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 Variable Skew 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. It's ideal for all the modulation duties in your modular synthesizer. I was allerted to an alteration that you can make to give this LFO a synchronization mode! That didn't really work for this LFO but more on that later further down the article.
This LFO has the following features: Stepless transition between Sawtooth to Triangle to Rampwave with one potentiometer. Sinewave. Pulsewave 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. So it can be a bit fiddly to calibrate.
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. You can take a look at the 8 step sequencer V2.0 schematic which also has an offset feature of my own design and it's the same design as used here. 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. The overall frequency range will go up a bit with a dual 15V powersupply of course.

LAYOUTS:

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. There's an explanation of the colour-coding of the wirebridges on the layout. If you're wondering why C4 is 10pF instead of 100pF as it is on the schematic, it's a change that Ray Wilson himself made. You can read it in the original text.

(Last revised: 21-Jan.2021 Updated the old layout with some components re-arranged and got rid of a jump wire.  28-Aug.-2021: Cosmetic changes, got rid of resistor colour coding lines. 

Stripboard only:

Cuts and wirebridges seen from COMPONENT SIDE!!

Here's the schematic for the Music From Outer Space LFO. I put in a 100K potmeter for the Wave shape function instead of a 50K as is shown in the schematic. This doesn't make any difference. It'll work the same but put in a 50K if you have one. 

The timing capacitors are C1 and C2 (two 10µF electrolytic caps) switched in series with their negative poles connected together thus forming a 5µF bi-polar cap. This is used for the low frequency setting. The high frequency setting uses just C3, a 100nF capacitor.

Make sure all potmeters are linear types. You can see that only one half of the LM13700 is actually used so it would be easy enough to turn this into a dual LFO. All you need to do is duplicate the LFO circuit and connect it to the pins that lay directly on the opposite side of the 13700 chip. You'll need to make a new layout for that yourself though. A nice exercise in layout making ^___^ 

Bill of Materials. As mentioned earlier, C4 has been changed from 100pF to 10pF by Ray Wilson himself on the MFOS website, so that's why it's 10pF in the B.O.M.:

SYNC OPTION:

There is a circuit design available on the internet that will add a synchronization option to LFO's with a triangle core. I have tried that circuit on this LFO but the timing capacitors in this LFO design are too big for this to work. However it will work on other LFO designs from MFOS. I have linked to the schematics for the sync circuit below so you can check it out. There's also a link to a video by Rich Holmes from Analog Output who shows some changes he made to the circuit to make it work better with his LFO. Very useful to watch if you want to use this circuit with other LFO's.

--- SCHEMATIC ----

--- GITHUB ---

--- ANALOG OUTPUT YT VIDEO ---

CALIBRATING the LFO:

Calibrating the circuit should be very straight forward. Connect an oscilloscope to the sinewave output and manipulate the Sine shape trimpot until you get a symmetrical sinewave. Make sure the wave shape potmeter on the face plate is set half way. Turn the symmetry trimmer until the waves look the way they should.

Set the DC offset potmeter so the output reads 0 to 10V peak to peak on those two outputs. That's the bit I added on myself so it's not in the schematic.

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:

This is not the board I ended up using. If you look closely you can see the 10pF cap is over pins 6 and 7 instead of 5 and 7 on the left TL084. There may have been more mistakes on it, I can't remember but the layouts are absolutely 100% verified so don't worry about it.

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. I think every synthesizer module needs at least one LED :)

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