The CrowBX
2023-11-28
I am going to write some things down about the CrowBX, a DIY synthesizer project I have been working on on and off since 2017. I will start by explaining what it is.
The CrowBX is a DIY recreation of the analog half of the Oberheim OB-X synthesizer created by Scott Rider AKA Old Crow in 2013.
The name
The name is the easiest bit to explain: it is a portmanteau of Old Crow (the creator) and OB-X (the thing the project is recreating).
The OB-X
Image from Retro Synth Ads.
The OB-X was a polyphonic keyboard synthesizer produced by Oberheim from 1979-1981. It succeeded the monophonic OB-1 and could be bought with either 4, 6 or 8-voice polyphony, hence the "X" in the name.
The most distinguishing feature of the OB-X was its digital patch memory. This was intended to allow live keyboard players to rapidly change the sound of their instrument in between or even during songs. Without patch memory, you would have to stop playing and carefully dial in the next sound from paper notes using the front panel knobs. With patch memory you could push a button and the sound of the instrument changed instantly. All this was made possible by a Z80 microprocessor.
The sounds themselves were synthesized with analog circuits largely built from standard components, with the exception of the Curtis Electro-Music CEM3310 envelope generator IC's (two per voice) which were specialized IC's for music applications.
The OB-X was quickly replaced by two subsequent sibling models which took advantage of new specialized IC's from Curtis that simplified the analog circuits of the synthesizer: the OB-Xa which used the CEM3340 voltage controlled oscillator (VCO) and CEM3320 voltage controlled filter (VCF) and later the OB-8 which used the same Curtis IC's but with a further streamlined circuit layout.
Sonically, the OB-X is the most unruly of these three which probably has to do with the looser tolerances of its oscillators and filters, because those sub-circuits were assembled from smaller components instead of being etched into silicon inside the Curtis IC's.
What makes the OB-X special, in my eyes anyway, is the extravagance of bundling together (up to) 8 identical synthesizers (the voice cards) with 2 discrete VCO's each. It takes a lot of calibration work to get all of those oscillators and filters to sound the same and you never quite get it right, which is part of the charm of the instrument. Later instruments became more reliable but also less characterful.
Here is an example where you hear all 8 voices of my CrowBX trying to sound the same.
Also see this nice video by Alex Ball showcasing the OB-X.
A DIY recreation of the analog half
Why did someone make a DIY recreation of the OB-X? As with most of the instruments from this time, the second-hand prices of surviving examples have been getting crazier and crazier through the years. At the same time the service manuals for these instruments, which contain complete schematics of the analog circuitry, have been widely available on the internet for quite a while. All it takes is a skilled and motivitated electronics engineer to create new circuit boards that implement the old schematics.
This would not have happened today because in 2022, a reboot of the Oberheim company released a new product called the OB-X8. This synthesizer offers an integrated package with the sounds of all three siblings (OB-X, OB-Xa, OB-8) in a modern instrument. Back in 2013, if you wanted this sound your options were to either pay silly money for an old-timer requiring lots of maintenance and repair, or to build a clone yourself.
Old Crow started writing about his project in this ModWiggler forum thread in April of 2013. I think it is a great example of how to start small and work your way up to a bigger and bigger project.
The OB-X is structured roughly as follows:
To recreate this, Old Crow started by recreating a single voice card, combined with a custom "host board" or "voice tester" control panel that sends analog control voltages straight into the voice card, skipping the Z80, ADC and DAC.
This resulted in a monophonic analog synthesizer that was already playable.
As the next step he made a 4-slot "carrier board" that bolted on to the "host board" user interface, so that you could connect 4 voice cards resulting in a 4-voice polyphonic synthesizer. This is what that looked like when I built mine:
Old Crow's next iteration was to design and manufacture an 8-slot motherboard PCB. This was too big to bolt to the back of the host board but could be connected to it with wires.
After this, the next step would have been a digital interface to control all the analog boards so far, but this never materialized. So because the digital part never happened, I tend to call the CrowBX the "analog half" of the OB-X.
At each step of the way, Old Crow had circuit boards manufactured in larger batches than what he needed for his own use, and he sold these circuit boards on the internet to enthusiasts such as myself. I will write another blog post about all the work involved to turn the circuit boards into a complete working instrument.