Possible alternative to Dual SVF board

Hello everyone. I just joined the community, and this is my first post. So, first of all, “Hi”.

With that out of the way, I wanted to let everyone in on what I’ve been working on. I think it’s a little early to say that this will work out for sure and that this will definitely be a new filter board that will be good enough to release to everyone. But, that is my hope and intention. And progress has been very good.

(Edit: Well, I wrote this all the other day, and it kind of took the form of a narrative, so it’ a little long, but it should get everyone caught up to where things stand right now.)

I built my first (only, actually) Shruthi with the SMR4-MKI analog board in the summer of 2011. I really enjoyed it, and still do. I was completely new to electronics at the time, and though I know much more now than I did then, I still consider myself new to electronics. Perhaps I always will. After seeing some of the other boards that were available, and the “Your Own Filter Board?” link (now gone) on the mutable instruments site, I started thinking that I wanted to make a filter board of my own. I actually started working on a polivoks filter, but didn’t get very far before the official Mutable Instruments one was announced. So I gave up on that, since there was no reason to continue down that road. I didn’t give much thought to trying to make another filter board for awhile.

Some time later I saw the BBC documentary, “Synth Britannia”. In interviews with Phil Oakey and Daniel Miller, both mentioned the Korg Minikorg 700S, and how important it was in their early work. This really stood out in my mind because it’s a synth I didn’t really hear much about online at the time. (This was before the Korg Volca series was announced.) When I saw a used one in a shop, although I was in no position to buy it, I did mess around with it a lot and knew right away that I loved the sound of the filters.

The idea of making a filter board came back to me, and I thought I would just make a one-off board for myself with the Minikorg’s fixed routing (High pass and low pass in series) to use with the Dual SVF setting in the firmware. Obviously most of the functionality of the real Dual SVF board would go to waste, but I could at least get the two more control voltages for the second filter, and I wouldn’t have to program anything into the firmware (I wouldn’t have been able to anyway. I don’t know any C+). I started work on it, and it quickly became apparent that making a true clone of the Minikorg filter, down to the same transistors, was more work than it was worth for such limited functionality. So from there, I started started simulating the MS-50 filter in Spice. (almost the same filter as the Minikorg, but it uses op amps rather than discrete transistors, has variable resonance rather than just “On/Off”, and doesn’t operate at weird voltages. In other words a much better fit for a Shruthi filter board.) I still hoped to be able to fit two filters on the board, so I tried to save some space by getting rid of everything that wasn’t absolutely necessary and reducing the filter to its bare essentials. I quickly and easily got it working in simulation on/-5V. Getting the highpass filter working was also pretty easy. At this point I started probing random places on the schematic in Spice, and realized that it’s also possible to get a bandpass response out of the filter. As far as I know, this hasn’t ever been implemented on any commercially released synth with this filter. Anyway, it’s a multi-mode filter which doesn’t require many parts, and those parts which are needed are all cheap and easy to get.

OK. I hope I’m not breaking any forum rules by continuing that first post here. If so, sorry. This is kind of long.

At this point I started thinking that it may be possible to actually get all the functionality of the Dual SVF board on a different board with a completely different filter topology. It would give a new variation on the sound, and wouldn’t require any rare ICs. Both good things. All that was left was to design the resonance circuit, the digital mode selection circuitry, and the amp.

The resonance of the two filters can be controlled with a dual OTA. It’s a simple idea, but getting it just right was a lot of trial and error, but I’m pretty happy with the results at this point.

The digital mode selection circuitry doesn’t require any changes in firmware. A state variable filter has one input but different outputs for each filter response. A diode ring filter, as is found in the Minikorg and MS-50, has separate inputs for each response but one output. So with the same 3 ICs used for the mode selection in the Dual SVF board, all the filter responses can be selected with diode ring filters, but the signal through the multiplexers is basically reversed. I’m not sure that made sense. It’s a little hard to explain.

The Minikorg and MS-50 both have a VCA unlike any other that I know of. Like their VCF, it makes use of a ring of matched silicon diodes, and requires very few parts. To my ears, I can’t hear any major difference between this VCA or any other, but I may not be listening for the right things. In any case, it’s something that hasn’t been implemented on any other Shruthi boards, the parts are cheap and easily available, and there’s no need to put another LM13700 on the board, just to have half of it go to waste.

For awhile I had this idea in my head, and I simulated it over and over with various tweaks until I realized that I had taken the idea about as far as I could without actually building the circuit.

So, I started breadboarding. It’s bigger than anything I’ve ever built on a breadboard before, so I worked slowly and methodically because I knew if I screwed anything up, it would be very hard to find and fix. At the beginning of this month, I got one filter working nearly perfectly on the first try. Sunday night, I finally had time to try to connect the second filter and the digital mode selection circuitry.

I am happy to say it works remarkably well, though not perfectly.

I still see room to tweak things, and some things probably will be tweaked. However, the breadboard (breadboardS actually) is so cramped that I’m reluctant to mess around with it too much. I consider it nearly a miracle that it works as well as it does, and I don’t want to push my luck fiddling around with it trying to get it perfect just to screw it up. Although it’s not absolutely 100% perfect, I know what places need work, so this week I’ve started laying out the circuit board. It’ll be easier to tweak on an actual board if I just socket the few parts that I think can be improved on, rather than trying to fit my fat fingers through this maze of wire on the breadboard without knocking anything out. If you take a look at the picture I’m attaching to this post, you’ll see what I mean.

Although it’s on two breadboards, the number of parts is roughly comparable to the the original Dual SVF board because there are many places in the circuit where discrete resistors can be replaced with resistor networks on the PCB. I’ve just started laying out the PCB. I expect that everything should fit. In a worst case scenario, the blinky eye ghost may get the boot. Using 1/8 watt resistors, or mounting the diodes vertically are also options, but I don’t think any of that will be necessary. It’s still early going in laying out the PCB, but I’m pretty sure everything should fit. We’ll see.

Hopefully, I’ll be able to get a few hours away from the wife and kid tomorrow to record a rough demo. I really wanted to wait until I had some sounds before I posted this, but I’ve got some down time at work now, so I thought I might as well post what I have so far. I can’t bring the breadboard to work with me though, so I can’t record a demo until I get some free time at home, and it’s a little tough as a family man with two jobs. Tomorrow looks promising though.

Here is a rough “to do” list of things that I want to improve on:

There is considerable mains hum. I think/hope this will mostly go away on a proper PCB with a ground plane. I’m not sure it will ever completely go away though. As of now, it mostly goes away when powering it from a 9 volt battery, but there is still a little just from having it in the same general area as other gear powered from the mains.

As I was breadboarding, I didn’t put much forethought into filtering the control voltages. I just put 100 nanofarad caps in the right places. I’m hearing more digital garbage than I’d like to. I haven’t hooked it up for awhile, but I don’t remember hearing any at all on my old SMR4 board. Using larger value caps should fix that I think. Hopefully, I can get rid of that without making the response of any of the parameters sluggish. Space permitting, I also hope to filter the 39kHz signal from the oscillators with an active filter as well.

Right now, there is a pretty annoying volume drop when using the filters in series. It’s not a big surprise to me because that is how I designed it to work, but now that I’ve heard it, I don’t think I like it so much. I was thinking that filter 2 wouldn’t work right if the amplitude of the input signal was too high. (I.E. when filter 1 is at full resonance.) I thought this volume drop was the lesser of two evils, but now I want to try getting rid of it to see how it sounds. It might not be so bad. Anyway, I have a few ideas of ways I could tinker with this. I’ll probably be able to put in a quick fix before making a demo.

The MS-50 and the Volca Bass have a few extra parts to extend the filters ability to self-oscillate to a higher frequency. I’m not sure about the Volca Keys, but the Minikorg and Korg 770 don’t have this and wouldn’t really need it with their limited resonance. If there is room on the board, I’m going to try to include this. I’m actually quite happy with the sound without it, but if there is extra space, I see no reason not to. (On an only slightly related note: My Volca Bass was a casualty of this project. R.I.P. My soldering skills weren’t up to the task of modding the SMD side of the board for a highpass response.)

Other filter boards use a large value resistor to quiet the VCA when no note is being played. However, this VCA is a different design and my initial impression is that this may not be needed, but I’m not 100% sure about that. There were a few times when I could just barely hear a little bit of the VCA bleed without any resistor, but most of the time I couldn’t hear it. (There is that mains hum though, so I can’t be too sure.) I haven’t really experimented with this, but if this resistor is included, it will probably be several mega ohms.

My ghost eyes aren’t lighting up and the relative control of filter 2 menus aren’t in the menu on my Shruthi. I imagine that’s just because I’m still using the firmware that was shipped to me with the kit in 2011. I really need to update that.

I should probably do a more careful tuning of filter 2 before making a demo.

And that just about brings things to where they are now. I know that was long, and maybe a little self-indulgent. Sorry.

Anyway, with luck, I’ll be able to post a demo tomorrow.

Looks absolutely epic, looking forward to hearing some demos! :slight_smile:

Very interesting to see another filter board development!

Quite an adventure!

I like that you are sharing with us your journey of invention.

Keep it up and lets hear some samples!

I’m definitely in for a new double filter board! Huray!

Did somebody say Mini Korg 700s ? Count me in :slight_smile:

Thanks for all the words of support everyone.

I’ve uploaded two very simple demos to soundcloud

Nothing terribly exciting just showing that the filter mostly works on the initial patch. And one more demo of going through a few presets, and then tweaking the filter, while a sequence from the Alesis Micron plays.

Oh also, I fixed that volume drop I was talking about earlier. This is much better.

very interesting to read you story and nice sounding demos!
keep us posted on the developtment please.

Sounds really sweet - I like it!

sounds great indeed!

Wow, that’s awesome. I’m amazed you’re able to even do this with two jobs and a family! Your wife must be a saint! I thoroughly enjoyed reading through your journey.

Hi again everyone. I thought I’d drop by just to make a small update on this.

I’ve been working on this in Kicad, and I think I’ve placed all the components in pretty good places so that the routing won’t get too wacky. I’ll get to work routing the traces pretty soon. I just want to go over the documentation for the Kicad suite very thoroughly one more time before I get started so that I don’t screw anything up and also to make sure that I don’t miss out on anything that will make this easier. I guess I should also brush up on good PCB layout in general. This is my first time doing something this big. As you can tell, I’m kind of learning as I go. I’m not sure if I’d be able to do this with the free version of Eagle, but honestly it doesn’t matter because no matter how many times I try, I can never get along with Eagle. But so far I’m really liking Kicad.

I’ve attached a picture of a 3d model of the board. As you can see, it’s a pretty tight fit. There may be a few more parts than on the dual SVF, but not that many I think. Also the ICs aren’t laid out in nice lines like in most (all?) the other Shruthi boards, but I can live with that. Laying everything out was kind of like playing Tetris, so it’s actually kind of fun. I imagine some things will get moved around a little as I route, but the final version will probably look something similar to this.

I guess that’s all for now. I’ll try to post a few demos from the breadboard processing external audio within the next few days.

Looks great!

It may be better to place trimpots with adjustment screws on the side facing out at the edge of the board, so that they can be adjusted without having to disassemble the controller board from the filter board every time. Even better if they face out on the sides of the PCB, so that their adjustment screws are accessible through the hexagonal mesh on the sides of the standard Shruthi case without having to remove the case.

Since the PCB is not that big ti might be sufficient to use Trimpots that have the screw at the side not at the top……

I didn’t quite get how the two filters are routed - is it a fixed series configuration?

That’s a good point about the trim pots. The thought never even crossed my mind. I just put the ones with the screw at the top on because they are what were on my SMR-4 mk1, and I had tons of them in my junk box. The problem is, there aren’t so many places that they can be moved to where you can access the side. The back is out because the jacks would block the screw. The power supply is on the right side. It’s already cramped, and it’s probably not a good idea to move that or to try to weave through it and stick the trim pots right in the middle of it. The left side has the sockets for connecting to the digital board, and they can’t be moved. I don’t think there’s any way to get around them.

That only leaves the front. Two of the four trim pots could easily be moved forward if we don’t mind getting rid of the ghost. But that still leaves two more. Those three ICs in front are for switching the filter modes, and they are placed in exactly the same place as Olivier placed them in the original. I figured he put them there for a reason, so I’m a little reluctant to move them.

I’ll give it some thought, and see what I can do, but it may in the end be necessary to connect the boards together with wires to tune the filters. I think it’s the same with the official Mutable Instruments boards, so it’s not the end of the world if I can’t find a way to tune them while screwed together.

Anyway, I’ll keep thinking about it. Thanks for the suggestion.

Oh, and to answer the question about the routing. Sorry that it may not have been clear in my first post. They are switchable between series and parallel.

Hi again. I thought I’d give an update on the progress of the board.

Yesterday I finally finished all the routing. I started off by routing little blocks of the circuit, and they were all routed very nicely and neatly. But when it came time to put them all together it got a bit more messy. In the end, as long as it works, I can live with it. I wish I had seen that video of Kicad’s interactive router in the Ambika thread earlier. I haven’t updated my Kicad in over a year, so everything was done without any auto-routing of any kind. Perhaps if there’s ever a 2nd version.

Although I didn’t think I’d be able to at first, I was in fact able to move those trim pots to a position where they can be adjusted fairly easily from the right side with the boards connected. (Not sure about when it’s in a case.) They are behind the power supply, but anyone with even remotely steady hands should be able to do it with almost no risk of shorting any pins on the regulators or LT1054 with the screwdriver because electrolytic caps are in the way.

Also, I’ve changed some cap values to clean up the control voltages and some resistor values to extend the self-resonance and avoid clipping at hi res with the filters in series.

So this week, and perhaps next week as well, I’ll be checking and double-checking and triple checking for mistakes, perhaps tidying up a few places where the traces can be made a little neater, and working on the silk screen art.

After all that’s done, I’ll order the boards. I would guess that in two weeks to a month they’ll be in my hands:)