Wednesday, January 22, 2020

3340 VCOs: Finished!

Happy day! The 3340 VCOs, based on TH's Maximus design, are good to go. Got some expensive panels back from FPE, along with PCBs that fixed the DOH! mistakes in Part IINASA: we have 2x VCO modules with DIY PCBs, DIY panels, and DIY mods, that work great.

To review:
  • Part I, where I create an V/octave switch using TI's REF02 IC, is here.
  • Part II: where I stuff the first set of VCO PCBs to come from China, and fix the dumb mistakes, is here.
  • Part III--love at last--wait, that's this post! it all works!  
The obligatory and probably fully unnecessary bench photos follow. 3340 yet.  I always test the boards for shorts before putting on the expensive ICs. Another tidbit: I had to use 25 turn trimmers for scaling, expensive but worth it--we want our VCOs to really be V/oct right?

Yeh Baby!  Yeh!!!!

Of note: about the REF 1V/octave switcher board. Yep, for these 2 VCOs I "improved" it (not a bug--a feature?)

Read more about the basic octave switcher for this here; to sum up, I could have used hand-matched 1% resistors, a rotary switch, and an op amp, like most everyone else, but wanted to try new things, and also hopefully get better performance in hot and cold environments. To that end, the PCB uses a REF02 chip from TI as a 5V reference. Next, high performance .1% SMD resistor arrays that stay put, value wise, in inclement weather are used to as voltage dividers.

Each stage of the V divider is wired to a switch; I ended up having to spring for pretty good switches....finally to an OP07 op amp used as a unity gain buffer.

So far so good?

There is a problem with my design first described in PART I, towards the bottom of the post, again part I of this post is  here.

To reiterate: TI's somewhat vague datasheet has an example where they suggest using a trimmer (not shown or really explained in their docs? But why, Brian, why? Come on TI, help out an old dumb rock and roller!) to set the exact output value of the reference chip relative to ground (pdf here--see page 7). To save money and PCB space, and to avoid the fear and loathing caused by having to figure out vague TI circuit examples, I skipped all that.

But: omitting this overall 5V ref trim was done at my peril?

No, happy ending!

Here's the issue: After wiring up the board: choosing GND (0V) as a voltage setting on the front panel V/oct. switch was a few mV off vs. REF02's 5V output. This makes sense--GND knows nothing about whatever the REF02 IC is producing at its output. And I have no way to trim the REF to be exactly 5V relative to Ground (I left that off, remember?)

And of course I can't change GND feeding the V/octave switching sub-circuit without some really stupid extra work.

That meant that when choosing the 0V octave setting, the VCO would suddenly go out of tune vs. the other VCOs in my rack.

OK what to do about our butt switch setting?

I didn't want to go back to square one for the v/oct board design. Instead, let's mod: I cut the GND trace on the octave switch board and used a 24 gauge kludge wire (brown wire seen in the photo above) to connect the GND rotary switch setting to the tip of a 3.5 inch jack. Ha! Take that! Now you have LFO, 2', 4', 8', 16' (I think 4 octave switching is good enough) and this new jack: "EXT". Into this external input you can blast FM mod, audio, CV, whatever, then choose your goofy mod signal, any time you want, with the octave switch. This turned out to be a fun addition to the VCOs user interface, and if I had thought through the ref board design with a bit more care it would have never happened.

Lesson learned: it pays not to think too much?

Note the whacked font for "LFO" and "OCTAVE SELECT"...I was so ocd'd out with getting the drills just right that I missed this illustrator flub! CRAP! I want my panels to be perfect; but a fellow synth club dude says I'm being hysterical....

The whole process of building all of this was a lot of work but enjoyable....

I have parts to build about 2 more of these VCOs....not sure I will right away, too many other things to do....

Good to have stable VCOs but perhaps too much time and money? You can get 3340 kits already good to go, EG: here and might want to go this route instead of what I did? Me, I'll take door number er....I mean door # two.

At some point I'll post the finished PCBs for this VCO on my website; the 3340 board schematic is not my design so i don't feel good about posting it, but Scott Sites and TH are cool so it's posted here.

I left out the suboctave and sine wave stuff but otherwise the PCB is a total match.

Time to move on; already working on the first DIY synth full kit I've touched in like 12 years. The kit thing so far has been a struggle, and my psychiatrist girlfriend questions whether it's time for me to not solder for at least a week. Too many fumes?  Can't say--ask the good doctor.

Until next time: have fun, then enjoy the post-Curtis glow.  See ya!

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