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.