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C16 - Build Log
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AlanP



Joined: Mar 11, 2014
Posts: 713
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PostPosted: Wed Sep 16, 2020 6:37 pm    Post subject: C16 - Build Log Reply with quote  Mark this post and the followings unread

This one... has a bit of a story behind it. The name is either Complex 16, or Crappy 16, depending on how peeved I am.

When I first started with Eurorack, I spotted Yves Usson's Fixed Filter Bank, and thought, that looks like an interestingly complex project. (This is probably what they mean by 'gateway drug.') I learnt quite a bit by doing this project, chiefly to do with how to use pin headers. http://alanp.info/synth/yusynthffb.html

I then found out about the Buchla 296 Spectral Sound Processor on Dave Brown's website (https://modularsynthesis.com/roman/buchla296/296psp.htm). This looked like even MORE fun but, at the time, Dave didn't have the schematics up, I couldn't find any, and it looked very intimidating. So I shelved the idea for the time being... but it did look like a lot of fun.

I subsequently built the Serge Resonant EQ Mark II in a kit from Elby Designs -- this used sliders, not pots, had individual outs, and was extremely musical, both with signal running through, and even just resonating on it's own.

Sin Phi released an utterly amazing tool on another forum that creates plots and component value charts for fixed filter banks, using staggered tuning so that each frequency band was more of a plateau with sharp edges at the corner frequencies, than a pointy peak centering on the center frequency. I worked up a PCB set for this.

Posted Image, might have been reduced in size. Click Image to view fullscreen.

"Simple 16." I learnt several things from this. One, putting the summing opamps on the back of the panel PCB (with the jacks and pots) certainly works... but it makes things very crowded, cramped, and untidy looking. Two, it's possible to do these with through hole components... but some capacitor values are a bugger to find (hence the gonzo caps) and it winds up taking up a lot of space. So, I had verified that Sin Phi's tool did a very good job, and I could move onto the other parts of the 296's feature set.

I decided to use 2164 based VCAs, as this would simplify things quite a bit, being able to use one chip for four channels, meaning four total, and not eight total, like for LM13700, or sixteen total, if I were daft enough to use CA3080. I used Neil Johnson's protection diode on the negative power rail, an input opamp for the control input for each VCA, and an output opamp on each VCA, all going to a pair of summing opamps (odd and even bands) at the bottom of the board. The audio into each VCA is just going straight from the filterbanks to the 2164 inputs, through a 30k-510R-560pF passive filter as per the datasheet.

After getting this working, I found that I need not have bothered with the output opamps dedicated to each VCA -- the 2164 has current outputs, so I could have tied all the outputs directly together (no resistors needed) and gone straight to the summing opamp. Live and learn. I also learnt that if the CV input is roughly 5V or so, the output is really nice. If the CV input is sky high, the output will be really distorted and square-wave blown out.

After stripping out the output opamp per VCA, I'll call that done.

Envelope filters per band... ugh. I honestly cannot remember where I got the one I used on my PCBs, but it does not work, and doing rework (cutting traces, running patches) on SMD is a complete pig of a job. I'm going to perfboard up some alternatives this weekend, and try those with a range of values with through hole components to test. (I can never get breadboard to work properly, so I will use perf.)

The ones I currently have seem to have some kind of PWM deal going on between +12V and GND, the more +12V is, the higher the output is supposed to be. Remember how I said that the VCA output's niceness depends on the CV? Yeah. Really bad. Once I have a good envelope follower worked up (the Digisound one looks good, I will test it on perf), I will work up a new PCB set.

I didn't include the Scan functionality from the 296. I can't think of an elegant way to do it.

Luckily (remember what I said about the Simple 16?) the panel PCB only has passive summing and output resistors, along with some MMBFJ201 JFets to crosspatch when I want them to, so I can reuse the panel-PCB as long as I maintain the same jumper positions on the two engine PCBs.

The top rank of jacks are the Envelope Follower Outs for each band (this output also drives the LED in the slider, to give a visual representation.) Next down is the Individual Outs per band, with the volume faders beneath that. Below the sliders are three sets of outputs -- slider output (odd, even, and both), comb out (odd and even), and VCA outputs (odd, even, and both -- Buchla calls this "program output" or similar.) At the bottom are the CV inputs for the VCAs per band.

This jack arrangement is so that if I have two, I can run 16 patch cables directly down for a 16 channel vocoder Shocked

Posted Image, might have been reduced in size. Click Image to view fullscreen.

This is the three main PCBs. Left - panel PCB. Middle -- VCA and summing PCB. Right -- filterbanks and envelope followers. I didn't include the input PCB, it's basically just an input buffer with volume, along with some toggles for the transfer thing.

Posted Image, might have been reduced in size. Click Image to view fullscreen.
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Skrog Productions



Joined: Jan 07, 2009
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PostPosted: Wed Sep 16, 2020 11:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

lots of soldering ! , very nice build log , looking good Alan Smile
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PHOBoS



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PostPosted: Thu Sep 17, 2020 1:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

Shocked impressive. had a look at the 296 looks nice with all the CV controls.
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AlanP



Joined: Mar 11, 2014
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PostPosted: Sat Sep 19, 2020 5:36 pm    Post subject: Reply with quote  Mark this post and the followings unread

Perfed up the Digisound 80-13 envelope follower, and it works pretty well. There is some lag on the attack, but I can live with that, I guess. Just need to redesign the second engine PCB, now, and then work up a panel for this.
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Ayab



Joined: Oct 27, 2015
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PostPosted: Sun Sep 20, 2020 11:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

Looks like you have put lots of hard work into this. Good work.

A description of how it sounds/how you have tweaked it compared to the Serge EQ you built would be interesting - and how you would use the extra outs.

Please let me know when it is done and you have finished the panel - if you sell pcbs/panels
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AlanP



Joined: Mar 11, 2014
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PostPosted: Wed Sep 30, 2020 3:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

Posted Image, might have been reduced in size. Click Image to view fullscreen.

This is the VCA/Summing board, after patching. Two opamps (long wires) had their inputs reversed, needing traces cut and patched, and the 2164's are currently bypassing the individual output opamps and are summed directly by the final opamp. I only included two because one, I only had two when testing, and two, I don't need all sixteen bands to be VCA'ed to test how a handful work together. I've got a new revision that incorporates all this Smile

To the right of that is the jack/slider PCB. Two slider LEDs had no ground connection, hence the two 'U' patch wires. I only had 50kB linear taper sliders on hand, and all the resistors are to change them to a 10kA audio taper. I've got a new revision with the added grounds, and also optional SMD resistor placements to adjust the slider taper/resistance (see the link below for more on this.)

https://www.diystompboxes.com/analogalchemy/emh/emh.html -- click 'Tapered Pot'.

Posted Image, might have been reduced in size. Click Image to view fullscreen.

For fun, this is a mockup of what the user should see in the end (with, obviously, a panel on top when done!)

Input -- the two toggle switches on top control whether the envelopes of odd control the VCAs of even (an 8 channel vocoder, if you have separate odd/even inputs patched), the top two Input jacks are gates in to control this as well. The bottom two jacks are Odd/Even inputs (normalled to each other), with attenuators above them.

As a side note -- photographing shiny things is a bit of a pig!
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Blue Hell
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PostPosted: Wed Sep 30, 2020 3:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

AlanP wrote:
As a side note -- photographing shiny things is a bit of a pig!


Oh you're not doing bad, quite nice shots. Aaand .. also ... you can diffuse the light source when using the cam's flash by using a small (makeup) mirror to direct the flash to the ceiling .. might help Smile

Looks like you'll have a very nice module in a while!

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also .. could someone please turn down the thermostat a bit.
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AlanP



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PostPosted: Thu Oct 08, 2020 5:23 am    Post subject: Reply with quote  Mark this post and the followings unread

This is how I test circuits! Breadboard never seems to work right, for me, so I like to perf circuits that I don't want to dedicate a pro PCB to. Thonkiconn PCB mount jacks just make this even easier for me.

Prototype of the Digisound 80-13 Envelope Follower. I did plan the layout (roughly) in Eagle, both to find a good layout, and also to remind myself of the pinout for the TL072. (Measure twice, and all.)

The socketed capacitor is marked in the original schematic as being changed to match the frequency of the signal being followed. I tried 100nF and 470nF, and there is definitely matching needed to get the best results. If I was doing a proper module for this circuit, I think I'd put a switch on the panel to toggle between three different capacitor values. I'm going to patch up some enveloped noise going through my 0.1 revision C16 and use the Direct Out for each band to figure out the optimum capacitor value for each iteration of this envelope follower.


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AlanP



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PostPosted: Fri Oct 09, 2020 11:14 pm    Post subject: Reply with quote  Mark this post and the followings unread

Laying out this panel was not fun. Lots of holes to locate, and they are laid out to be evenly spaced within the constraints of my Eagle license (hobbyist 7.5.0), not for nice spacing that snaps to the default grid settings.

But the printout seems to match up to my rev 0.1 boards. I'll generate gerbers and order the next set of boards (as well as this panel) after next payday.


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AlanP



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PostPosted: Fri Oct 16, 2020 6:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

Right, new revision of PCBs and panel are being fabbed, so I needed to test which envelope follower capacitor value worked best in each EQ band.

You can see my little group of test caps I was swapping in and out of the sockets on the perfboard Smile


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AlanP



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PostPosted: Sun Nov 01, 2020 6:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

Posted Image, might have been reduced in size. Click Image to view fullscreen.

The new revision PCBs arrived, including the panel for this. Usefully, the old revision panel-PCBs (with the jacks, and controls and stuff) all work with the new revision engine PCBs, so I don't need to desolder sixteen sliders!

I still have to populate the two new engine PCBs. Boy, is that going to be fun...

(Oh, and putting the 56 odd jack nuts on was about as much fun as you think it would be.)
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AlanP



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PostPosted: Tue Nov 10, 2020 7:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

EngA is finished -- the sixteen VCAs, and the final mixers for the various outputs. I think it looks quite spiffy!

EngB, on the other hand, needs 99% of work still to be done on it.


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AlanP



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PostPosted: Thu Dec 03, 2020 9:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ooof. Work has gone busy, really busy. This week, overtime monday through friday, and eight hours on Saturday tomorrow. On the one hand, money for parts isn't an issue. On the other hand, time to solder said parts is an issue.

I've soldered up EngB, the board with the filters themselves and the new envelope followers. If I ever do another take on this project, the filters are getting their own, dedicated PCB so I can just solder them up once and then do new revisions of the other submodules.

The good news is that the followers work nicely. I plugged in a VCO to the Input, and as I messed with the frequency of the VCO, I could see the lights on the panel travel up and down the spectrum (as opposed to the old followers, which lit up whenever the heck they felt like it.) One or two LEDs aren't lighting up, so I need to test whether it's the filter for that channel (everything follows on from the filters), or whether it's the follower. I'm going to do that when I have the energy... stupid overtime.

Once all of this board is verified good, I'm then going to go through the gain stages -- the gain on the input buffer (currently set to 3x gain... too high, I'm pretty sure it's blowing out the VCAs), and the gain on the various output mixers (faders, VCAs, comb) so that they all come out at roughly the same p-p levels. Should I set the fader out levels so that when the faders are at max, the gain is x1 (compared to the input), or so that they have a bit more gain if you dime all the faders?

I also need to test the spectral transfer switches, and see if the resistors are passing the right amount of signal (or if they're going to distort those VCAs... again...)

Making progress, slowly, but then I've wanted this module for years. I can wait a bit. It's probaby vanity, but I also think these circuit boards are looking dead spiffy!


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AlanP



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PostPosted: Tue Dec 29, 2020 10:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

The bad news is that I did something really stupid that I don't understand with the spectral transfer system. It uses a bunch of Jfets (MMBFJ201) to connect the envelope follower out of one channel, to the VCA CV In of the paired channel. At the moment, like an idiot, I have the gate either at +12V (through a 100k resistor), or at ground (though same 100k resistor.) Because I'm an idiot, I didn't do my basic research on this.

The good news is that hopefully, it's just going to need a new revision of the Input PCB, the nice, simple PCB that is just one 30x100mm pcb with no extra guff.

The annoying news is that I think I may end up having to... *shudder*... breadboard something.
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