Monday, December 10, 2018

ReverselandFill "Aconitum" Noise Board--Augmented with C1406HA VC Attenuation--Finished!

Hello again fellow readers and audio nerds.  This post: Time to finish off the Reverselandfill "Aconitum" board for my DIY modular synthesizer.

If you want to skip what's below and just hear what the module sounds like finished, that's here.

Also: Part one of this post is here. You are going to want to at least skim the first post or the rest of this one may not make much sense....

Getting to it: after a PCB swap with Reverselandfill's Martijn Verhallen he was nice enough to send me the eagle schematic for his analog noise mixer so I could understand how it works.

His "Aconitum" noise board is designed something like this (it's more complicated, but this is the basic idea):
Simplified block diagram of reverselandfill's Aconitum noise board

An NPN "avalanche" transistor gets sent through a couple of extremely musical passive variable filters, then mixed with two inverting op amp stages in series. There are accommodations for jacks and pots on the PCB, making it a good fit for your Eurorack modular synth.

NB: Martijn must have worked really hard on this design...."box stock" it's a really good sounding board. The filters yield extremely useful timbres for things like rain, wind, surf, everything between, all the way to frying bacon and back.

But "in spite of good tone, I can't leave anything alone?"  For extra headaches I stuck C1406HA attenuator SIPS in series and added a 4th input for external sounds like this:

I could have used VCAs for this.... but trying something new was more torture and thus more desirable.

See more about C1406HA's from my last post about this project; in general the C1406 is an easy to implement voltage controlled dual attenuator SIP IC--in goes audio and CV; out comes audio levels based on CV--the more CV, the more attenuation.  It's like a VCA that can only cut amplitude, not boost it.

One of the challenges for this build (other than some bonehead mistakes on my PCB--more about that in a bit) is that I use FrakRac and Martijn, like many more sane DIYers, uses the much more popular Eurorack format. So I had to piggy back my board below his to make this fit in my rack:


Thank goodness for 4/40 hardware?


Once I got all that bolted together, I separated out the boards and panel and got to work with stuffing and wiring them.

Aconitum board is on the right; my custom panel and quad attenuator PCB on the left.

Let's get building (finally): First I wired up the Aconitum board and powered it up so I could hear it:



Good news here: Martijn's board is flawless--no mistakes--nevertheless, he emailed me a few times to say he is improving it....cool! Martijn also PM'd me to say that he owes the legendary Thomas Henry and Ken Stone for design ideas for the Aconitum board.....if you get your hands on this board, for me it worked with +/- 15V without any mods, even though it's designed for +/- 12V.

Next I wired up my REV1 C1406HA quad attenuator PCB fresh off the DHL boat; and this is where my problems began:

The original schematic--anyone interested, I will post it in Eagle and PDF....


C1406board Rev1: a Plague of Locusts.  Here I've already fixed the power silkscreen issue.....

I rushed this design and fab and made some really stupid mistakes:

  • the V++ and Ground silkscreens on REV1 of the board are drawn backwards. Really?  Um, yep! Traces are OK but the silkscreen says to wire the power supply's V++ to Ground and visa versa.  Doh!! Nothing blew up, but my board obviously didn't work at all until that was straightened out. It took me an evening of head scratching to figure the errors of my ways....it was such a dumb mistake it took me a while to realize, hey, everything is backwards!
  • I forgot to put 100K resistors at the "output side" of the 1406 output decoupling caps. Since these caps are tied to a TL084 (super high impedance inputs right?) there was nowhere for the current to go so the decoupling basically didn't work at all, and I still had a DC offset at output. That's something my first tech boss always told me: always give current somewhere to go!  You'd think after 30 years I'd get this right, but, nope. OK kludged some 100K's on the bottom of the board....now the cap coupling works--fixed.
  • the C1406HA can't really support 10V P/P.  So its input needs to be gained down, while its output needs to be gained up. That remained unfixed; I just have to watch pegging the C1406, but I guess sometimes the pegging doesn't sound that bad (but often it does).  I'll put 6db gain at output I think on the next run. See R21 circled below--this is the standard 6db gain op amp configuration, that's the idea.

Some schematic detail about the resistor issue:


You need resistors (R13, R14, etc) to have upstream capacitor decoupling work!

...and my original design didn't have these!


OK I kludged/fixed all those mistakes.  Yes it's a pain to have to kludge prototype boards, but perf boarding all this would have been way more work.....

For those who want to share the pain: I have a revised board ready to be fabbed--as usual, any takers on trying the improved one out? no takers? OK I will get to checking REV2 one of these days, but the REV1 board works, anyway, after I have fixed all the DUMB ASS MISTAKES. I might make a smaller stereo version as well....

Moving on:





I wired the two boards together--a lot of wiring, but nothing too difficult.  

All done but no panel decal; ready for testing

The good news is after all of this I got (I think) some really good sounds out of this board.  Take a listen on Soundcloud here.

Details about the demo:

As usual the demo was made fast, fast, fast.  If I can do this in a few minutes, you should be able to better in say 15 minutes right?

00:00 to 00:28
: Surf part I. Each noise source in Aconitum went though an its internal passive filter then through the Aconitum's built in mixer. The wave crashes were produced by a slow random gate generator which was fed into an ADSR, which was then used to produce CV; that CV attenuated noise ("Tone1") using a C1406HA CV controlled attenuator. Tone1 was set to a very bassy, boomy filter sound. NOTE: I cheated a bit on this one: a tiny bit of phase shifting, reverb and bandpass filtering was used in series with the output to make it the aconitum sound a bit more "other world" but you can get good surf sounds stock right out of box (listen to the last cut of this demo?).

00:30 to 00:45: LFOs feeding the 1406 attenuator. Each of the 3 Aconitum noise sources had a different LFO input attenuating noise. The output of the Aconitum's mixer section went into a vactrol bandpass filter, getting rid of very high and very low frequencies so your speakers and ears aren't destroyed.  With some screwing around I probably could have gotten close to the Apocalypse Now "Helicopter Whup sound", maybe for another day.

00:45 to 01:00 same thing as 30 to 45, but different LFO settings. The "sprinkler" sounds were created by using a CV controlled LFO.

1:05 to 1:30: I put a drum machine snare and kick outs into envelope followers and used that to trigger ADSRs which fed the C1406HA attenuators. The kick and snare noise sounds you hear are substitutes for a 707 kick and snare. The hat sound was fed into the "external" attenuator channel and passed through to the aconitum's mixer.

1:35 to 1:45: A byproduct of setting up the gun range sound (which comes next). Fast Random gates hitting the noise attenuators and also throwing the sound into a stereo panner that was very quickly panning between L and R channels. Produces a frying bacon sort of sound.

1:45 to 2:15:  The gun range. Each output of the aconitum (noise, tone1, tone2) was fed into 3 VCF's and 3 VCA's.  The VCA's were modulated by ADSRs, each of which was driven by its own random gate. this went into an external mixer and into my recorder.

2:15 to 2:40: Same as 00:00 sound but I changed parameters to make it sound more like traditional surf.

BACK TO THE BUILD: Final step after testing and seeing what it can do was to decal it.  


Ok that's it for now.....Overall this modified board swap module was a lot of work but I had a lot of fun building it, and it's already become a really useful addition to my DIY modular synth. I think it sounds really good!

Not sure what I will work on next but the MCU world is calling me again. My girlfriend (who btw is a mental health professional) says I have become too obsessed with DIY, and I need to take a break, so I am almost done w/ week one of a two week lull.  Did I breathe too many fumes? Maybe! And always remember: "Sanity is relative".

Thursday, November 29, 2018

FrakRac +/- 15V Power Supply, drop in Replacement for the Original

Hi again: Quick one this time.

When it came time to pick a format for my modules, I had a lot of choices: Doepfer's "Eurorack"; "5U"; and so on.  I went a bit outside (I do sometimes) and chose PAIA's offering, FracRak.

Maybe because it's about the same physical size as the API 500 "Lunchbox" format; 500 series is a good compromise in size--the knobs aren't super tiny and the panels aren't huge.  With Frak or the 500 series you can throw your rig on the front seat of your car without issue, or just get your girlfriend to carry it on her lap?  Try doing that with a Moog 55, right?

Or maybe because a PAIA 2700 kit got me into this whole thing to begin with when I was like 12?

Because I spoke to John Simonton in 2005 and he was one of the nicest guys you could ever talk to?  Yeh that's it.

OK good news is that Fracrak is pretty cheap n easy for the case and power supply (which using their 9700 universal power supply, could be built as +/-12 or +/- 15).

For non-critical apps I wanted something a bit beefier than PAIAs "wing" 9770 supply I whipped up my own drop in replacement for powering FraKracs:


The clone board--bolt it right onto your FrakRac case.....
My design has the advantage of being able to add heat sinks then screw them down to the PCB vs. have them "flap in the breeze", offers a bit more cap filtering options, slightly better reverse voltage protection for the regulators and some other basics. This is a $5 for 10 PCBs fabbed in china type deal--why not right?  It probably won't work for super critical applications, say a bank of analog VCO's.  For that you will need better regulation, but for the basics this quick replacement gets the job done.

the original PAIA supplied PCB.  Super low parts count--them Voltage Regulators are looking shabby.....

For this powersupply you can get more information on my website including gerbers, PDF of schematic and board layout, BOM etc.



OK, If you want something more stable/better for VCO's let's say, I'd suggest sticking to PAIA's 9771 Psup...maybe I will modify the design above for more beef as well....that said I have built this board a few times and have successfully mounted it inside some of my FracRaks. Works.....

About the board: The cap values are not critical--I have used 1000uF or 1500uF for C3-C4, 470uF or 100uF for C2-C6, left off C1 C5, 100pF for the caps close to the output headers, etc. I mostly use up whatever I have in my junk box that's reasonably close value wise to what's here.  Good enough.

If you are a more advanced DIYer and want to get a freebee of this board, personal message me through the electro-music forum (my handle is cslammy).  Maybe we can do a swap?

Coming soon: More noise projects with voltage controlled attenuators.

Friday, November 16, 2018

Synth Gates, Interrupts, and Arduinos; a Million and One Uses

Yes once again: more Arduino Control Voltage Fu.

Continuing from last time...I am focusing on another corner of the Arduino for audio world:  interrupts. The idea; your code is running, we want to interrupt things (say, a note has just been pressed) and have something happen (say, play a note). When you are done, go back to whatever you were doing (like, wait for another note).

With further ado: the coding examples here were tested on an ATTINY85. All the interrupt information you see is available for most if not all Arduinos but the code will be different (because, for instance, a Nano has more than 8 pins).

Goes like this: When I started using Arduinos for Synth DIY, it quickly became clear I needed to figure out a good way to accept a gate input signal and control my code with it. Duh! We synth guys use 0-5V gates all over right?  But let's not create a loop like this: did someone press a key yet? No? How about Now?  No! Um, Now?  No!!!! Now?  Yes!!!! Etc. all that waiting around ends up sounding, well, not very musical.

Instead, you want to press a key, and then as close to real time as possible, you want something to happen.

So: what's a really good way to do this? Use interrupts....specifically change pin interrupts--then using a boolean variable to trap if the gate signal was going from 0 to 5V or from 5V to 0. That worked great for me.

Turns out it's not hard to stick all of this into your ATTINY85 sketch, once you know the secret:

First declare this:

void setup()
{

    // (pin declarations, variables, whatever blah blah....
    //but then this...

    GIMSK = 0b00100000;    // turn on pin change interrupts
    PCMSK = 0b00010000;    // turn on interrupt on ATTINY pin PB4
/* you can enable more than one pin for this.....
 a "1" in the above PCMSK line means use pins as interrupts, following this logic: 0bxx54321 where
the "54321" business are physical pins on an ATTINY. 
*/

    sei();                 // enables interrupts


Wait a minute, what are "pin change interrupts"? Information is here but: Easy! 

That means, you're using external hardware interrupts--change the voltage present at this pin, relative to ground, and you can immediately change what your Arduino is doing.

Added bonus: with "pin change interrupts" you can pick the pins you want to use for interrupts. Including more than one or two or three pins on the same chip at the same time. And, depending on the Arduino you use, the pins allow you create up to 3 completely independent interrupts, each of which can do something different! Handy!

BTW, an incredibly good explanation of the the Arduino interrupt universe--from the Atmel ATTINY to the big huge crapping ones--is here.

(The "change pin" moniker threw me for a bit. Shouldn't it be a "pickyerown" interrupt? Whatever.)

OK after your GIMSK PCMSK  etc. you have declare an "interrupt vector"--this tells the ATTINY what to do when the interrupt is thrown.

Here I am reading whatever voltage is present at PB4 pin and assigning it to "gatemaker", a boolean variable I do declare.


void setup()

volatile boolean gatemaker = LOW;

// blah blah, other stuff here....

ISR(PCINT0_vect)
{

     gatemaker = digitalRead(PB4);
}

This code can thus act on incoming data--a gate rising edge, a new midi note on, etc., fast--very, very fast. More musical!

So--do that like this and this like that:

void loop()

if (gatemaker == LOW) 

 // I see a falling edge, 5V to 0V, on my gate signal.  When I see that:

///do something; charge a cap, turn on an LED, change pitch, //whatever.....
}

//but if I see 0V to 5V......

if (gatemaker == HIGH)
//do something even still more different; discharge a cap, turn off //an LED, don't change pitch, whatever.....
}

Wow! That's really easy! A million and one uses: ARs, gate to trigger, trigger delays, on and on. Trap when your gate signal is going up or down, then do something fun. NNNNNNext!

Again this is useful for ATTINY but by studying Arduino coding on line can be done with all the chips I've seen in this large family--so, whatever other Arduino widget suits you.

This ATTINY methodology took me a few evenings to figure out, mostly because there are so many different documented ways to get at the interrupt paradigm, but this whole change pin interrupt/boolean method seems to work every time and is simple, simple, SIMPLE!!! For the Nano, there are two pins already all set for interrupts, and the code needed is the usual super simple stuff we all expect here in Arduinoland.

Back to Audio: Final thing to do is protect your ATTINY interrupt pins from an obnoxious gate signals from (say) your 15V power supply (oops!  Patched my modular wrong....blew up that ATTINY....magic smoke....yeh baby yeh!)

For that I used this circuit fragment plugged into the PB4 pin used above; as always, many ways to do this, but this simple NPN circuit fragment worked for me. Transistor is a 2N3904....this inverts the gate, so you have to code accordingly.




OK for now that's it. I created an ATTINY based module I call "ConBrio" allowing a gate to speed up up up or down down down. It uses this whole pin Change interrupt thing extensively. It works on the bench but I am trying to make it "more musical" whatever the hell that means. I figure I will finish the code and hardware for that and post it soon.

Until then, don't breathe the TINY and have fun!!!




Thursday, November 15, 2018

ATTINY arduino for AUDIODIY--Pros and Cons--Debugging these Tiny Spuds!

From last time: I am waiting for C1406HA voltage controlled attenuator boards to arrive from China to get reverselandfill's noiseboard going.

Any day now....

 In the meantime I thought I'd keep working on my Arduino skills, this time working with an ATTINY85.

Part one of my ATTINY exploration is here.....



Cutting the BS and getting to it--pros and cons w/ working with an ATTINY vs. all the other Arduinos out there.  Why use it at all?

Pros:

  • ATTINYs are super cheap. I bought 4 of them for $12 on eBay.  If you can work with SMT, it's even cheaper.
  • They are super small. Tiny? Sure. In Eurorack land where everything needs to get tiny, that's good.
  • You can program a few of them to do the same thing and then drop them into your projects like you just fab'd up custom IC's. Gate to trig? Gate delay? One Chip LFO?  Yep.  
  • Most all the important commands you can get out of a Nano or whatever are supported here.  Pin change interrupts for instance?  Yep. I'll get into that in the next post, which I'll upload tomorrow.
  • You can get an all in one programmer for about $16 USD complete with handy built in blinking LED.  But watch out for this gotcha....this one drove my crazy....on Linux or mac programming code into an ATTINY works OK. Then suddenly quits. Like you unplugged the programming board...but you didn't. Why? Turns out to be a USB permissions issue. To fix follow the instructions in this link or just do this : open your terminal and run lsusb from terminal. Then chmod the crap out of the usb ports with this: sudo chmod 666 /dev/bus/usb/0xx/0yy where xx and yy are the device ID's used for the ATTINY programmer.  You can make that change permanent by following the advise here.  Otherwise the "write to chip" commands in Arduino IDE become caloric; i.e., they donut work.


The AVRTINY programmer.  $16USD!!!


And now....Cons:

Biggest issue, con number one! There is no easy way to debug these damn ATTINY rascals.

What does this mean?

On the Nano, you use serial print commands for this, to see if a variable is working the way you want, for example:


void setup()
{
  Serial.begin(9600);
}
x = 100; // or whatever other bozo value you are trying to determine

Serial.println(x);
delay(2000);


If you expect to see a 100 here and get 0 in your serial monitor, that tells you you screwed up your code!

On the ATTINY, this debugging methodology is not so easy. That's because serial commands are not supported by default at all!

The workaround: There are lots of fixes for this. After digging I found a nifty add-in library called TinyDebugSerial. I liked it because it was easy to implement. And if you read this blog at all you know I like easy.

You can get the add-in here.

Drag the files into your Arduino library folder, then try this as code:


 #include <TinyDebugSerial.h>

void setup() {           
mySerial.begin(9600);
TinyDebugSerial mySerial = TinyDebugSerial();
}

void loop() {
mySerial.println("hello world!");
}

OK, serial print data, streaming at 9600 baud, now shoots out ATTINY85's "pin PB4" (that's pin 2 for those of us in the DIP8 real world). In this case "hello world" over and over.

But....Where to hook up the TX out? So you can read it, Elmo??

I ran it to a RX pin of an Ardino UNO via a 220 ohm resistor--hooked the UNO up to a 2nd USB port in my Linux laptop--fired up a new IDE window for the UNO--set the USB port in the IDE for the uno--and (finally) read the serial strings out of there. Worked!

Using an UNO to get tinydebugserial output from an ATTINY.  Wire it like this....

Con #2:
there just aren't that many pins to work with. This is an 8 pin chip and 2 of them are for power.  1 more is for reset, and it can do other things, but not easily. Not much more to say there. Sorry.

Con #3: the pinout designations are odd for ATTINY.  I found for analog in I had to use something like "A0", "A1" etc. For digital I had to use "3". I had to check the pinout diagrams online more than a few times, and for me, screw around a bit....

For instance, analog Pin A2 is tied to physical pin 3 and A3 is tied to physical pin 2.  But why, Scottie, why?


Con #4: ATTINYS are slow, slow, slow, SLLLOWWW and then they are slower than snot!

Overall--I almost gave up a few times--really, for audio DIY why use ATTINY at all and not just a Nano? Or Uno? Or Due? Or  Dookay?

Not sure I can answer that....but then I came back to it...I bought the damn tiny things--I just have to figure it out.

After a few evenings I did get it working, and with debugging set up could start to do useful things. So yes there are probably more cons than pros. I programmed it anyway. Guess it's just the way I am.
Doesn't mean you have to be that same way.

Soon to come--using interrupts to track CV gates with these little turds.  Until then.....well you  know.

Friday, November 9, 2018

Reverselandfill PCB Swaps and the Crazy C1406HA Attenuator Chip. Thoughts about the old days.

History Lesson....

How it was back in the 80's: I'd get in my Station Wagon (or later, in a van, or a plane), get to the seedy club, and if the vibe was good, swap conversation, drinks, and, well, other things, with my new found friends.

If said friend was a fan, tour caterer, flight attendant, promoter's daughter, and/or exotic dancer and liked wire-thin synth and backup vocal dudes with Brian May hair, (a few did, most of them liked the drummer better) try even harder to swap....well, you can guess right?

This was the 80's and I'm oversharing....so let's fast forward 30 years.

My Brian May hair and Ford Country Squire are long gone. I don't drive to clubs a lot nowadays; instead I get up early and control/alt/delete a PC.....mess with Eagle and whatever audio PCB I am trying to finish, look at audio websites and peruse PDF data sheets extolling obsolete OTA IC's.

Feeling social? Not often, but if so, perhaps I will email like minded audio folks, many of whom live halfway around the world. And if the vibe is good, we swap....

(.....wait for it?)

.....printed circuit boards!!!

(Welcome to my World.)

One such swap occurred recently with Electro-Music mensch Martijn Verhallen from reverselandfill.

Martijn and his site are chock full of creative ideas and music.  To name a few: he does crazy things with video synthesis; mounts synthesizer PCBs on slabs of wood, and produces noise/dark electronic/soundscape  music. 

I sent Martijn a couple of Lunetta VCO variation PCB's as well as an ASMVCO.  In trade he airmailed his "Aconitum Noise Mixer" PCB along with a CMOS based noise PCB with on-board patching called "Noise! Oscillator".

I have slim to no white/pink noise capabilities in my homemade synth so let's build the Aconitum first.

ReverseLandfill's Aconitum Noise PCB

Aconitum combines a few analog staples--the "get noise out of 2 pins of a gained up transistor" and the "two inverting op amps in series mixer"--along with active filtering--a bunch of pots and jacks--into a small board.

I could build this box stock, which would be fun, but it's even more fun to mod it.

How about adding CV control of the noise levels? Why not?

There are a hundred ways to do that, an easy way: use VCA's. I have already messed around with that--the Farm VCA and Irwin VCA for instance. But, as per my initial push into DIY, I want to see if I can do something new (for me anyway). Let's try using a chip that you may have never heard of. I know I hadn't!

It's called the C1406HA:




 As far as I can tell, he audio consumer mass market was the C1406HA's target. Fair bet whoever designed this IC never thought it'd end up in a synth.

I say SIP it!  SIP it good!

So it's a voltage controlled stereo attenuator.  OK, the 1406 might useful for what we do, but buying these IC's was a shot in the dark--I was putting together an order for Goldmine (switches and caps) and saw this, on a lark I bought ten of them--they cost pennies--figuring, if they didn't work, toss em.



I breadboarded my first C1406HA and--they are super simple and work great! The CV to audio curve, to my ears, is very musical; the distortion is there but not too bad, certainly acceptable for mixing noise and grunge.

Proof of Concept for 1406 was 5 parts--the chip and 4 caps!

If there is any issue at all with the C1406HAit's that the channel separation is butt--the spec sheet says 60db but really I doubt that, to me it sounds a lot worse, but for attenuating noise signals, who cares? Maybe the crappy channel sep makes the thing sound better?

I spent an evening creating an Eagle "experimenters board", which I will get fab'd soon. It will (if it works?) accommodate two 1406 chips, which allow control of 4 audio signals.


The C1406HA Experimenters Board.

In  case you're curious: where can you buy this chip? As far as I can tell was been discontinued during the Eisenhower administration (a bit after, but close?)  Goldmine no longer has them.  Damn!  I found more in China at affordable prices--UTSOURCE in China, had them for 50 cents each USD and $4 for postage.  I have no idea if they will ever show up, much less work. But DIY is about gambling--both money and time? 


UPDATE! The UTSOURCE chips showed up and appear to be just fine. The PCB I created for the C1406's work but has a lot of errors. Read Part II of this build, where the module is finished and Doris gets her oats? here. Until then, don't breathe le fumes!

Monday, November 5, 2018

SMALL B.O.W.A.L: Creating DC Bias Offsets with a Control Voltage

Last time I was extolling the virtues of creating DC offsets for control voltage and audio signals.  This can add color to LFO signals, wave shapers, modulators, and other devices.

To review, what does this mean?

It means you start with an AC signal like this:

Figure 1: standard Sine Wave
And you offset it a few volts above ground so it looks like this:


Figure 2: sine wave with DC bias offset

The B.O.W.A.L. circuit from last time let you tweak the bias with a potentiometer.

But! Wouldn't it be nice to be able to do the same with a control voltage?

Turns out it's not hard.  We start with the same PCB:



Then add some wiring instructions which you can find here. And yes I realize that the PDF is hard to follow (crappy handwriting).  Hopefully this next diagram helps--it summarizes the 4 main wiring mods you need to add to hook the op amp stages together to make this work.


Wire all that up and you have 2 fully independent, buffered bias offset circuits; feed in CV and out comes the input sound or another CV, offset by the amount of CV at the "bias" input.  Easy!

Some bench photos. Means not much to anyone but me, but it makes me remember the zen times at the bench putting this module together. That's the good thing about being a blogger--as long as you're not overly offensive, you can post any damn thing you want.

Like Big B.O.W.A.L., Small B.O.W.A.L. worked the first time without kludges, tweaks, and changes. Two in a row.  I am trying to stay humble.

The wires here turn four channel BOWAL into two channel Small Bowal

Skiff?  We don't need no stinking skiff!




B.O.W.A.L. brothers....


Bench B.O.W.A.L.


Still with me?

Interesting synthesizer patches may be imminent when using CV to offset an audio or control voltage....a lot of times you want to get rid of DC offsets, but not always. Hear what Small B.O.W.A.L. sounds like here.  As usual, no effects, tweaks, tricks etc. for this demo.  This is all bias offsets using the circuit in this post.

details about the demo:

00:02 CLASSIC MOD F/X
  • Slow LFO goes to Small bowal input.
  • Small bowal CV input source (for bias offset) is from an AR
  • AR is set to immediate attack and moderate release.
  • Post-gate LFO climbs down from an offset of about 3V above ground to zero volts. 
  • Sent this offset LFO CV signal to a mod input of a VCO.  VCO set to ramp wave  
  • So you hear the warbly mod pitch descending as the CV offset goes from around 3V to 0V.
  • This is a popular mod trick for modular synthesis. A bit tired perhaps.

00:20 Distortion unit becomes waveshaper
  •  Sine wave to small bowel audio input.
  • DC offset of this sine wave with slow LFO, maybe 1V P/P
  • Feed this signal into input (1) of the SON OF ZENER
  • The other channel (2) of son of Zener is a triangle wave.
  • You end up with something that sounds a lot like a Serge Waveshaper meets VCA.  I have no idea why.

00:55:  Goose a Korg MS20 style ring modulator 
  • 1K Ramp wave > Small Bowal input.
  • Small bowal CV input is modulated by ADSR.  
  • So ramp wave is starting with a  DC offset and falling back to 0V
  • That's fed into X input of the ring-mod.
  • Y input of Korg Rmod is ramp wave, unadulterated
  • You end up with the ringmod effect that is not audible, then suddenly very audible.

1:21: Odd things happen when you DC offset audio into a balanced modulator.
  • Not everything will sound good here, you have to mess around.
  • pulse wave (audio) offset by SB. 
  • SB bias offset modulated by slow LFO. 
  • SB audio output to Y of 3080 or equivalent balanced mod. 
  • X in to B-M is another pulse wave. 
  • Balanced mod out to VCF > VCA
  • You can get all sorts of very odd sounds from this.  If you have a balanced modulator, without AC coupled inputs, try messing with putting DC audio offsets to feed X & Y inputs.

OK next up I have some guest circuits from reverselandfill.org.  Stay tuned!

Monday, October 29, 2018

Bias Offset With Adjustable Level, AKA "B.O.W.A.L."--EZ Op Amp Circuit that worked the first time!

Almost nothing makes me happier then laying out a PCB, getting it fab'd, stuffing it with parts, mounting it to a metal panel, racking it, and having it (wait?  wait?) work the first time !!!

NO MISTAKES!! No kludges, no cutting traces, no gnashing of teeth. Yeah!!!

What is up: While I had the Zener Distortion Units on the bench I found this: if I offset the audio signal(s) feeding it by a few hundred millivolts or more the zener unit's sound at output changed quite a bit.

I also found that in my growing DIY modular synthesizer I had very few ways, if any, to create this said offset.

Let's get building! The usual: researched, opened up Eagle, designed, checked, sent it off to JLCPCB for quick fab.....

While we're waiting for the PCBs to come back to the US now what?

As Frank Zappa used to say "let's examine this phenomena!"

Q: What do I mean by offset?  

Say you have a sine wave, and it's centered on 0V DC (ground).

It will look like this on a scope, or, if you hunt around Google and copy illustrations to your blog:



The bias offset will "elevate" the sine wave to something more like this; and it can go either way; apply a negative DC offset and the sine wave is dragged below zero, but here it is above:


At first glance, this doesn't seem like a big deal, but in terms of modular synth, it can be useful.  Put your waveforms through the bias offset device, change its bias level relative to ground, and put it through a ring modular.  Ha! Sounds different!

Q: Still not convinced, but anyway, how to design this? 

There are lots of ways; but like most every other analog designer out there, I like using op amps.

On my website I already had a non-inverting design for this, but it requires small value offset trimmers or pots, say 1K or 5K, which might draw too much current if you add say 8-12 of them to a design.  I don't use battery power but if I did you probably don't want to do this!


I don't spend a ton of time worrying about power efficiency but still, it's good to do things right. OK, lets put a bigger value trimmer in for the 5K above....let's make this trimmer 100K to keep our power supply completely happy, and the resistors in the op amp's negative feedback network are also 100K.  But wait, now, the gain doesn't come out right--the offset resistors screw up the output:

Audio goes to +; this circuit draws relatively little current--but-your gain will not be what you expect!

What I figure: an inverting solution ends up working better when the power supply draw is critical. .

Enter B.O.W.A.L....a simple quad inverting then re-inverting op amp PCB with 2x quad op amps.

You can see details about PCB layout on my website but basically the circuit board looks like this:


While a single gain stage (repeated 4x) looks like this:



For my own B.O.W.A.L. I added a brain dead AC coupler to undo whatever the offset circuit does, creating a "star belly off machine", entirely wired to the front panel.  The Offset pot above can be a trimmer for fixed applications, or a pot on the panel so you can adjust the bias on the fly.

Aside: a really good guide for setting op amp bias offsets can be found here: MIT's "how to bias an op amp".  (Note that my undergrad application to MIT in 1980 got the big goose egg. I wonder why?)

So anyway this PCB got back from China and I populated it, selected an unused a front panel, got some pots from Small Bear going etc:



Designed up a front panel for Lazertran using Adobe Illustrator:




As I said! worked first time. Yeh!



BEWARE: if you amplify and/or offset your incoming signal too much, your op amp will squash the dickens off some or all of your incoming signal, turning it into an ugly square wave. To my ears, this isn't good sounding distortion, this is really bad sounding distortion. At best an op amp can gain something up to slightly above its V++ and V-- rails, and usually less. So keep your amount of BOWAL output flushable....

And: if you are really concerned about input impedance you may want to put yet another buffer in front of your B.O.W.A.L. or change BOWALs R values.  The incoming sound will travel through all the feedback resistors from input to output, which might yield a different input impedance than you want.

Update!  The +/- 15V DC bias settings weren't working for me sound-wise--it was too easy to squash incoming signals, which sounded terrible, so I did this easy mod to the 4 bias pots (in my build, these are inner pots of dual concentrics from Small Bear:)

"To PCB" means wiring to the center tap of the bias trimpots on the circuit board.  The down arrow is -15V


I used different resistor values between V++, the pot and V-- to allow for flexible settings

UPDATE to the update: After messing with this design a bit more, I realized the circuit would be more musical if I could control voltages to determine the bias offsets. And that's easy, buffer an incoming CV signal and feed it to each of the op amps' non-inverting inputs. I thought about modifying B.O.W.A.L. to do this, but the front panel was already done and labeled.  Instead, I  created a 2 channel variation of this same PCB that has CV to DC bias offset: "Small B.o.w.a.l".

OK that's it.  Remain biased at all times! Until next time, go MITEs and don't breathe the fumes!



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