a ring modulator. So a very feature rich design and a design with very few components so not much can go wrong. It uses a TL084 quad opamp chip and a LM13700 OTA chip.
I even managed to add a little extra of my own design; all the outputs go from -5 to +5 volt but I added two extra outputs for the saw-triangle-ramp wave and the sinewave that goes from 0 to 10 volt. There was room on the circuitboard to put a little TL082 on and make the two inverting buffers with DC offset potmeters. It's a bit cramped on the stripboard but it works fine. I'm sorry there's no schematic for these additions, I did it from memory. Remember these signals are inverted so the waveshape potmeter works the other way around for these waves.

I made a stripboard layout that will fit the normal format of 24 by 55 holes. All the component numbers are the same as on the schematic below. Un-numbered components are those I added myself for the DC offset function. It's a bit cramped on the stripboard but that's because I added it as an afterthought. I'm glad I did though because I use those outputs a lot
(All potmeters viewed from the front)

(Last revised: 04-Feb-2020)

Here's the schematic for the Music From Outer Space LFO:

 It was easy enough to put together although I did manage to forget 2 components, but finally I had it done. Then it was time to test it. Wouldn't you know it, I couldn't get it to work. So I applied the first rule of troubleshooting: Thou shalt measure voltages. Sure enough, my dual voltage supply was broken. I connected it to the power supply of the synthesizer and it suddenly sprung to life!

Here's a video of the finished LFO panel with an added AD/AR envelope generator from the LookMumNoComputer website: (no audio)

I added the AD/AR because I had room left on the panel but more importantly to compensate for the fact that this LFO has no sync input. I can now use the AD/AR to trigger a filter when ever I press a key on the keyboard. I even made it with a big arcade push button with internal lighting just like the one from LMNC, because I thought that looked pretty cool and you can press the button to get a loud filter resonance reaction (for instance). If you want to know how to build that I can recommend checking out the LookMumNoComputer website. Click here and you'll be taken right to the AD/AR page. The LMNC AD/AR does have some inherent design problems which means it's not very good with trigger signals. That's why I recommend to build the Thomas Henry designed AD/AR, using the 7555, chip instead. Click here to go directly to that page.

3 Months of building and soldering and all that and the result is pretty darn amazing! I am really proud of what I produced here and of how well it sounds!!
I could not have done it with out the help of the awesome folk over at the "Synth DIY" Facebook group. If you are thinking of building one of these, I can really recommend joining that group.

I named my synth the "Bergman-Berlin" because Berlin is the capital of electronic music in Europe and because I love the city. I've seen it when the wall was still up and I really need to go back. (If you live there, invite me over!, LOL)

Alright, thank you for joining me on this epic voyage and stay tuned for more. Because the beauty of this hobby is that there is always more stuff to build :)

Please, share, subscribe, follow, stalk, rob etc. etc and see you on the next one. :)
Oh, and if you have any questions and/or comments please post them below in the comment section. Leave me a comment anyway and tell me what you think of the website. I can really use the feedback. Tell me what you like about it and what I can do better.

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