OBX voice card (with non-custom CEM3310 equivalent circuits)

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oldcrow
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OBX voice card (with non-custom CEM3310 equivalent circuits)

Post by oldcrow » Fri Apr 26, 2013 1:34 am

Ever since playing MC's OB-X at AHMW, I got it into my head to try and make one--gotta be cheaper than ebay! This meant making my own voice cards, first in the original form factor to test first on a "host board" to power and provide pots and switches for the voice board under test, then later in an actual OB-X. Since the vintage cards used CEM3310s and discrete everything else, I made the EGs in non-custom form using JH's VC_HADSR as a design reference (it was inspired by the 3310). It works on a breadboard, so they're in the CAD schematic and the "hard" part of the layout is done: the VCOs. I squashed the circuits a bit in order to open up some room on the board as my EGs were going to be a tad larger than a single 16-pin chip; they each use 4 chips, 4 transistors and a handful of passives. Throughhole parts as my eyes have gotten too bad to hand solder SMT very well (but I have a full SMT factory line for that if I ever make more than 8 of these ;)

The CA3080s are replaced by 13700s, which are dual 3080s. I do not bother with the darlington buffers. The CA3086 transistor array is replaced by two LS318s, and those mount over SMT TC resistors (see A3A, A3B in image). I do use some SMT bypass caps as the original board features no bypass caps whatsoever, save for the DC filter caps on the local voltage regulators.

More later. --Crow

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Post by Jarno » Fri Apr 26, 2013 5:21 am

Looks great!

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Post by moogah » Fri Apr 26, 2013 9:03 am

This would be a dream come true! The OB-X and Xa have long been my favorite old polys :)

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Post by spneca » Fri Apr 26, 2013 9:43 am

```````

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Post by EMwhite » Fri Apr 26, 2013 9:55 am

Same here, I'd be up for 8 or 12 of these if you ever getting around to fab'ing a board.

I bought a OBX voice card off eBay with plans to simply restore and make into a MU module. I can build just fine and move a schematic to breadboard but have zero skills when it comes to working one of those fancy design tools; please let us know how this progresses.
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Post by glmodular » Fri Apr 26, 2013 12:58 pm

I would also be interested in a set of 8! :yay:

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Post by oldcrow » Sat Apr 27, 2013 3:07 am

Layout done. I need to verify the dimensions of the mounting holes and connectors, and I need to create the correct library part for the Molex bottom-entry connectors, but otherwise done. Now to make a breakout board for it. Well, after I sleep. ;)

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Post by moogah » Sat Apr 27, 2013 10:42 am

:nana:

This makes me so happy :0) Any chance you'd be interested in making a version with the 2p/4p switchable filter ala OB-Xa? I would program the everloving shit out of a microcontroller to see a neo-ob become reality!

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Post by numan7 » Sat Apr 27, 2013 11:22 am

:omg: what a wonderfully awesome idea, oldcrow! :hail:

;hmm: i think i'd be down for 4.... and would like to mount them behind 5U/motm panels as individual voice-modules, patching a motm 650 as the voice-controller, should you decide to offer these for sale.

:popcorn:

cheers
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Post by negativspace » Sat Apr 27, 2013 12:21 pm

That came together quick!

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Post by oldcrow » Sat Apr 27, 2013 2:34 pm

I will experiment with other filters after I verify this standard version works as expected. I left space at the top and right end of the board for extra filter parts, so there can eventually be a 2 pole / 4-pole revision. Programmable switching means I have to find connector pins for the control signal; there are 4 wired but unused "X" signals that were meant for something on the VCOs, probably pitch and pulse width modulation from a 2nd LFO, I might use some of those for extra filter control.

These KK-156 Molex connectors are not my favorite, we used them when I designed factory instrumentation in the 1980s. They had a knack of tarnishing rather badly. The last time I used them was on the original MOTM-480 prototype to make swapping of filter elements quick. I will have to ponder a proper card edge for a future version.

--Crow
moogah wrote::nana:

This makes me so happy :0) Any chance you'd be interested in making a version with the 2p/4p switchable filter ala OB-Xa? I would program the everloving shit out of a microcontroller to see a neo-ob become reality!

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Post by oldcrow » Sat Apr 27, 2013 2:45 pm

I will definitely sell the bare boards. I am not building up more boards that have 200 resistors apiece than I absolutely have to. I am in the electronic manufacturing business and can farm out assembly work relatively cheap, however--it is just a matter of logistics as they like to have all the parts sent to them along with the boards. But that is down the road a bit.

--Crow
/**/
numan7 wrote::omg: what a wonderfully awesome idea, oldcrow! :hail:

;hmm: i think i'd be down for 4.... and would like to mount them behind 5U/motm panels as individual voice-modules, patching a motm 650 as the voice-controller, should you decide to offer these for sale.

:popcorn:

cheers

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Post by oldcrow » Sat Apr 27, 2013 3:12 pm

Analog is easy. :lol: No 0.4mm pin pitch SMT here. This took about 15 hours of actual CAD work. --Crow
negativspace wrote:That came together quick!

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Post by aladan » Mon Apr 29, 2013 1:05 am

Wow - fantastic!

Even just five or so years ago if you'd told me that people would soon be offering DIY OB-Xs*, 2600s and VCS-3s then I reckon I would have been somewhat skeptical. We really are living in an unbelievable golden age of synth DIY!

So, yeah, count me in for a poly's worth.

Cheers,
A.

* Ok, taking some small liberties here - obviously someone's going to have to design and build processor and other support circuitry to get the complete thing :)

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Post by beyourdog » Mon Apr 29, 2013 1:54 am

I'm as well amazed by the obstination and talent of the people who deliver such Projects and the quality of the documentation provided, which surpass often the original (For the VCS3 clone for exemple...)

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Post by Jarno » Mon Apr 29, 2013 3:16 am

oldcrow wrote:Analog is easy. :lol: No 0.4mm pin pitch SMT here. This took about 15 hours of actual CAD work. --Crow
negativspace wrote:That came together quick!
But no 4+ layers to route on either :hihi:

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Post by suitandtieguy » Mon Apr 29, 2013 3:36 am

holy shit this is amazing, Mr Crow!
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Post by oozitron » Mon Apr 29, 2013 6:10 am

I'm amazed at how fast you put this together Scott... it was just a couple weeks ago when MC's OBX was the center of attention (mine at least!) in the library. Call me foolhardy, but I would choose it over any polysynth :banana:

I would love to have a 4+ voice OBX. Even if it wasn't programmable.

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Post by doepferiano » Mon Apr 29, 2013 10:28 am

please count me in for 4!

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Post by EMwhite » Mon Apr 29, 2013 11:01 am

Re: being programmable, I'd be thrilled with a set of controls which are hard wired (single Filter Cutoff knob wired through a buffer to all voice cards, for example) and forget about the complexity of patch memory.

I dont have an understanding of the original intent of the project. Was it for voice board replacement of originals for those that have an OBX synth with wonky, or an incomplete set of voice cards, or is it a pure DIY affair to be implemented as each individual sees fit?

If the latter, a missing piece is the voice distribution/dispatch/voice stealing control. I have MidiPal and it works well but as the name implies, it's Midi only. Will be buying the ACXsynth kit from Hexinverter but it will only do 4 with the ability to daisy chain a bit of an unknown at the moment (the code is owned by a bloke in France and my understanding is that the project is held up waiting on some fairly basic additions to key priority.)

I suppose that a large ExpertSleepers compliment for all cv, gate, and envelope duties is a possibility but excessive in terms of cost and complexity in my opinion.

Any ideas?
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Post by negativspace » Mon Apr 29, 2013 11:07 am

oldcrow wrote:Analog is easy. :lol: No 0.4mm pin pitch SMT here. This took about 15 hours of actual CAD work. --Crow
I realize it's not a daunting layout for a seasoned pro... nevertheless, usually things don't materialize here in a matter of hours - even 15 of them.

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Post by Isaiah » Mon Apr 29, 2013 11:53 am

EMwhite wrote:Re: being programmable, I'd be thrilled with a set of controls which are hard wired (single Filter Cutoff knob wired through a buffer to all voice cards, for example) and forget about the complexity of patch memory.
+1
Is it as simple as wiring a voltage divider to the same point on all four voices, or will there be a separate control PCB to handle parameter CV distribution?


I dont have an understanding of the original intent of the project. Was it for voice board replacement of originals for those that have an OBX synth with wonky, or an incomplete set of voice cards, or is it a pure DIY affair to be implemented as each individual sees fit?
I'm also keen to know this.


If the latter, a missing piece is the voice distribution/dispatch/voice stealing control. I have MidiPal and it works well but as the name implies, it's Midi only. Will be buying the ACXsynth kit from Hexinverter but it will only do 4 with the ability to daisy chain a bit of an unknown at the moment (the code is owned by a bloke in France and my understanding is that the project is held up waiting on some fairly basic additions to key priority.)
I was thinking about the ACXsynth/Hexinverter Poly MIDI-CV converter, too.
Four voices polyphony isn't a problem for me.

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Post by Isaiah » Mon Apr 29, 2013 12:04 pm

I'll just leave this here...


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Post by marvkaye » Mon Apr 29, 2013 12:11 pm

I'd be in for 2 to 4 as well, depending on final price, if and when this happens. Very nice!!

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Post by oldcrow » Mon Apr 29, 2013 4:24 pm

I am doing this because I want the classic Oberheim sound, the one before everything went CEM chips. I started with the voice card because it is the actual synthesizer and I had annotated documents from MC to work from--he corrected drawing errors in the service manual schematics and so on.

The next board to make, for testing a voice card, is a manual patch manager. On it would be a panel control for each parameter, as well as connections for external jacks to provide places to apply signals such as LFOs for pitch and PWM. I would make this manual patch "breakout" board such that it would attach to a front panel say 6U wide and have the voice card plug onto the back. While it is mainly for me to use as a test platform other folks might see it as a module sort of in the original SEM category.

A programmable-preset patch manager would of course use a microcontroller and a host of multiplexed CV storage buffers, provide a local VCLFO and noise source--all the elements the original OBX patch manager did. Of course there is also a panel digitizer to consider, but unlike single-CPU systems where for example the OBX's Z80A ran everything, I can split the workload across "smaller" microcontrollers for each task.

I know that to implement the polyphonic OBX I will also need to build a key assigner. At this point I would revise the voice card to have an actual card edge connector so as to use say a Vector card rack enclosure and rack eight voice cards, patch manager, key assigner, power supply, etc.

It is a lot of work, but nothing so far is that daunting. When I did instrumentation design, I worked with the Z8 microcontroller series extensively and used one to make one of the oldest MIDI retrofits I can think of for the Korg Polysix in 1985. I would keep the patch manager and key assigner as separate boards with their own microcontrollers, having them share a common MIDI interface to pick up MIDI messages and decide which board is supposed to handle it.

This divide and conquer method is, for me, the best way to realize all possible parts of the project.

Crow
/**/
EMwhite wrote:Re: being programmable, I'd be thrilled with a set of controls which are hard wired (single Filter Cutoff knob wired through a buffer to all voice cards, for example) and forget about the complexity of patch memory.

I dont have an understanding of the original intent of the project. Was it for voice board replacement of originals for those that have an OBX synth with wonky, or an incomplete set of voice cards, or is it a pure DIY affair to be implemented as each individual sees fit?

If the latter, a missing piece is the voice distribution/dispatch/voice stealing control. I have MidiPal and it works well but as the name implies, it's Midi only. Will be buying the ACXsynth kit from Hexinverter but it will only do 4 with the ability to daisy chain a bit of an unknown at the moment (the code is owned by a bloke in France and my understanding is that the project is held up waiting on some fairly basic additions to key priority.)

I suppose that a large ExpertSleepers compliment for all cv, gate, and envelope duties is a possibility but excessive in terms of cost and complexity in my opinion.

Any ideas?
Last edited by oldcrow on Tue Apr 30, 2013 12:37 am, edited 1 time in total.

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