Possible Shruthi-1 mods

I’m contemplating making a shruthi, if I can afford it off my student loan (I’ve figured out what I can afford per day, so if I’ve succeded in keeping my costs low enough over the last couple of weeks, I’ll spend the difference on building a shruthi). To keep costs down, I’m planning to get most of the components from farnell, as I get a student discount there. I’ve got a few of the neccary components already, left over from other projects, such as the LM13700s from my A-level synth.
Anyhow, I thought I might look at doing a few mods as I go. I’m not sure what’s worth trying, and which aren’t really worth doing. I’d quite like to add a few of the SDE mods, maybe do a PCB for them. What are the disadvantages of doing these mods?
Another thought I had was to try adding a D-beam esque control to it, by adding an IR proximity sensor on one of the CV inputs (something like this: http://uk.farnell.com/sharp/gp20a41sk0f/sensor-distance-analogue-output/dp/1618431). I suspect I might need to bump up the signal from that a bit, as it’s only about a 3v swing, which won’t really get the most from the ADC. I might also add a joystick or knobs on some of the other inputs.
Is it possible to re-name the CV I/O ports within the firmware, so rather than coming up as “CV1” on the screen, it comes up as “D-beam” or “X-Axis”? Likewise, is it possible to directly alter the value of the output CVs, or are they only accessible through the mod matrix?
Can anyone see any problems with patching in a PT2399, with the feedback loop going partially through the main filter? Is it possible to sync it to the arpeggiator etc. on the shruthi? (I figure that’d need some sort of uC reading the midi clock pulses or something, and I suspect it’s probably more trouble than it’s worth, but I’ll ask anyway)
Also, what’s the cheapest compatiable & worthwhile OLED display that anyone’s found? I like the look of them, but all the ones I’ve seen have been in the £15-20 range, which is a bit on the steep side for me when I can get an LCD for <£3.
Additionaly, is it possible to slim down the shruthi at all? I’m contemplating re-positioning the audio & midi jacks, would that allow the two PCB to be mounted closer together? I’m planning to go for a similar look to xiwi’s version of the noise toaster (https://www.flickr.com/photos/52082118@N05/sets/72157633108503187/) and I suspect it’d look best when kept fairly slim.
I think that’s everything I’d thought of so far; I was half considering trying a valve VCA in place of the normal one, but I suspect that’s not a wise plan.
Appologies for the long list of probably stupid questions about modifying a synth I haven’t even purchased yet, let alone built. I don’t know how much of this I’ll carry out; I just figured that I’d be best getting a bit of info before I order everything. Also, I reckon I can probably do a neater job of the modifications if I plan & integrate them from the start.

Also, Thanks to everyone who gave me advice with my A-level project (http://mutable-instruments.net/forum/discussion/3566/eurorack-standard-specs#Item_85) & on career advice (http://mutable-instruments.net/forum/discussion/3854/careers-in-electronics-etc-#Item_18); I managed to get A*AAA on my a-levels, with full marks on my coursework project, which has got me into the course I wanted at the University of Leeds.

@mushroomglue: I think the best thing for experimenting with the CV inputs would be to add well protected 1/8inch stereo jacks. You can use the tip to send the signal, the ring to send the +5V signal, and the ground sleeve to ground.
The signals sent to the Shruthi should never exceed 5V or be negative as they are directly connected to the MCU.

Also, the CV in’s are available thought he mod matrix and can be atenueverted just like any other Shruthi signal in the mod matrix.

Check the mutable wiki page for lots of modification info.
I think it’s better to just build it normally first and mod it later. At least you know you are coming from a working start point.
I also think Reichelt will be cheaper for lots of parts than farnell even with discount.
Just use a normal LCD as Oled can cause noise problems and since you plan mods that may cause problems.

I think the trouble with Reichelt for me would be delivery; Reichelt’s in germany, by the looks of it (I’ve not heard of the company before), wheras Farnell’s just a couple of miles up the road for me.
I didn’t know that the OLED’s cause noise problems; if that’s the case, I’ll stick to LCD. I can always swap it out later.
I was planning on getting it going as-is to start with, to make sure I’ve got a stable platform for modding, but then get in to adding a few extras before I do a case.
I was thinking of adding using the CVs to directly control a few parameters; That’s why I was asking about naming, and if they could be directly edited, in the same way as, say, the filter cutoff, or whatever. I don’t have much CV kit at the moment, so that’s why I wanted to integrate some controllers, rather than just adding jacks. I’ll connect any leftovers to jacks, but I’d like some extra controls, too, even if they’re just pots. I’m guessing I’d be best buffering any CV inputs with an op-amp or something, to keep it all safe.

The LCD is the best choice as OLED does cause noise at varying degrees with different filters.

The CV on the Shruthi is best used with the provided 5V and ground pins. External control voltages will not track at one volt per octave. So adding controls that can vary that 5V signal like a soft pot or a photo resistor is best.
You can always prototype these input devices with an Arduino as they have a 5V ground pin and a way to read the voltage of the signal.

If you really want to use the CVins with external gear you best install a Buffer using a MPC600x OpAmp with Shruthis +5V/GND rails so you can be absolutely sure no excess or negative Voltage will ever hit your precious AT-Mega.
With a 28ct MPC6004 and you can buffer all 4 CV ins. eMail me a copy of your Immatrikulationsbescheinigung™ and ill send you one for free :wink:

I’d only really be planning on breaking out one or two of the CV ins to external jacks, and I’ve got a few assorted op-amps knocking around that’d probably do the job. Thanks for the offer, though.

You really should use a protected RailToRail OpAmp so you have the whole CV Range and can be sure it withstands to high and/or negative Voltages…… get a 600x, they are crash tested be the god of killing chips - me.

I googled the MPC6004 and found, that maximum input voltage is VDD+1V down to VSS-1V.
This is taken from the absolute maximum table.

All i can say they work perfectly here in all my modules with whatever negative voltage you throw at it….

fcd72: are you really using them in the buffer/follower configuration?

try the blue module…… I use them as Buffers via 100k, with 1M tied to GND. Works great so far.

what about a classical resistor-zener combination to protect against overload and a simple opamp afterwards (probably not even needed if going directly into the adc)? Sounds safer to me than using an opamp outside its spec. That will work for a some devices and maybe even for some time. But once you order them from another distributor it might be completely different.

Two schottky diodes can also do the trick. Use a low resistor (say 1k) because the AVR ADC inputs have a low-ish impedance in the ~10k range…

Can you explain that a little more (I mean the schottky part)? I’m not getting it…

Check the Frames schematics.

Regarding the “slim down” part.
what makes it “bulky” is mainly the 2 midi-jacks. I had some similar idea and thought of replacing them with something else and build some kind of adapter. (Only problem is that I probably would loose the adapter all the time :wink: )

I even went so far that I started to make a whole new PCB-layout with SMD-parts to be able to have a cutout for the display. That lead to next “problem”, witch is make the controller board compatible with the standard filter. Of course i had a go at a SMD-version for the SMR4 filter.

Maybe I will have a go at this “not-quite-a-Shruti project” some day

Couldn’t you replace them with panel mount jacks? loosing the PCB mounting’d allow a little more flexibility on positioning, so you could mount them on top, or over to one side on the back.

Well, panel mount = cabels = blah :wink:
A din 5 jack is “bulky” so either I accept that, and get som air inside the box, or i go the “apple way” and make it as flat as possible with some non standard "midi-jack+ adapter (“shruthi-air”? ;))