CEM3320 Based Voltage Controller Filter. RTFM.

"Read the freaking manual"--"freaking" may not be the right word.

This last week I built my second clone of the Sequential Pro-One VCF, reconfigured slightly for Eurorack:

After 2 revisions: works but more effort needed.

ABOUT THE IC

The CEM3320--the IC upon which this post's voltage controlled filter is based--is a strange beast. 

Read about its developer here; essential reading about the CEM3320 can be found on Electric Druid's page, here.

Specifics about the CEM3320 and the Pro-One, a classic mono synth that used this IC, are also found on Electric Druid's site, here. 

Mod wiggler pages focusing on this IC are found here and here.

PAIA (one of my favorites) made an experimenter's board for the CEM3320...cool! Get the PDF here.

And--didn't find this until a few days ago--a schematic for a complete DIY Pro-One VCF can be found on dslamnig's site, here.

Fortunately finding 3320's for sale is easy. Original early 80's CEM3320's are unobtanium, but there are clones and work-alikes available from CoolAudio and Alfa, and I read that CEM has reissued it.  

UNDER THE CERAMIC

To learn about what makes this chip go: two datasheets are found online--the 2 pager (here) and a 6 pager (here).  

At the heart of the 3320 lies 4 active filter building blocks--a current to current gain stage, whose input is held a diode drop above ground, feeding a pin to tie a capacitor to ground or to an input source, and after that, a buffer. 

A feedback resistor ties output to input. 

With no signal at input, the gain stage is set for .65V (diode drop) at input and .46*VCC at output. Ohms law reveals that for 15VCC, Rf should thus be 100K. Details here.

Resonance is taken from the IC's capacitor-coupled output and fed back into the initial gain stage. 

The amount of resonance feedback is controlled by an OTA--in this case, easier to implement than say a CA3080; current limit the OTA's resonance control voltage to a 0-12V DC signal and present it to Pin 9.

In general, even after reading, the 3320 didn't make a lot of sense until I redrew the layout for a few common use cases' resistor configurations. This information was already available online, but the exercise helped me better understand the IC.

Bandpass. Yes, the CEM3320's 4 active filter blocks have odd pin assignments.

Highpass

Lowpass

THREE TIMES' THE CHARM?

I have already created 2 revisions of a Eurorack Pro-One type VCF, sent them off for fabrication to this blog's ever-humble sponsor, PCBWAY, who sent PCB's back to me lickity-split, to see if my layout yielded a working filter or a smoking piece of charcoal.

For the first revision: sadly, its PCB layout turned out to be a non-starter because it contained a stupid error: the Pro-One schematic indicates that these 3 critical resistors get tied to the negative rail:

 .....but I tied them to V+.  

Very Nice!  

This foobar was indeed beyond all repair--it would have meant too many trace cuts and fixes. The REV1 build went into E-waste.

Fortunately after a redesign the good folks at PCBWAY rushed Rev 2 PCB's back to the USA.  

Double happiness is getting boards and stencils from the blog's patient sponsor, PCBWAY....please help this blog by checking them out.

 Stencil + paste + SMD parts, then I put the PCB on a hotplate. 150C > 215C > 150C did the trick.

Parts is parts

Testing....

More testing....

So--did REV2 work?  

Lo and Behold. Nope. 

If I had read the frigging datasheet with a bit more care it might have, but, well, I didn't.

In fact, looking over all the links above, what I was thinking? There was a ton of information about how to wire up a CEM3320 and I apparently ignored most all of it.

One critical detail missed: to control VCF's cutoff frequency you send the CEM3320 an inverted control voltage. Meaning: lower voltages meant higher cutoff frequencies while higher voltages meant a lower cutoff frequency. 

The suggested CV range is 155mv to -25mv. 

Did I read this before starting the layout and build?  Nope. 

For revisions one and two I had a CV mixer at input, then inverted the inverter:

For revision 2 I used a 24 gauge jumper wire to go from R4 to pin 6 of the TL074:

  

Which ended up looking like this:

                                                          

With this fix and some component value changes the filter now worked--it swept, got the famous shimmery resonance sound, and sounded funky with fast decay.

But the wiring fix bugged me; I finished a 3rd revision to clean up this mistake.

To not have to do a major PCB redesign I left a quad op amp on the PCB, even though only 2 op amp stages are used. Sorry.

The modified REV2 sounds pretty good, but not great, it may be a bit too bright with some unwanted grittiness when resonance is approaching oscillation.  

I noticed that changing the voltage rails (from -12V to -9V, from +12V to +15V, and so on), made the filter sound, well....different, and every now and then, to my ears, better; more tweaking of REV3 is in order. 

I'll do that in the next few weeks and post again. 

Until then avoid the CEM-icals.

AD5761: Arduino Library and S/H Mux--one worked, the other sucked

 

Two goals since last time: see if I could create an IC library for Arduino sketch, which was surprising easy, and breadboarding an Arduino based bipolar audio Sample &Hold multiplexer, which I never got working. 

A basic sample and hold circuit, aka S/H or just SH. The HOLD COMMAND is provided by a microcontroller switching multiples of this S/H circuit one after another in rapid succession, allowing a single DAC to produce multiplexed output voltages.

At the bench I used an AD5761 16 bit bipolar DAC IC (previous post here), an Arduino Micro, a library for the DAC I wrote for Arduino Sketch (get the library on github, here), and, for the mux switch, the ADG413 analog switch featured in previous post here.

Writing the sketch library took a lot less time than anticipated.  

I used the Helge Langehaug's sketch here as a starting point and combined it with embedded C code created previously--post is here. 

To get the C library working with an Arduino sketch IDE:

• I put the IC C code for the AD5761 into a single .h file: AD57651
• I put the .h file into the same INO directory and the setup() n' loop() Arduino .ino sketch file.
• I #included the library file at the top of the sketch's INO file
• I compiled.

Worked first time--after initializing the DAC I could enter values into an AD6751_out() function and sure enough, correct voltages were present on my scope. 

You can get the working library file and a sample .ino sketch on github, here.

Whole thing took less than an evening to craft. Joy!

With confidence soaring I used the same library to create a 16-bit four-channel sample and hold. 

It's the idea found in this previous post, but employing Arduino sketch .ino/.h files instead of embedded C.

Guess what: the AD5761/ADG413 based S&H circuit never worked. At all.

I could get pretty steady sampled DC voltages out of the circuit, but the voltages at output were never close to being accurate, especially when voltages went below ground.  

I speculated that the issue was a combination of trying to breadboard the design (I read that accurate S/H circuits need to have caps and buffers close to one another which didn't happen here), using the AD5761 in bipolar mode instead of 0-10V, which I hadn't tried in a mux design before, and the lack of precise timing in the world of Arduino--I constrained my sketch to use only delay(), digitalWrite(), Serial.begin() function calls and the rest, in the best Arduino fashion; I could have called out Atmel registers in my code and slammed away, but I might have well have used embedded C for that.

And/or some really stupid mistake(s) I never figured out.  

After a too many evenings (four? five?) I gave up; easier to use unipolar dacs then level shift them with op amps.  

Or, use 3 AD5761's in parallel.

Or, buy a 4- or 8-channel DAC with the mux built in.  

Or, whatever.....

Thinking I had an issue with the ADG413 I tried a CD4051 Mux. Nope, that sucked too.  

The S/H worked, sort of, but the voltages were never close to accurate. I stripped things out and tried to use a Salae logic analyzer to sort the issue. Nope, it wasn't SPI.   

OMG, too many wires, too much mess.  

Overall: breadboarding is NO FUN. 

I am probably returning to ditching breadboards and getting small quantities of PCB's for experimentation from the blog's faithful and patient sponsor, PCBWAY. 

If said PCB's don't work because my design is crap, well, I will pitch them and try again.  

Even with the tariffs, I think time- and money-wise I would have come out WAY ahead this time. 

Overall a win in library land, but this was a lot bench time spent chasing what I thought was a cool idea with zilch to show for it. 

Enough! See ya next time.

Analog Devices ADG413--Incredible Audio Switch IC

Two of my favorite social media (S&M) things: IMSAI Guy's Chip of the Day vlog and finding a great surplus electronics deal on the web. 

This time I do both: my own chip of the day (more like chip of the quarter?  I blame my day job...): an audio/video frequency switch IC I found online at a great surplus price, Analog Device's ADG413BR.

MUX AND FLUX

Audio/digital switch IC's--MUX'es or "multiplexers"--are extremely useful for what we do. The classics contain them; inside Roland's Jupiter 8 (schematic here) and Oberheim's OBXa (here) you find CD4053 and CD4051 mux IC's. 

Details about MCU control of this IC family can be found here and here. 

The ADG413 is a 4-channel switch with superb specs: 50V rail-to-rail supply range compared to the CD4051's 20V, for instance.  

The problem with these high performance switch IC's is their expense. Digikey US sells the ADG413BRZ for $9.08USD in small quantities, compared to 79c each for 4051's on Amazon. 

I'll stick with CD4051's?

Not so fast!! An AudioDiWHY favorite, The Electronic Goldmine, had ADG413BR's on sale recently for $1.79USD each. The 413BR is an obsolete version of the current BRZ IC. The only difference I could see was that the surplus parts were not RoHS compliant. 

(Better to use non-ROHS parts than throw them away--but if they have to go, please get rid of them sensibly....)

An aside: I love The Electronic Goldmine. Hobbyist-friendly surplus stores are a dying breed with many of the great ones like Haltek long gone. Goldmine keeps fighting the good fight. Sign up for their 4AM daily email flyer (here)--good way to start your work day. 

Lifetime buy....

The IC's arrived quickly. I got them ready for the bench.

SOIC: an adapter was needed for breadboarding

Ready to roll....

QUICK WORD FROM THIS BLOG'S SPONSOR

Many thanks to PCBWAY for sponsoring the AudioDiWHY blog and for all the help they've provided over the years. You can help out this blog in a big way by checking out PCBWAY, here. 

As soon as I get my first ADG413 based PCB laid out I'll send the gerber to PCBWAY along with any other fabrication needs, including 3D printing and assembly. 

PCBWAY is super fast, super friendly, and does great work. Please consider using PCBWAY for your next audio project. 

MAKING BALLOON ANIMALS

With two ADG413's soldered up, time to see if I could lay out basic switch circuits.

I still hate breadboarding....

....but I'm slowly getting better. I have found extreme patience, along with using a quality breadboard and checking my work early and often, gets the job done.

Using the 413 to switch LED's off and on (doh). Seems working!

 

Wiring used for ADG413 tests. "IN" represents logic inputs. Yes, the 10K pulldown was necessary; I used 12V for Vdd, GND for VSS, and 5V for logic but found the IC accepted a surprisingly wide range of voltages for rails and logic. Nice! Also, Pins 6/7 are one of 4 I/O pairs and for all I/O audio or DC voltage can flow source to drain or drain to source, makes no difference. 

For breadboarding sanity I used a CD4069 hex inverter for ADG413 pins "IN2" and "IN3". Using an MCU control I'd do this inversion in software.

 A few ideas....

The ADG413 let me create common switch configurations without needing a lot of logic signals:

1-to-4 multiplexer:

Dual SPDT's:

4 SPST's:

DPDT:

1:8 multiplexer using a 74HC595. Use more IC's and get as many channels as you want.

Good tutorial for the 74HC595 IC is here.

OUTTRO

OK enough for now. I bought a lot of ADG413's so (hopefully) they make it into future designs--hours of fun! 

In the meantime, ROHS well, AI well, and above all: don't breathe the fumes.

Dual 13700-based LFO: Mo' Hotta, Mo' Frya

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

========

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; 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, 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? 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! 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 fine gauge 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.Flipping the TL074's is a clever fix but energizes my OCD buzzer.

I will have another round of boards fabricated and try again. Stay tuned, and don't breathe the fumes.