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 :)



Please, share and follow this blog and see you on the next one. :)
If you have any questions and/or comments please post them below in the comment section or post them on the  EddyBergman Discussion and Help Facebook group.

If you find these articles useful and you would like to support these efforts and future projects you can buy me a Coffee. There's a button for that underneath the main menu if you're on a PC or Mac. Otherwise you can donate some change directly by using this PAYPAL ME link. Thank you so much! Your support is really appreciated.

No comments:

Post a Comment

Comments with links will be automatically deleted.