Sunday, 29 December 2019

Synthesizer Build part-13: THE LFO (MusicFromOuterSpace version).

A very useful, good working and simple to build LFO for square-, sine- and triangle-waves plus a stepless transition between ramp- triangle- and sawtooth waves. A good LFO for beginners to build too. I still use this as my main LFO.

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. 
Frequency Range with switch in 'HI' position = 1 wave every 2,39 seconds to 84 waves per second (239mHz to 84Hz)
Frequency Range with switch in 'LO' position = 1 wave every 7 minutes and 46 seconds to 1,43 waves per second (1,43Hz). The readings you will get will differ a bit from mine due to tolerance fluctuations in capacitor and resistor values.  
Squarewave pulsewidth (or dutycycle) goes from 1% to 99%. The pulse width of the squarewave is set with the same potmeter that controls the shape of the other waves. It also influences the shape of the sinewave.
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: normally this is a bi-polar LFO meaning all the outputs go from -5 to +5 volt but I added a uni-polar feature with two extra outputs for the saw-triangle-ramp wave and the sinewave that go 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. I'm sorry there's no schematic for these additions, I did it from memory, but this feature is included in the stripboard layout. Remember these 0 to 10V signals are inverted, so the waveshape potmeter works the other way around for these waves.
Unipolar LFO's are particularly useful for modulating the pitch of a VCO when you want to set the tuning very accurately.
This LFO is meant to be used with a -12V/0V/+12V powersupply but it works equally well on a -15V/0/+15V powersupply without any changes needed.

Here's the layout, wiring diagram (All potmeters viewed from the front). The layout is verified. I recently built a second one of these LFO's to use as a standalone signal generator and it all worked first time. The green wirebridges indicate connections to ground:

(Last revised: 21-Jan.2021 Updated the old layout with some components re-arranged and got rid of a jump wire. Also a mistake corrected. 28-Aug.-2021: Cosmetic changes, got rid of resistor colour coding lines. 18-Jan.-2022: Marked positive pole of the two electrolytic caps.) 

Print only:

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

Bill of Materials:

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!
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.

Here's a high resolution picture showing oscilloscope screenshots of the different waves.

Here are some pictures of the stripboard with wirebridges and with components:

Here's a picture of the panel I made for it. Like I mentioned earlier, it is combined with an AD/AR, the version that uses the 7555 chip. I used multi-coloured LEDs on the outputs to indicate positive and negative cycles of the outputs. There's no practical reason why I did that, I just thought it looked cool :)

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