Friday, October 18, 2019

CEM VCO Part II: Maximus Troubleshooting, and the DOH! Count

From a couple of posts ago: My ASMVCO's are getting old and drifting in pitch.  Might be time to ditch 'em. how about three new VCO's based on the repop of the Curtis 3340 VCO chip?

Module ready to test (MRTT).  If the module works OK for a couple of weeks I will create a FPE panel for this and ditch the front panel here.

OK got rev one of the three boards back from China, got it stuffed and put behind a temp panel. It works!  Holds tune for at least 7 octaves, I have trouble hearing the really high stuff no matter what but let's assume that works as well.

Ho-MORE DOH-KIE (about the design): I am using 3 interconnected PCBs to try to minimize hook up wire: a main board (3340 chip, buffers), an octave switcher (see previous post here--btw, the  TI REF02 based add-one-volt-per-rotary-click seems to work great so far, guess I got lucky?), and a PCB to hold the "Thonk" type PCB 3.5" jacks. The Thonks connect to the main PCB with a 10 conductor IDC ribbon. The octave selector was supposed to mount to the front panel, but I melted my cheapo switches, so for this rev 1 prototype I used a clunky junk box NOS rotary switch wired to the board, post for the octave board madness is here.

OK, the main board is based on  "VCO Maximus", another classic from the mighty Thomas Henry. Another good page on the Maxiums for Eurorack is here.

I chose this 3340 design vs. all the other ones out there because a guy I know in my synth meetup group already built a couple of these on strip board; he says Maximus works and sounds great. I omitted things like the sub octave circuitry and sine wave conversion from triangle; I already know how to do that, and have plenty of sine waves and sub oscs in my modular setup now, so these 3x VCO's will just be pulse, saw, and triangle. Good enough!

Old trick: print out the PCB 1:1, lay it over the panel, tape it, punch it and drill.  The drills will line up.

OK so how did rev one of this module do? Considering that this is a pretty complex board, happy to say: not too bad; I got it all to work in about a day. I had to kludge things, I made mistakes, but so far everything but the sync in, which may be broken or just sounds crappy, is on the air. Update: sync is fixed.  I had the DPDT switch wired wrong. It stays in tune, the waveforms look good on a scope, PWM, linear and FM mod all work. Happy day.

Also the pots turn the right way (clockwise for more of whatever), that's good. I was worried about that.

"Obligatory Bench photo"

I modified a left over alubase 2u Frac panel I had lying around so as to not have to test this module with circuit guts flopping everywhere. The drills on the junk panel didn't line up with the PCB's for this project; that's what Dremel tools and drill presses are for.

Almost ready to test?

OK let me go over the mistakes. I document this (and everything else in my blog) mostly for me--if I write this down I might not keep making the same mistakes.

And for levity (?), instead of only bitching I am going to rate my mistakes with Homer Simpson DOHs.

First mistake: Four DOHs! the ramp wave output was busted, I just got a 15V DC at Output, no saw wave, no audio.

Using my scope I could see the saw out worked before the buffer, but not after.

But why, Scotty, why? I traced everything out--no mistakes. I pulled the SMD chip with a HAR and replaced it, same issue. Turns out I did a butt dumb thing: I had an op amp stage flipped, so what should have gone into inverting went into non-inverting. I missed this while translating the Maxiums schematic from the web to Eagle. I have been making this flub since I started laying out PCBs and I am a bit surprised I missed this when proofreading my work--well, I did. To sum this up:

 The moral: Always check Op amp + and - inputs. Get em backwards on stripboard, PCB, whatever, and the damn thing will never work.

I fixed this by lifting parts, soldering components together above the PCB, and using hook up wire to put things back where they needed to go. After a day on my bench all of that broke (resistors detached from traces, solder connections shorting together, it was horrible) so I ended up creating yet another small daughterboard for the ramp buffer. So much for cutting down on hook up wire? not pretty, but it works.

Second mistake TWO DOHS:  I used the "net names" feature in Eagle to connect a pulse pin connector to the jack trace, but I called the trace "PULSE" on one side and "8" on the other. Naming nets/traces in Eagle CAD can make your layouts easier to follow but you have to get it 100% right. When you incorrectly join nets, Eagle won't tell you you made a mistake--it will dutifully disconnect things and your board won't work the way you want.

Oddly I have never made this net names mistake before now, but Gabba gabba hey! This was fixed by cutting a trace and using a single 22 gauge kludge wire. Now I have a working pulse at output.  Oh yeh, see how pin 10 is unused?  Why didn't I put the ground on that? Instead I had to run a hookup wire for ground. I add that into the 2 dohs calculated. I will fix it next time.

No, Elmo, these 2 traces won't connect!

Third mistake: ONE DOH  
I forgot to connect ground to one of the pins of the rotary switch PCB.  I will let myself off the hook on that, I was kludging stuff here to accommodate the NOS switch replacement for the bang good crapola rotary switches, and I just missed this.  I also forgot to put a ground run into the 10 conductor ribbon. Wait, I already said that. Both easily fixed in rev 2. Yes there will be a rev2.

Fourth mistake: SIX DOHS!  OK this one gets 6 out of five. I left the entire buffer for pulse out of the REV1 PCB design. I have no idea what I was thinking. Did I think it didn't need a buffer? Overall I  wasn't looking at the Maximus schematic carefully enough I guess. I fixed this by digging into my junk box of tiny buffer PCB's and added the circuit fragment you see below as a daughter board to the main PCB. Since this was buffering a square wave I used a junk box CA3140, which may not have made any difference at all.

OK I think that's it. Last night I plugged in the new VCO and it worked very well. I may not make a lot of design mods for rev2, rather just all the fixes, but who knows. Stay tuned.

Saturday, October 12, 2019

Gate Delay Part II: On the Bench--And It Works!

Hello, back from Arduinoland: this time I am continuing work on a gate delay and gate to trigger/gate modifier. I hope this gizmo will allow good control of not just delay but gate duration, and whether or not the gate follows real world note offs--my modular has nothing that can do any of this now. I also have a vague idea that if I can provide some sort or OR and XOR logic I can get get echos, randomness and whatnot.

(Part I, where I ponder the basics of this project, is here....)

The logic I am trying to lay down for this synth module goes like this:

As I was coding this (I used Arduino skretch) I was glad I took the time to write out the logic chart.Getting my mind around the code would have been really hard to write otherwise.

Some of the features I am going for:

  • A pot to control the delay between real world gate's start and when the output gate starts.
  • A pot to control the length of the gate at output.
  • CV control over both the parameters delivered by these 2 pots
  • A switch that says: no matter where the delay pot or delay CV is set, when the real world gate is over, the output gate is over too.
  • Another switch that does the same for the output gate--when a key is let up, the gate goes off, no matter where any knobs are set, any CV comes in, but only after the delay time has passed.
  • I want to fit this on a 1u Frac panel; better yet 2 of 'em in a 1u Frac panel. So the PCB has to be pretty small.
  • LEDs to visually track input and output gate
  • Buffering for input and output gates with 2x NPN transistors
  • Buffering for incoming CV and zener 5V protection 
  • Use SMD op amps to save space

To save space I used an Arduino Pro Mini, a first for me, 5V version. This is an easy Arduino, set it up as described here. Works just like you'd think, and it's smaller; I needed to buy a programming board to make it go, nothing was documented well, as I have come to expect with these Aliexpress knock offs, but I got it all working, soldered on the pins, dropped it into a breadboard ran the blinking LED demo, in about a half hour. Minimal muss and fuss.

USB to serial programmer. To save $$, uses a C340 clone chip. Been there, done that!  

One of the main takeaways: you can hook the V+ and GND from the programmer to the Pro Mini and the ProMini's VCC on board pin will become a 5V source you can use for pots, logic etc. I don't know why it took me so long to find that out, I couldn't find that right away online, I just had to hook it up and mess with it. Yep it works.

So time to port the code I had started in VBB. moved to Nano, tried unsuccessfully to make go on an ATTINY85 (not enough pins) and finally settled on a ProMini.

I built a switch for this:

If any of the 3 switches are down it's 5V, up is gnd.  5V is current limited by 100 ohms; the 5V pullup resistors are 10K.  The "output" are repurposed wire wrap headers; it's easy to shove 100mil headers into those receptacles for prototype madness.

Adding 10K pullups....
It took me maybe an hour to round up all the junk box parts for this, drill out the perf, solder it all together.  May come in handy for other projects.

Once I had this working on the bench and was reasonably happy with the sketch code I spent a couple more days (off and on) messing with the schematic for a small dedicated PCB:

Found out again: micro switches for any electronics project, if you buy quality, are going to be expensive. The two switches here could cost as much as $8USD each from Digikey, no sweat. That's a lot of dough. Not sure what I will use yet for this PCB; after some online searching I ordered 100x ALCO TT11's on Ebay for $100 USD, new they are about $7 each, so let's see if this "lifetime buy" works out. If it does, I'll make a custom Eagle device for the TT-11, as I can't find one yet online. If not, I will have wasted about the same $ as I spend treating my old fart cover band dudes to breakfast at Denny's. $100 gone, but I'll live, which isn't how I feel after breakfast.

OK for this gate delay, here's the schematic so far:

So what's next?  I get the lifetime switch buy by (bye bye?) the end of next week, I have to get the board fab'd, and then see what flubs I made. Once I kludge that up and get it working (yeh) I can maybe make a front panel.  And once it's all going I will post all of this, including the code, on my website.

So there will be a part III! Until then, don't delay breathing the fumes.

Saturday, October 5, 2019

CEM VCO Part I: REF02 Based Octave Switcher

I've been using three DIY ASM VCO's for over 12 years now, and they work fine, but they drift a bit in hot weather, the PWM on one keeps conking out, and they collectively use 9U of Frac, a lot of space.

Time to get some new VCO's going to complement the 13700 VCOs I built a while back....

How about basing them on reissued 3340 chips?

You may have heard 3340's before, they make VCO builds easy and fun: the tough parts--temperature stability, PWM, synch, 1V/octave response, etc., are all sorted within the guts of one chip. These ICs along with other Curtis offerings were popular in the late 70's to early 90's, in the Prophets and OB-X's and whatnot. Then they became harder to find for years; now with modular craziness they have been cloned and/or reissued and are back with a vengeance.

OK let's get building! But!!! Let's not start with the thing that makes noise, let's start with a peripheral if you will--an octave switcher. The MiniMoog has that; the SH5 has that, the MS20 has that..... 

On the SH5 this feature is called "Range". the rotary switch lets you quickly choose octaves: clunk clunk clunk--I like it!

That means somehow producing 0V, 1V, 2V, 3V etc, accurately, and presenting this stepped voltage as a modulation source to the 3340 VCOs.

But how, Scotty, how?

I studied schems for Minimoog and a few others, and based on that, here is the idea:

OK, the reference voltage IC, in this case TI's REF02 5V, feeds a voltage divider consisting of 10K .1% resistors that stay stable (I hope!) in spite of  hot and cold, an octave select rotary switch, and finally a buffer for the whole mess, so, a decent performing op amp wired for unity gain. 

In this case for the buffer I will use a TI OP07--mainly because I got a bunch of OP07 SMT's cheap from Tayda.

Breadboard Time: I had a REF02 DIP in my junk box as well, no idea where it came from but it's been there a long time. OK, let's put this into the unflappable Radioshack learning lab with some 10K 1% resistors. It seems to work!  I can assume the rotary switch won't add any resistance (right?) and wiring up an OP07 as unity gain buffer is a no-brainer, so I didn't bother bread boarding those.  

Seems like the main idea is sound; Moving on....

Whipped up a schematic in Eagle:

....and came up with this board design:

I want to keep each VCO to 2u Frac so I decided to use SMD for as much as I can for this board. Did this to save space and make things more challenging.  

OK sent that off to China for fab, and wham, bam, thank you Mr. Lamm--it's back!

Dude let's stuff that puppy!

( I know--IC2 is backwards! Damn those safety belts...fixed in the photo below)

What you see on the sea of green:

  • The Yellow box out is the REF02. 
  • The red shows the location of the voltage dividers, I used an incredibly tiny 4x 10K .1% 10ppm resistor array  from Vishay (ACASA1002S1002P1AT but you knew that right?) and a 10K .1% itty bitty whatever SMT.  
  • The Blue box shows the location of the OP07 op amp.

Here it is in a vice ready to test:

....and that's where the problems begin:

  • The rotary knob selects the wrong voltages, at least partly because the Eagle footprint I used matched the geometry of the bang good cheapo switch, but not the way the switch is wired up electrically. So what should be pin 3 on the switch is really pin 1, 2 is 2, 3 is 1, 4 is 12, and so on. A mess! I checked three times to make sure the switch would fit physically, but assumed the pin out would match. "When you assume"....OK need to fix that and redo the board.
  • I burned up a few REF02s and one OP07 because there is a mistake in the silk screen. From the photo above: on the bottom left where you see GND: The V- and V+ are silk screened backwards!!!  OH NO!  Easily fixed, just wire opposite the silkscreen, but still....
  • I burned up another REF02 because I shorted out its input supply voltages, this was due to sloppy wiring from my bench power supply. that one was avoidable!
  • OK with all that fixed, here's the bad news: The output voltages are just WRONG. Wrong, wrong, wrong!!!
To address these issues I used a Sparkfun hot air rework station to extract the SMD parts one by one. I lifted some traces under one of the REF02's by overheating, so scratch that entire PCB, but at least the parts can be reused.

The evening is not going well! After a lumbgurius amount of cursing, fear and loathing I discovered  the problem has something to do with the rotary switch. When I removed it from the PCB I could put a DVM on each step of the voltage divider ladder and my DVM would read 5V, 4V, 3V etc, to about 3/500th of a volt, which for me is probably adequate. But why the switch (or the buffer? The traces? Something else?) is introducing crazy errors in output V is a mystery to me. I have to redo the board anyway to accommodate the bang good switch pin outs but still....not sure if fixing that will fix the voltage issue, why would it? 

OK to really knock this dead I confirmed that the rotary switch was at least part issue; after removing it from the board and spinning it I would get unreliable continuity from its inputs to its output. 

Wiring it up back up to the board with pin out errors fixed didn't help things at all, as you'd expect. I felt that it was extremely easy to overheat the switch's contacts, which might have happened when soldering the part to the PCB. I am pretty sure I melted the switch's guts with what I think to be normal soldering. Sorry!

I also noticed that the unity gain buffer daughter board was having its own reliability issues, so I swapped the OP07 for an SMT TL071. To my amazement I found the TL071, purchased from Tayda, didn't have any designation re: where PIN1 was (no dot, no line--nothing) which may account for the bargain basement price of USD35c each. I figured I had a 50% chance of getting it right so I flipped a coin and dropped it in with the mgr. logo facing where pin1 should be--If you were the manufacturer of an op amp would you want your logo close to pin 1, or near the chip's butt? 

OK now blasting something like 1 to 3V DC into the unity gain buffer board's input, the tiny op amp PCB works flawlessly.

But still: Employing the rotary switch you see above the circuit's output remains spotty--works occasionally but most often, no dice. The score so far: Octave board -10, me -30. Neither side is doing well. 

Not one to give up! Let's knock this dead--is it the only switch, or something more?  

I got an old 6 banger from my junk box:

After throwing in this ancient rotary switch the circuit works perfectly!! No hiccups, no dropouts, no issues. 

Test time: I soldered a 3.5mm jack on the unity gain board and a set screwed a chicken head knob on the repaired REF02 circuit's NOS switch, and tested it using a Mutable Links to accurately sum the output of my octave generator and the output from a Kenton Midi to CV converter.  Plugging this into a couple of 1V/octave VCO's the octave tweaker prototype worked well enough, I feel, to proceed with redesigning the PCB for a more reliable rotary switch. I had to bump a VCO's fine tune slightly after changing to a new octave every now and then, but I have to do that now on my beloved 1978 Minimoog and ancient Roland SH5 as well--so: good enough.

Bang-Good-Riddance: It's time to put the cheapo Banggood rotary switches into the e-waste recycle pile. Once again I spent all day trying to fix a crap parts problem and maybe I saved $10 on the "affordable" component. Was it worth it, Elmo? Haven't I asked this same Q before??? 

Oh yeh, and I also have an entire VCO to build. Stay tuned.

Thursday, September 26, 2019

The new "Virtual Breadboard" App and The Delight of the Delay

Note: The backstory about Virtual Breadboard, version 6.0.x, one of my favorite electronics software programs for AudioDIY, can be found in the earlier post here. You might want to glance through that before reading today's blah blah ginger blah blah, otherwise what's here might not make sense.

Hello Again: this time I'll be writing about the "new look and feel" VBB.  It makes getting Arduino projects going on easier than before and as I see it, easier is always good.

You can only get the new VBB app from the Microsoft Store as far as I can tell (and yes, it only runs on Windows). So: Sign up and give 'em the big data.

Link to get the app at the MS store for the new VBB is here. Sorry Mac and Linux users, dig out that crappy Windows 10 laptop, throw some Mutable stickers on it, get on over to Starbucks, and get with it.

Cuentas? The new VBB offering is free from the MS store, and freebee is usable right out of the box--unlike VBB 6.0.x the for-free version includes a working Ardiuno Duo SIM. Yeh!

To get the tired but mandatory blink LED going, follow the tutorial here.  But I can make it even quicker:
  • create your Arduino virtual breadboard with an LED in D13 and power it on
  • load up the example > basic >"Blink" sketch from Arduino IDE (yes you have to use an Arduino IDE outside of VBB still), 
  • compile your sketch to hex (export > export compiled Binary)
  • Find your export (sketch > show sketch folder would be one way)
  • drag the hex file over the Arduino in the SIM.
  • Blink away!
Poof! It's that easy, and for me it worked.

If you change your code, same as VBB 6.0.8: you don't have to power off your SIM and restart it. Copy the new hex over the Arduino SIM icon and changes are seen pretty much immediately. Yeh!

But other than this blog, what is really free? For an extra $30 a year you get more cool VBB components, like a (possibly indispensable) terminal for debugging Arduino using serial.begin() type code, a square wave function generator, a very cool function creator (still want to do a post just about that!) and some other new features I've not gotten into yet.

So with that in mind let's get to some AudioDIwhY.....goes like this: Over the next few weeks I am going to simulate then build a gate delay circuit for modular synthesis.

This is something that could be done with 555 chips or whatever, but as far as low parts count, nothing will beat an Arduino for on-off things like this, right?

SIM of my Gate Delay

Here's the idea: a gate signal is delayed with the "DELAY" pot from in to out, and its length at output is determined by the setting of "LENGTH". So far this is all pretty easy 555-chip stuff.

But here's what's a bit trickier maybe: there are switches that follow or ignore the incoming gate off status and start things over as needed. So if you are in the middle of a delay, the EOGD switch is set to follow, you have nothing at Gate out, and the input gate signal suddenly goes low, the circuit starts over.

Same if you are in the middle of a delayed gate, whose length is determined by the LENGTH pot. In follow mode, as soon as the gate goes low, everything starts over and waits for a new gate signal no matter what. In ignore mode, whatever is happening with Gate off is ignored.

The whole idea is to provide more flexibility with how gate-offs are handled.

How will this sound with real gates, EGs and what not?  I have no idea, but I need some gate delays for my modular rig and am not sure there are any others out there quite like what I describe here.

Let's get coding! Instead of winging this, I drew up a logic chart (click on the full size version below)

I'll be referring to that as I program the SIM.  There may be mistakes....

And one last thing: the new VBB has a lot of the same shortcomings (features?) of version 6.0.8.  This is still primarily an digital only sim, so things like this simulated DVM in this "analog" setup won't work:

But if you monitor an Arduino analog port it will:

Update 10-12-19: I have the gate delay sketch working on the bench.  Part II is here.

Up next: I am sending boards for a 3340 based VCO off to fab soon, so while I'm waiting for that I'll try to write some code for the gate delay and post it. For clarity I probably won't use "pure C" which you can read about here, but who knows. Until the 3340 boards come back: No fumes!

Tuesday, September 17, 2019

Balanced Quad Line Driver--Saving Dough!

For me the why in AudioDIWHY is never to save money. By the time a board gets fabbed, stuffed, tested, debugged, a front panel made and so on, the $USD I fork over to Doepfer or Boss or Behringer for a more professionally made version of the same thing seems like a frigging deal!

But maybe not this time?

(Note: if you need to know what this "balanced audio" thing is all about, a good quick explanation from MOTU can be found here).

The back story: I had four unbalanced synthesizer ties in my home studio that needed to get jacked up about 12db and balanced. I  was using two Aphex 124's for this but one went dead, no idea why.

Both units were getting old and took up more room in my rack than I liked, so I priced out various replacement active balanced audio line converters but all seemed a bit pricey: Intelligel's is about $260USD for quad; $40 for one channel if you want to buy a tiny Intelligel I/O widget that goes into their skiff rack; Whirlwind's offering is about $130 for 2 channels, Aphex's level shifter is about $300USD for stereo (yes, it's I/O and O/I so it will cost more) and so on.  Nothing too affordable here!

Granted some of these have extra features like level meters, but really all I need is to balance the audio and get about a +12db shift. How to do this cheap?

How about this: buy some THAT line driver IC's, RAP2CV jacks and TRS 1/4ers from Ebay, get five PCB's prototypes fabbed in China for $5 USD, hit the junk box, and have at it?  OK!

Is Audio DIY getting old hat? This all feels so familiar! I designed the PCB in Eagle (I had to conjure up a Neutrik TRS package--I added this to my madbean JACKS.LBR file--I can share that if you are interested in adding this TRS part to your Eagle project, comment below.....)

Is that the DHL plane flying over my house with PCBs from China? At 3AM! Could be!!! 

Unwrap the boards, pull the parts, insert, solder, power, test, repeat.

The PCB for this project uses through hole--if I had used surface mount THAT IC's I would have saved another $3USD or so, but when I drew this up I was too chicken. Next time?

The heart of this circuit is a THAT chip, the 1646. Small quantities are available from Mouser for about $4.50USD each; so for quad you are looking at $18. The TRS and 3.5" jacks (you may want to redesign via Eagle for RCA input? TS? Whatever?) are easy finds via Ebay or maybe at a surplus place.

Everything else I used--other than the jacks and 1646 chip--were from the junk box--really all this is is an op amp buffer/gain stage in series with the 1646. Not sure it could get much easier.

Applications. Running audio long distances is best accomplished using balanced audio, so right away you'll get with the THAT 1646. Another common application for an audio line driver is converting a consumer audio device, like a CD player, to the professional "+4 balanced dBu" spec. If you have -10dbV gear that needs to get amped up to +4dBu I calculated that the op amp (IC1 in this circuit) needs to provide about 4x gain, along with gain provided by the THAT IC. I tried to make the design easy to mod--drop in different values in the op amp gain stage, or make it unity gain using the "BYPASS" test point hooked up to the top of R2--whatever you need.

So for -10dbV to +4dBm conversion, try 50K for R5, 250K audio taper for R6, and 100K for R2. Tweaking that on my bench got me close to +4dbU from a -10dbV sine wave test tone.Veeola!

A good article on op amp only equivalent circuits for balancing and unbalancing audio can be found here. For balancing a synthesizer output, which is my ray-zahn-detre, it might be closer to the values you see above: all 100K. That's what I used anyway and it worked.

It only took a few hours to build 2x of these (4 channels total) and it all worked the first time. Yeh!

One more benefit: I have 2 identical boards, 4 channels total, that are easy to A-B.  I decided to put fancy and expensive Burr Brown OPA624's  (since I'm saving money here right?) and 1% resistors in one (I have no idea where the BB's came from, these are all old junk box parts, but I might as well use them?), and ancient but not too horrible CA3140's and 5% resistors in the other. Will I be able to hear a difference? Try different op amps, listen to 'em, argue with your audio geek friends about what sounds better, and have fun right? I doubt it, but I will let you know.

Warning: Butt-Ugly front panel! For testing and tweaking I modified an old 2u Frac panel I had lying around and used Ptouch tape for labeling. No, that's not how it will look if this device continues to get used (so far it works great so it probably will). I am ahead on budget here, so I will spring for a $45ish panel from Front Panel Express.

Yes, if you add up the hours I have with design, stuffing, testing etc. I think it might still be more expensive then Intellijel, but still, maybe not.  Remember that unlike Burr Brown IC's the fumes are free. The mask may not be.

Saturday, September 7, 2019

Skiff to my Loo! My first SKIFF Friendly Synth Module--SEM Style Attenuator Completed

Don't know why, for some reason I had to start using SMD parts and start designing and building skiff friendly modules.

"Skiff" in synth speak means the whole module enchilada is about 30-40mm deep or less.

Wait, do I have to? No! That's the thing about AudiDIWhy, I can do this any way I want. But all the big boys make audio modules that you can slip through a doorjamb, why can't I? The whole world is moving away from depth right?

I already bought some tools for SMD one off fab--read about that here....of course it can be done!  

First skiff is an SEM style attenuverter. Blog post part one, where I stripboarded a butt-ugly but functional (and highly skiff unfriendly) Tom Oberhiem inspired attenuverter is here. Today's post may not make a lot of sense if you don't skim through that first--the CTPOT is a cool part, also I add some easy bias offset tricks....

Or don't.....Screw it! let's finish this SEM sucker off.

First, I used my SEM inspired attenuverter schematic in Eagle (used to create the strip board mock up) to create a PCB for the project. The Thonk 3.5mm jacks for Eagle come from the very useful MusicMachineModular lbr file, which you can get here. Thonk you.

This pretty easy stuff: the only tricky thing was finding an Eagle device definition for the center tap pot. I searched high and low and couldn't find one so I had to create the device myself, a tutorial about how to do that is here:

The center tap pot....

In Eagle the CTPOT now looks like this the dude on the left:

For 10 to the -50 percent of the world's population: I can post the Eagle device library (lbr) with this handy pot included. Comment below. Not holding my breath.

OK with this Eagle part good to go, I whipped up the board:

Also, I had to fab a small daughter board for the CTPOT since it sat too low on the  PCB.

The board accommodates 2x CTPOTS, so for this project I sawed the board in half.

Now what? I realized after sending the gerbers off to China that I should have laid out in the pots and jacks for the main PCB in a more logical way--say every 200 mils or whatever. That would have made laying out the front panel a lot easier. But I was in a rush and I didn't. I'll deal with that later....

PCB is back from China, let's get the board going--I laid in the op amps and a few resistors (through hole--not ready to go all in w SMD I guess?)

Jacks etc? Yep. With Skiff there is no wiring, or not much, so it's a fast build. But the board didn't work first time because the TL084 was soldered in upside down. Arg! I wicked it out, tossed it, and tried again. Joy: now the attenuverter works.

Independent bias offset on each side; normals for easy inverting of whatever. The joy of fuming!

Front Panel Fu: Since I didn't lay the pots and jacks out in a smart way, they landed on the PCB where ever, I ended up taking caliper readings of all the dimensions of the completed skiff board, laid out a potential front panel in Illustrator, and printed out a mock up on a paper sheet, finally cut it out with an xacto. That way I could see if my measurements were butt.

After about 8 tries: the paper mock up fits. Joy!

OK final step is a FPE panel for this. Yep, 10 days later it's back and it fits.

Done!  You can get Eagle files, etc etc for this on my webpage, here.

OK on to whatever is next....

Overall I'm sorry to say I put this SMD stuff off because I was scared to try something new. Turns out it's probably easier and quicker to fab your DIY projects this way in some situations vs. through hole and hook-up wire. And you can use your finished module as a bookmark.

Live and forget?

OK that's it, until next time: don't skiff the fumes.

CEM VCO Part II: Maximus Troubleshooting, and the DOH! Count

From a couple of posts ago : My ASMVCO 's are getting old and drifting in pitch.  Might be time to ditch 'em. how about three new VC...