EFM LFO2--Old Skool Fun

Readers: If you'd like to build the project featured in today's post, please go to PCBWAY's Community pages--gerber file (main board); gerber for jacks board, front panel gerber, KiCAD project/pcb/schematic/library files, a B.O.M., a link to GitHub simulations for the design and more are here.  

You can also help out this site immensely by checking out PCBWAY using the link here. Thanks!

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I needed a break from programming, MPU's, logic, and whatever else, and stumbled upon a cool design here from EFM ("Electronics for Music"). 

I decided to build it:

To further break with past Eurorack projects I built this "EFM LFO2" in PAIA's Frac Format, giving the build a large format look and feel: +/- 15V rails, panel mount potentiometers, and hookup wire.

I had forgotten how much fun it was going old-school--I could try out different things, slap in modifications, and quickly recover from the stupid mistakes.  

DESIGN

LFO2 caught my eye because of its extensive use of OTA's and the different ways current was passed to the CA3080's amp bias, pin 5. 

Using Falstad I simulated the different sub-circuits that fed  bias current to the circuit's OTA's: get the simulations at GitHub here. 

Falstad sim for the LFO delay sub-circuit

If you want to learn more about how to incorporate OTA IC's into your designs a great resource is Thomas Henry's book "Making Music with the 3080" here; highly recommended.

The circuit also incorporated a CD4007--an interesting IC, 6 MOSFET in a 14 pin DIP. 

It clamps and buffers an incoming gate signal to control the LFO's onset delay--yep, CD4007, an IC with 1001 uses.

Overall: LFO2 combines EFM's Tom Gamble signature economy-with-coolness--a clever  design that shows off Mr. Gamble's considerable skill as an analog audio design engineer.

The project used 3 PCB's:  a 3.5mm jacks board found at PCBWAY's community site here; a front panel, and a main board.  

I copied the EFM schematic into Kicad 8 and laid the main board out on a 99 x 99mm PCB.

You can get a 99x99 board from my sponsor PCBWAY dirt cheap: 

GO PCBWAY!

Shout out to Serene and the good folks at PCBWAY for help getting this project off the ground. I use them for all of my PCB fab and have also counted on PCBWAY for metal work, 3D printing, and assembly. They do all this and a lot more.  

They always do great work, are extremely affordable, and ship super fast. Please help this blog and check 'em out: here.

THE BUILD

I employed the same design and fab techniques found in the previous post here, but employed a SMD stencil (a good video for using stencils is here--go to 38:10--showcasing the technique used), stencils are available from PCBWAY of course.

IMPORTANT! For what we DIYers do, pick the "non-framework" option when you order your stencil--for instance, at PCBWAY the order choice looks like this (I figure all fab houses offer this option):

Happiness is a batch of new PCB's:

3 boards and a mask....

The build went  quickly, but of course didn't work first time (WFT). 

Here are a few useless build photos:

To add to LFO2's old-skool vibe I used JSTs, hook wire, and a Stratocaster 3 position switch. Fender Stratocaster switches are a standardized part, with lots of clones for sale; any decent Strat 3-position pickup select switch should work for this project.

I didn't have a .005uF (5nF) 1206 cap so I stacked 2 2.2nF 1206 SMD caps, which was surprisingly easy to do, I soldered on one 2.2nF cap, put the other on top, and reheated. 

    

I used DIP IC's, not SOIC. Again: old skool, dude, old skool.

"Wires R us"

MISTAKES, I'VE MADE A FEW

But then again too few to mention? Nope. Lots to mention.

I misread the schematic; the "range" pot to the left is a 10K trimmer but no, I made it panel mounted.  

So....I soldered a 10K 25 turn trimmer into the JST footprint (perfect fit, I got lucky).

I thought a first the circuit didn't work at all, but once I turned this "cvrange" trimmer down pretty far things started to work. 

But...now I have a big drill in the front panel ("Range") that doesn't need to be.

I fed a 50K "range" pot to pin 3 of the EXT JST.  Feeding this range pot was V+ from the Euro 10 pin power header; GND was provided to pin 1. I had to add a 10K resistor between the wiper and EXT to make sure if the pot was set fully counterclockwise--to ground--the +15V supply rail wasn't sent there as well.

This meant when nothing was plugged into MODULATION CV2 the range knob provided 0-15CV to the OTA controlling frequency and the frequency of the LFO at output increased dramatically.

If I revised LFO2 I'd put +12V or +15V or whatever to the normal of one of the mod pots....but the mod to fill the useless "range" drill worked. 

I also had a considerable amount of trouble remembering that when I looked a potentiometer from behind its pinout is 3-2-1. From the front it's 1-2-3. I had to re-solder a few pots.

I also mistakenly plugged one of the pots into the EXT JST during testing. The EXT JST is for normaling this module to others and could be omitted. This mistake created some very strange behavior at output until I unplugged the errant pot.

 

MODS

Only one: The random voltage output bias offset increased with the "Level" settings, which some synth heads might like, but I didn't.  

I put a 10uF cap in series with the 3 position switch input for SQUARE, which knocked its DC offset output to ground regardless LEVEL's setting. 

The 10uF cap turned the random signal into not quite a random changing square wave, more of a randomly changing slightly sawtooth wave, but: good enough. 

Maybe even better, the cap adds an analog vibe to the random output.

If I do another revision of this module I will add the 10uF cap to the PCB design.

PERFORMANCE

This is a cool LFO with a early Roland modular  feel. The delay goes from 0 (no delay) to maybe 20 seconds. Cool! The main issue is that some level settings, which control the overall output amplitude for all the LFO's waveforms, distort the triangle wave and can push the square wave output to a DC rail voltage. Not sure how to fix that quickly, but I can live with adjusting the LEVEL pot to get the output I want. 

The frequency at output goes from slow < .1hz to a nice fast-modulated sound--satisfying!

Overall, this was a really entertaining project, and I needed a fun build after some not-so-fun builds. 

Going forward I may create more large format/frac/ with wires, guitar switches, pots, and whatever else. Sure it may not work for the other guys in my synth group, but it works for me....so,why not?