Obie-Wan – Episode 1

Introduction The Oberheim OB-1 was the first analog mono synth with presets in memory, closely followed by the PPG 1003 Sonic Carrier. Announced in January 1977 at Winter NAMM and released back in November 1977 the OB-1 is a rare synthesizer, even though hundreds were made (500 in total?), it is even more rare in the UK where Oberheim had no distributor and the UK price was over £1200.

The OB-1 was innovative in 1977 but its market started to be eroded in 1978 as analog poly synths with patch memory entered the market. Whilst the Prophet 5 and  OB-X were considerably more expensive, the Jupiter 4 (1978) and 2-voice Pro Mars were closer in price to the OB-1. With only eight presets the OB-1 looked dated by 1979 as did its pre-microprocessor design. Moog entered the preset mono synth market in 1981 with the Source, but ditched real time control to save costs.

The two oscillators have a SEM heritage, and the VCA uses a CA3080 OTA. but the legacy ends there. The filter is a world away from a SVF with a Roland-like 2 or 4 pole OTA filter. The envelope generators were initially OTA based but replaced with CEM3310’s in the Mark 2 OB-1.

The modulation options are fantastic, with noise as one of the sources. The OB-1 is a precursor to the OB-X and OB-8, which harness the full set of CEM chips to great effect.

Why an Obie Wan?  I had the idea of building a replica of the OB-1, initially as a wide 84HP euro rack module.  So why is the OB-1 an attractive synth to replicate:

• Variable waveform VCO’s.
• Sub octaves on both VCO’s.
• OTA 2/4-pole filter (not a SEM SVF).
• Fast and snappy envelopes (CEM3310).
• Fantastic bass and lead sounds.
• 8 user presets, upgradeable to 64!

How hard could this be?

Inside an OB-1

Analog Boards The first and easy task was to design a set of analog PCB’s, one for each major component (LFO, VCO, VCF, VCA+ENVS). The schematics are easily located and I have a high resolution copy of an original service manual. Each PCB is 70 x 105mm and this is familiar analog electronics which is easy to get right. A set of PCB’s were ordered in late February 2021. The LFO board will need to be revised to include the performance controls – see below.

Digital Boards The OB-1 uses a ton of CMOS chips to scan the switches and pots, store presets, strobe the memory and S&H’s, and light the switch LED’s – as well as controlling the analog synth. Although the Z80 was launched in July 1976, and I remember the impact very well as I was studying electronics in the UK – it took another year for the Z80 to be accessible to synthesizer manufacturers, as software development was needed. This was more expensive to set up than using CMOS digital logic, as a a computer development system was needed to write the software!

The Z80 powered the first set of poly synths in 1978, starting with the Prophet 5 and the Jupiter 8, as well as being used in the Emu Systems 4060 polyphonic keyboard. Prior to this PPG, SCI, ARP and Oberheim were using CMOS logic in their synths. and making use of high cost RAM memory back in 1977, which seriously constrained the size of storage.

OB-1 with 48 Presets

The OB-1 is frugal on memory and uses just 1k bit of CMOS RAM, although toward the end of production a factory modification raised the preset number to 48. A rotary switch on the lower left panel selected which memory bank (1-8) was in use, with 8x the CMOS RAM. The limited RAM size means that serial processing and 6-bit values for potentiometers is required. The Oberheim OB-1 case is large and for a reason, there are a lot of circuits in there!

Game Change Replicating all the CMOS logic in the OB-1 processor board was daunting and I knew it would take up  too much PCB space to make the project viable. Thanks to Tauntek the CMOS processor has been replaced by a modern PIC18F2525 processor on a smaller PCB. Whilst this PCB is still too big for my project, it can be redesigned to fit. The new processor PCB effectively upgrades the OB-1 into the 21st century with 64 preset patches, no need for a battery, although we still have the 6-bit data values with 64 values and only half of the MIDI standard of 128.

Waldorf KB37

The KB37 Host The Waldorf KB37 is a fantastic piece of design and engineering, very well made and with space for 107HP of Euro Rack. I initially used it with my SH05 modules, but it seemed wasted on a mono synth with no integration of velocity and aftertouch. Patch cables were needed everywhere and it was just a bit messy. I nearly sold it, until I released what it true role was – to host my OB-1!

Fitting the OB-1 into the KB37 was not as easy as I first thought. There is only 25mm of depth in the right hand side of the KB37 to clear the power supply. This restricts the PCB depth to use one layer, two with care. There is a Waldorf PCB at the bottom of the case facing the rear with the 5V and 12V regulators, Doepfer style power headers and the digital circuits for keyboard scanning and MIDI.

I decided  to tailor my design to the KB37 and it is not transferable to a rack mount case. I think the advantages of having the keyboard and performance controls outweighs the limitation of not being an 84HP super wide module. The KB37 brings MIDI in and out and instantly makes the OB-1 MIDI capable, it includes an on board arpeggiator with glide and the aftertouch and velocity from MIDI or the keyboard can control the OB-1 filter cutoff and the volume by using an updated performance control module.

More in Episode 2!