Dual 13700-based LFO: Mo' Hotta NOT betta (Shorted OTA? No, worse!)

Readers--this is a work in progress, you can see the schem and pcb on the one page website here.

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Hidee Ho! This time I tried to turn a working OTA LFO design (post here) into a dual LFO/small PCB, based on a SOIC SMD LM13700 OTA chip. 

Guess what?  Damn thing didn't work, the LM13700 kept heating up, and I had no idea why. Update 9-14-25 ModWiggler to the rescue! got it working; please see the update at the bottom of this post.

"Seems not working"

 Started as a PCB n' stencil, from the blog's sponsor PCBWAY

Lately I have been using SMD non-framework stencils extensively; speeds up fabrication

SPONSOR TIME....

PCBWAY has been instrumental in keeping this blog up off the ground.  

Serene, my totally friendly handler at PCBWAY, told me to tell you: this month they are having specials in addition to their usual great pricing: 

1. Purple Solder Mask: In September, it's free--they won't charge an extra fee for purple cool color PCB's. That's to say, for 10 purple 2-layer 100x100mm PCBs, it's only $5USD, rather than $38.43USD.

2. TPU(FDM) in 3D printing: TPU is good for you! This super-useful material starts from $12.45USD, but in September, it's priced from $7.96. The heavier the weight, the bigger the discount, which can be up to 80% off. Go Getim!

And as always--PCB has all sorts of cool services tailored to what we do; you can help this blog immensely by checking PCBWAY out via the link here. Thanks!

USELESS (?) BUILD PHOTOS

Back to the blown up dual LFO....

Maybe if I look at these pictures long enough I'll find a build fault. but really, it all looks pretty good to me.

Glooped paste and picked/placed parts pre-bake--looking good I think

As usual, I heated up the plate to 215C, let it sit a minute, then back down to 150C. I am always surprised how well this works and how forgiving the process is. 

Next I soldered in THT parts like trimmers and a few caps:

Finally soldered the edge connector:

Test time...nope! 

My power supply immediately went haywire--the voltages maxed out at about .25 to 2V while the current went to the 30mA limit on each rail. Not good. 

NASA we have a short--the 13700 got really hot, very quickly. 

Removing the 13700 the short went away but (obviously) the LFO's didn't work.

So why doesn't this work (besides having removed the LM13700) ??

Was the 13700 a cheap fake? No. I replaced the LM13700 with chips from two different vendors (Digikey, Tayda), same problem; I removed the PNP transistors thinking maybe I had a I-abc issue, nope.  

I spent a couple more evenings troubleshooting, wishing the design was through-hole (I usually am glad it's not) so I could lift up IC pins. No love.

There was a time in my DiWHY journey where I would have kept slamming away on this until fixed, but I have built so many LFO's at this point that I realized: the hell with it, I can redesign it, pitch it, and/or move on. 

Above all, I had to make sure I was having fun, and more evenings troubleshooting this didn't feel like it'd be that.

I posted to the modwiggler DIY forum to see if those folks had ideas--maybe they'd save my bacon, they had before. 

If not, this one might be one and done for the dumpster.

update 9-13-25: Modwiggler to the rescue! In short order synthiq saw the problem:

If you look at the schematic (here), the power rails to the quad op amps are flipped!!!

That's at least part of the problem, right? 

Not sure why this foobar heated up the OTA and didn't smoke the TL074's, but, things definitely wouldn't work as-is. 

I wanted to test the rest of the design. I removed the TL074's and put on new ones flipped 180 degrees (quad op amps are fully symmetrical--so this counter-intuitive fix worked--ha! flipping quad op amps to solve audioDiWhy stupidity is another awesome tip from MW's resident genius "guest").  

Finally, I used trimmers TR1 and 2 to set the frequencies to about 200hz, then used 4.7K for R21 and R14, and 39K for R7 and R20, to get the output waveforms closer to 10V P/P. Finally tested CVs at input --which modulated the LFO's frequencies. HA!! WORKS!!!

Post TL074 flip: no more short circuits--made me very happy indeed.

Battle damage post-fix. Lots of flux spoojim visible. As I fix repair SMD boards I find myself using no-clean flux more and more. For SMD work it's crucial, the flux inside the fine guage solder doesn't get it done. No clean leaves a mess; I honor it by not cleaning.

Testing post-fixes. I will lay this board out again, having to flip the TL074's is a super clever fix but energizes my OCD buzzer--how I roll, I guess.

CV Generator Part II--"Seems Working"

Readers: If you'd like to build the CV Generator featured in this post, please go to PCBWAY's Community pages--gerber file; KiCAD 9 project/pcb/schematic/library files, B.O.M.'s, and more, are here.  

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

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Just got back from a long and much needed vacation with the psychiatrist wife--back home and back to it. 

This time I improved the ATTINY 1624 based CV generator from this previous post--good news: hey, it works.

Synch issues with my camera made the OLED appear to drop lines of text but in real life the super cheap OLED display looks good.

Reviewing its features: 

• Three -10 to +10 DC CV outputs, independently controlled from potentiometers (not rotary encoders). I like the way pots feel for this application; in general I dislike membrane switches and rotary encoders.
• A 5V momentary switch for creating triggers with a dedicated output and LED.
• Practical use of the ATTINY1624's super affordable high resolution mux'd AD conversion (details here towards the bottom of the post)--to see if I could get >12 bit oversampling using the analogReadEnh function in sketch (it works!)....This is killer feature that's surprisingly under-documented.
• Eurorack footprint--this is going to go into my bench rack.
• Test Points. TP1 can be used to test the MCU via a blink subroutine, or maybe some future expansion. TP2 should be adjusted to 2VDC using the 20K trimmer for bias offset for the final buffer stages.
• A TX/GND serial port. For future expansion? Who knows. But I figured why not.
• A port for UPDI programming. See previous post here (halfway down the post) and IMSAI guy video here.  

For firmware I used Arduino Sketch and IDE and the Greiman SSD1306ASCii library (here).  Firmware creation for the generator was pretty easy. By bringing analog data from the pots in at 16 bits, I could ditch a bunch of LSB's and get reasonable readings on the OLED that didn't jump around a lot. 

Some simple math converted the right shifted 16-bit value into a x = -10 to10 value for the OLED:

float scaleToPlusMinus10(unsigned int x2){

  x2 = x2 >> 8;

  return -10 + (float)x2 * 20.0 / 256  ;

}

The module consisted of 2 PCB's provided by this blog's patient and helpful sponsor PCBWAY: a main board with hardware on the front and the SMD components (1206 and SOIC--not too tiny) on the back, and an 8HP Eurorack front panel.  

I did a lot of the heavy lifting for the design during the initial build (post here). So this "rev 2" is a prettier version with a few improvements.

And--thankfully my measurements this time around were correct; everything fit.

USELESS BUILD PHOTOS

Always happy to get new PCB's from my sponsor, PCBWAY.  Even with the tariffs these guys are still way cheaper and faster than anything I can find in North America. Go A's!

I used an SMD stencil, solder paste, and a hotplate. This has become my go-to assembly process for most of what I do these days--it's fast....with some practice SMD hand assembly has become easier for fab, mods and troubleshooting vs. through-hole.  

I gloofed on a bit too much paste but fortunately it's a pretty forgiving process.

For the hotplate, I heated it up to 210C and it sat there for a couple of minutes, then cooled it to 150C.  This was all done manually from the plate's front panel. There appears to be way to automate this--there is some sort of temperature curve firmware in the hotplate--but I can't find any instructions for this in English. Sorry.

The hot plate process is surprisingly forgiving but I ended up with a few solder blobs on the SOIC IC's. Fortunately these can be quickly fixed with solder wick. 

Firmware test....

For the OLED I de-soldered its edge pins and used longer pins instead. This allowed the display to stand off from the PCB an appropriate distance. After putting items into place I tacked soldered the edge connector pins, removed the front panel PCB, and finished soldering.

As far as accuracy--the completed module is pretty good but not perfect. For what I do--cranking CV into whatever module I am messing around with on the bench it is definitely good enough. For better accuracy I'd probably need to use a rotary encoder and a 16-bit DAC. Maybe a future project?

Overall, fun times; I might build a few more of these. 

Until next time, don't breathe the fumes.

3 Channel Bipolar CV generator Part I: Didn't Fit; Built it Anyway

Readers: this is a work in progress but you can get the files associated with this post from Github, here. 

(Update--Euro 'version 2' of this module is working, post is here).

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Back again! 

I wanted a synthesizer module that accurately output and displayed 3 independent control voltages ranging from -10V to 10V DC.  

The early prototype used a 3D printed front panel conforming to PAIA Frac format. Camera Synch issues meant my digitial camera couldn't capture all 4 lines of the working OLED but in real life it works.

There were a lot of ways I could have done this--using a multiplexed bipolar digital to analog converter (post for DAC here; multiplexer here) but instead employed the 16-bit ADC features of the impressive ATTINY 1624 MCU, covered briefly in this previous post. Overall, design for version 0.1 was simple; I used the pots configured as voltage dividers, buffered by op amps, took the 0-5V wiper voltages from the pots and connected that to ADC inputs of the ATTINY, and had the MCU feed a 128 x 64 OLED display to convey the 3x voltages.

Finally I used inverting op amps to get 0-5V DC from the pots to the -10 to 10VDC needed at output:  

One of 3 identical buffer subcircuits. A simulation of this simple inverting buffer/amplifier can be found here; I have posted the files for this prototype at Github, here. When the design is further along I will post files to the PCBWAY Community.

CONSTRUCTION

I laid out the design in KICAD 9 and used Arduino Sketch to (quickly) create the firmware. Because the ATTINY isn't blessed with buttloads of flash memory I used Bill Greiman's memory-friendly SSD1306ASCii library (here) to provide functions needed for the OLED. 

I sent the gerber file to this blog's trusty and most-excellent sponsor, PCBWAY. Very soon the boards were back. They do great work! You can help out the blog by checking them out here.

The prototype used 1206 SMD components, a stencil, and a Eurorack formatted front panel. After years of trying different workflows, this combination seemed a decent mix for fast prototyping, flexibility in terms of modification, and small size. 

After unbagging and inspecting components I discovered a major mistake: the PCB was too tall to fit into a Eurorack or Frac enclosure:

I contemplated pitching the entire thing and starting again, but instead consulted the guys in my audio geek discord group. Could this be fixed? No, but, one member suggested I try to build it anyway, to shake out the first batch of mistakes, before going to Version 2. Good idea....

I got building....

Used my $35USB hotplate for SMD, works great.

 

Original design incorporated Eurorack power, but I adapted this to Frac Power using edge connectors, 22 gauge hookup wire, and shrink tubing. This allowed a fair amount of twisting and bending without breaking wires or shorting connections.

Same idea for the rest of the hookup wire....

Initial tests showed way too much current draw for the positive rail--should have been more like 4-5mA, but here it's 230mA. I pretty quickly traced this to a 78L05 regulator being installed upside down, due to using the wrong Kicad footprint--I used the 3N3904 footprint, not 78L05, so Pin 1 and 3 were flipped. Whoops. I pulled the 78L05 using a Hakko FR301, flipped it, and resoldered. Surprisingly this snafu didn't blow up the MCU, nor the regulator.  

With the power problems sorted, things seemed to work--albeit not terribly accurately -- off by as much as 5% at times--but OK for the first attempt.

I laid out a quick-and-dirty front panel for the PCB using FREECAD 1.01.

....then mounted the PCB to the 3D printed front panel:

Version 0.1, unpowered, in the flesh...

Version 0.1 seen through the lens of open source AI (ComfyUI + Flux Context Dev with a 12GB Lenovo/Nvidia RTX2000 GPU).  This first and only pass through the AI algo took 50 seconds and beat the holy crap out of what I could have done with an hour of organic intelligence and Photoshop--if this doesn't scare the hell out a tech + graphics professional, I don't know what does.

OK, quick post but enough for now. I will keep plugging away at this project and hope to have a revision with better accuracy in the coming weeks. Update 8-28-25: Eurorack "rev 2" version of the generator is complete and works....please see the post here.

So--until next time: don't breathe the AI.

"LFO MINI": Small and Simple CA3080 modulation source

Readers: If you'd like to build the project featured in today's post, please go to PCBWAY's Community pages--gerber file; KiCAD 9 project/pcb/schematic/library files, a couple of B.O.M.'s, 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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Carrying on with the latest "keep it simple" and "just say no to AI" posts: this time I designed, laid out and populated a small CA3080 based LFO PCB. Works!

 The basic OTA > op amp core was stolen from EFM's LFO2; see the previous post here; an online simulation of the current source for the 3080's "Iabc" pin 5 is here.

Laying this out was easy, but I had to make a few decisions that seemed a bit arbitrary:

• I used Through Hole Technology not SMD. I don't do a lot of through hole work any longer, but have so many THT parts lying around, I felt like I had to use them?
• I had a tube of NOS CA3080's so I used a few. If you build this LFO you'll need to find this IC. The DIP version of this IC is no longer made, but, as I am writing this post I see plenty for sale on Ebay, Amazon and elsewhere--it's not unobtanium, at least not yet.
• Inputs rails were +/- 12V, but the circuit should work fine with anything from maybe +/- 15V to +/- 9V.
• Inputs and outputs were put on 3 pin 100mil headers.

One modulation input was current limited by R5 and brought out to the "CV I/O" pin, but additional modulation sources could be easily added--wire up 100K resistor(s) between your CVs and the "Q1BASE" wirepad:

Be careful! If you leave off the 100K resistors and plug your additional CV directly into the "Q1base" wirepad you risk blowing up transistor Q1. Make sure to current limit the additional CV with a series resistor as shown.

From here this was an easy build:

I got 10 LFO minis from this blog's loyal Sponsor, PCBWAY, for $5USB (excluding tariff and import fees).  You can greatly help this blog by checking 'em out here.

Testing....

The LFO worked first time--almost.

I had a short between two of the bypass cap pins (C1) initially and the CA3080 got hot; once I cleaned up the sloppy solder everything was fine. Fortunately I didn't smoke the CA3080.

Using only the 10K trimmer board for modulation the LFO goes from < 1hz well into audio range. To calibrate, with no CV at input, I hooked the square wave output to my scope and turned the trimmer until the frequency started to change (it took several turns, and I thought maybe the circuit didn't work, but it did--I had to keep turning). I ended up settling in at about 200hz without any CV.

Then, with +/- 12V CV added, I got a wide range of frequencies, from reeallly slllowww to well up into the audio range--this is a the hallmark EFM design: cool and simple.

 

Output for triangle is about 6V P/P, for the square it's close to the rails. Since I designed the LFO-MINI to be a daughterboard, I figure whatever I plugged it into could gain up or down these output amplitudes, so for now this was good enough.

What's next? It would be easy to make this a dual super-small LFO using SMD parts and an LM13700. 

Overall, this will find its way into some "quad mod" type modules swirling around in my head, that combine random voltage sources, audio inputs, a couple of these LFO's, and who know what else. 

Until then, don't breathe the fumes.