OB-X Single Voice/Controller Card Build Info Page

Hi all and THANK YOU for being a part of our first of hopefully many DIY synth projects! 

This page will replace the need for email blasts and will keep all info in one place. 

Since I will be adding to this page as needed, the info will be organized by dates rather than subjects, so if you’re new to the build, it’s best to read the whole thing.


(original email blast info edited for web page)

The board you will/have received will have 6 cuts and 1 jumper already performed. However, an additional few jumpers will need to be performed after a few components have been added. I will post a video below with the jumpers needed after 6 resistors are installed. That said, make sure to get some kynar or other thing wire for these jumpers. 26-32 AWG wire will do. 
As for the BOM’s and lists, please see the links and descriptions below:
All parts listed with mouser as the source will be in the mouser cart linked below.
All other vendors will need to be ordered manually. Some of these parts can also be had from rosensound.com now. 
The FET transistors in the spreadsheet can be either the J112 from mouser or 2N4302 from eBay. You don’t need to buy both. Using J112 is easy, you just need to twist some legs on their way in.

Mouser cart:
My only note on this mouser cart is that you may want to add a couple extra of some resistors, like 47k, 100k, 470K, etc. Just 1 or 2 extras will do. We may make some small resistor changes for pulse width summing on the controller board, but those values should do.
We’ll do our best to update the cart with parts as they become obsolete. Sometimes basic resistor part numbers go out of stock even though they have the same thing under a different part number that has some esoteric parameter that doesn’t usually matter as the only difference. Please let me know if anything appears low/out of stock so we can send a substitute part number and update the cart
OB-X card build docs:
If you’re new to electronics, please do some light reading about diode/transistor/capacitor orientations 🙂
Note: We discovered some small errors in this doc but will be correcting it. Specifically, the FET transistors are listed under NPN, and the voltage regulators at the bottom left of the board are also listed under NPN. 
The PSU issue has been the main reason for delay, as it appeared to be a possibility it couldn’t be had, and I was sort of right. We have tested the product with a bench power supply, but of course not every has that around….
Some background if you care to know: we know there are some ready made supplies available that are *close* to the +/- 19V needed, but the variances were too great. The OBX needs the voltage to be exact in order for the oscillators to oscillate. So, with the help of my friend Jeff Turzo from Overstayer Recording Equipment, we’ve designed an adjustable power supply instead that we will email you about in less than 2 weeks. This will cost *much* less than any suitable commercial supply available, and we will be providing it to you guys at cost. It will be quite cheap.

6/27/2020 Update: PSU and other stuff

Sorry for the delay y’all. That’s a phrase I use often, because often I suck at predicting my availability while also meeting shop deadlines and having 2 kids…  

I’ve ordered the PCB’s for the PSU. For our Rev 1 buyers, we’re going to be offering this at cost, which will likely land around $25 or so. Luckily we were able to design something that’s much simpler than I originally thought, and it doesn’t get hot or anything. It uses a 18VAC wall wart and via basic creative circuitry results in an adjustable supply that properly powers the project. It will be mounted in a project enclosure with the MOLEX stuff coming out one end and the wall wart plugging into the other.
To touch on PSU pricing to Rev 1 builders and subsequently Rev 2 builders… we are not factoring the cost of the PCB into the $25 price tag. The PCB will be including as part of the overall kit going forward. The project enclosure, wall wart, and components will simply be added to the BOM for Rev 2 buyers (BOM to be updated when all parts are confirmed). We believe it will cost Rev 1 builders substantially less to get the “kit” straight from us at cost to save on shipping from various vendors. 

Mounting the new PCB in a project enclosure properly is the next small hurdle. I designed the PCB’s to fit into a project enclosure from Jameco based on datasheets but with this kind of stuff you never know if thats the most accurate, so once we verify that, we’ll blast again that it’s available and make it all happen. Time’s like this I wish I could just drive to All Electronics nearby and grab what I need, but COVID has put a major damper on that 🙁

We realized (embarrassingly) that the controller card is missing silk screen for the I/O. When we ship the supply stuff, we will also be providing a thin sticker label to compensate for this error. 

Resistor Values:
I’m working to figure out a better resistor value scheme to allow adequate calibration and range for the PW and also to shut off VCO2. This is due to the way the original OB-X shuts off oscillators: When an oscillator is “off”, the computer signals the voice card to switch to pulse wave and also sends 5V to the pulse width control node to effectively null out the oscillator. This isn’t a major issue, but I do still need to adjust some pull up/down resistors on the card. 

Gate Levels:
I’ve noticed that the OB-X card does not reliably sequence from a SH-101, which spits out a gate voltage of about 4.7V. The gate must be 5 or above for it to trigger reliably. I assume it may be possible to add a diode to lift the ground potential to “cheat” your way into 5+V, but I haven’t done that just yet. I imagine most other sequencers nowadays use gate voltages higher or exactly 5V. 

Maybe I’m biased because I made the thing but it sounds really sweet and works quite well! I’ve been sequencing it with an Oberheim Mini-Sequencer and it really not only sound like a 1 voice OB-X but also just a great sounding synth. The Sync and X-Mod is super sick. In a lot of ways this makes me wish the actual OB-X could easily disable 7 voices to only hear one card (this can be done, but it needs to be opened up). I can’t wait to hear what y’all make with this! You can hear this on our instagram… we made a “highlight” section on our page with the various videos I’ve posted of this project making sound. https://www.instagram.com/rosen_sound/

9/12/2020 Update: PSU Is Ready!

After needing to adjust the design due to very haphazard project box experiences, the design is now ready for prime time! Photos below….

Notice the SMD regulators… that was the main design change. Using through hole LM317/LM337 would have either required a larger board, or a larger project enclosure… Either way resulting in a large project box. The current draw is low enough that these regulators are sufficient. They’re big enough packages to hand solder, too. 

One side of the board has easy pads to connect a power switch, the AC jack, and a fuse.The other side has the output voltages with extra pads to make calibration a bit easier. 

We will let everyone know via email how much it will cost to have the PSU shipped to them. Again, it will be at cost which is quite low. The most expensive parts are the enclosure ($5) and the wall wart which isn’t much more.

All that’s left is:

1. Resistor values for controller card
2. Stickers
3. Gate levels (if needed, likely not). 


Stay tuned…