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 ("MRT's").  If the module works OK for a couple of weeks I'll create a FPE  front panel for and retire the kludge panel.

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: I am using 3 interconnected PCBs 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 build 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. A good page on the Maximus 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 told me 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.

So how did rev one of this module do? Considering the complexity of the board, happy to say: not too bad; I got the basic VCO to work in about a day. I had to kludge things, I made several mistakes that needed fixing for this revision, see below, 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. The VCO 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.

A

"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 it works, but, 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. Let's begin:

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. "I only give negative feedback!" 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 connect things as instructed, but 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 yep--it all works. I may not make a lot of design mods for rev2, rather just incorpoerate all the fixes, but who knows.

UPDATE: I had more PCBs made correcting these mistakes, new ones don't need kludges!  Yeh! post is here.

UPDATE 11-23-19 After a few weeks of having this VCO in my rack the top end v/oct scaling was still not quite where I wanted it to be (it was close, but was still a bit off when I went say 5 octaves above middle C).  After a fair amount of messing with the trimmers to set low and HF scaling, I replaced the 12 turn trimpots with Bourns QV36's, which are 25 turn.  They cost $3USB each from Digikey but it's worth it. Now with the more precise timmers the VCO scales much better, so I am going to add that to the BOM--25 turn pots.

UPDATE: All well and good, but this initial VCO ended up smoking and almost burning down my house. Too many kludges! Glad I was there to cut its power. Otherwsie I'd be writing this update from the public library's public PCs. Post about smoking this VCO is here.  

Stay tuned.

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 than a nano; I needed to buy a programming board to make it go, nothing was documented all too 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 and ran the tired ass 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.  UPDATE 11-8-19 the TT-11s are here and appear to be exactly what I need. I'll check the PCB for this project and the design a few more times, will send the board out for fab, see if it works, and do a "post part III" for this. 

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.  

UPDATE 12-12-19 it works!!  Gate Delay Part III, where doris gets her oats, is here, the website write up for the project, with schematics, gerbers, etc., is here. Front panel Express Front panel is done and fits, yeh, no more ptouch dreck.

Until then, don't delay breathing the fumes.

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 breaks, 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 Voltage Controlled Oscillator ICs?

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 one IC. 

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 the resurging popularity of modular audio synthesis they have been cloned and/or reissued and are back with a vengeance.

Let's get building! 

Let's not start with designing 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 chunk chunk--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 in spite of  hot and cold, an octave select rotary switch, and finally a buffer for the output voltage, which means a decent performing op amp configured 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 at a good price from Tayda.

Breadboard Time: I had a REF02 DIP in my parts box as well, no idea where it came from but it's been there a long time. OK, let's put that chip into the unflappable Radioshack learning lab with some 10K 1% resistors. 

It seems to work!  I can safely assume the rotary switch won't add any resistance and wiring up an OP07 as unity gain buffer is simple, so I didn't bother bread boarding those.  

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. I did this to save space and make to things more challenging.  

OK sent that off to China for fab, and it's back:

( I know--IC2 is backwards...fixed in the photo below)

What you see:

• 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) and a 10K .1% SMT resistors.  

• 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 began:

• The rotary knob selected the wrong voltages, at least partly because the Eagle footprint I used matched the geometry of the banggood rotary switch, but not the way the switch is wired up electrically. So what should have been pin 3 on the switch was really pin 1, 2 is 2, 3 was 1, 4 was 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 I needed to fix that and redo the PCB design.

• 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.  DOH! Easily fixed, I wired it opposite the silkscreen, still....dumb mistake.

• 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 was the bad news: The output voltages were 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, thus ruining the entire PCB, but at least the parts could be reused.

After a lumbgurius amount of cursing, fear and loathing I discovered the problem has something to do with the rotary switch itself. 

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 was probably adequate. 

But why the switch (or the buffer? the traces? something else?) was introducing crazy errors in output voltage is a mystery to me. I had to redo the board anyway to accommodate the banggood switch pinouts but still....I wasn't sure if fixing that would fix the voltage issue. 

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. 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 thought to be normal soldering. Oops....

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 bemusement 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? UPDATE: Not Tayda's fault, **my fault for not researching this carefully enough**. the Tayda chip has, upon close examination, a sloped side and a flat side. The sloped side "points to pin 1" on SMD chips. Read about how to find PIN1 for SMDs on the webpage here.

OK now presenting 1 to 3V DC into the unity gain buffer board's input, the tiny op amp PCB worked flawlessly.

But still: Employing the rotary switch the circuit's output remained spotty--it worked occasionally but most often was unreliable.

Not one to give up....I got an old 6 position switcher from my junk box:

After throwing in this ancient rotary switch the circuit worked 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 felt, 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 bin. Once again I spent all day trying to fix a problem with a poorly made component and maybe I saved $10 on the "affordable" component vs. something more reliable.  Not a good decision.  I am finding good hardware is worth the extra money.

And I also have an entire VCO to build. Stay tuned.

UPDATE: I got new boards made for the REF2 based octave switcher design. For this I planned on using prepurchased, relatively expensive switches: NKK MRK206's. There is an eagle symbol for it, that's one of the main reasons I used this switch vs. all the other ones you can get from say Digikey.

Got the boards, got the switches, guess what! The expensive switch DOESN'T FIT!  Turns out the NKK MRF206 symbol fits the MRK206 switch and visa versa.  I think whoever was creating the NKK library for Eagle got the 2 parts mixed up. 

Go figure--the lesson learned again: never trust an Eagle symbol! Print out the PCB before fab, lay the parts on top, and make sure everything lines up.  Oh well...toss the boards and try again.....

Pins don't line up!

UPDATE 12-8-19: PCB has been redone (again) to fix the pin issue above. For the new board everything lined up and worked. But--a design flaw I hadn't considered--1V, 2V, 3V all all within 3/500mV to each other, but GND is one of the selections as well, and it's not privy to the relative tuning coherence for the rest of the output V's.  So 1V is 0.95666, 2V is 1.95623, 3V is 2.95362, etc., which is fine, since there is a "master tune" tweak on the main VCO board, but also, GND is -6mV!!  I am thus concerned that the "2 foot" setting will be out of tune with the other octave settings. I can't see why it won't be, but I guess we'll see.

Finished Octave switchers, with NKK fancy 6 position switches.  

UPDATE 1-5-20: not a bug a feature! I cut the trace allowing me to choose ground as one of the "V/octave" settings and wired a jack to it. See the update immediately above--GND is not in lockstep with the switched voltages so using it as a switch setting was not feasible. But 2' or whatever isn't a useful setting anyway. Now, with this EXT switch in play, we have LFO, "external", 2', 4', 8', and 16'.   

UPDATE 1-22-20: VCO is done and the ref board (with the mod above) works! More info in the finished VCO post, here.  Part II of the VCO build, where I troubleshoot stupid mistakes on the main PCB, is here.

UPDATE 3-22-23: Hope this is the last update?  I just realized a dumb mistake I have been making for my VCOs incorporating the switch in this post: in the pipe organ world, 2' is the highest frequency, not 32' which is the lowest!  So, the panels I made have this silkscreen on incorrectly. Doh!

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. Update--fabbed, stuffed, and works, see post here.

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!

Balanced Quad Line Receiver

For me the "why" in AudioDIWHY is never about saving money. By the time a board gets fabbed, stuffed, tested, debugged, a front panel made and so on, the money for a Doepfer or Boss or Behringer for a consumer version of the same thing might seem cost less.

But maybe not this time?

Finished Module with FPE front panel

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

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

Both Aphex 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 a +12db boost. 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.

Is Audio DIY getting old hat? This all feels familiar. I designed the PCB in Eagle (I had to conjure up a Neutrik TRS Eagle device--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 parts--if I had used surface mount THAT IC's I would have saved another $3USD.

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?) are easy finds via Ebay or maybe at a surplus place.

Everything else I used--other than the jacks and 1646 chip--were from my parts 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, and 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.

A good article on op amp only equivalent circuits for balancing and unbalancing audio can be found here. For balancing a synthesizer output, you may need to use the resistor values you see above: all 100K. That's what I used and it worked.

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

Still need to add bypass caps, do that after the basics work.

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?) and 1% resistors in one, 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 them....Update: can't hear any difference. Maybe it's just me, my ears, too much loud R&R. Perhaps it's almost always OK to use whatever you have in your junk box as long as it doesn't smoke.  

Butt Ugly!

Warning: Butt-Ugly front panel! For testing and tweaking I modified an old 2u Frac panel 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. 

Update: FPE front panel is done--see the opening photo for this post.

UPDATE 12-28-19: after finishing this board, I found out that Sparkfun has something like it: here. No op amp to adjust incoming gain though; all SMT. 

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.

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

Update 9-23-22: I was unhappy with the way I laid out the bias+ and bias- buffers subcircuits in the circuit below, so I redesigned the attenuverter PCB.  Build the newer one--not this one--I think it improves things in terms of flexibility. Post about the rev 2 PCB is here.

For some reason I have started using SMD parts and start designing and building skiff friendly modules.

"Skiff" in synthesizer terms means the module is about 30-40mm deep or less.

That's the thing about AudiDIWhy, I can do this any way I want. The world is moving away from depth?

I already bought some tools for SMD one-off fab--read about that here....  

This time I build an SEM style attenuverter. Blog post part one, where I stripboarded an 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 an interesting component, and I added some bias offset tricks to the original design.

 
First, I used my SEM inspired attenuverter schematic in Eagle, capturing 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).

The only slightly 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:

 

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

Also, I had to fabricate a small daughter board for the Centertap Potentiometer since it sat too low on the  PCB:

The board accommodates 2x CTPOTS; 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. 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:

With Skiff there is no hook up wiring so it's a fast build. 

But the board didn't work first time because the TL084 was soldered in upside down.  I wicked it out, and soldered in a correctly oriented replacement. Joy: now the attenuverter works.

Independent bias offset on each channel; normals for easy inverting. 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 took caliper readings of all the dimensions of the completed skiff PCB, laid out a potential front panel in Illustrator, and printed out a mock up on a paper sheet, then cut it out with an Xacto knife.  

After about 8 tries: the paper mock up fits. 

Final step is a FPE panel for this.

Ten days later it's back and yes, it fits.

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

 

I initially 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 instead of using through hole and hook-up wire. 

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