Oberheim OB-1 Replica


Overview The AMSynths “Obie One” is a close replica of the original from 1977; the first or possibly second programmable analog mono synth. So why is this an attractive synth to replicate?

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

The original OB-1 came out in 1977 and is quite a large synthesizer, even if it only has a 3 octave¬† keyboard, and it paved the way for the “go large” OB polyphonic synths.

Overall Design The OB-1 replica is designed to fit into the Waldorf KB37 keyboard and euro rack system.  There are three panels or sections;

  • MODULATION with LFO and velocity and aftertouch control
  • MAIN with the VCO’s, VCF and ADRS’s
  • FIX with a BBD chorus

Behind these panels there are PCB’s for the analog circuits and pot/switch scanning. On the base of the KB37 lives both a SYNTH PCB and a PROCESSOR PCB which are connected to the panel attached PCB’s by ribbon cables. The 12V Doepfer power bus has been used to power all the PCB’s.

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. 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.

The OB-1 replica is tailored to the KB37 and it is not transferable to a Euro Rack system. 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.

Analog Boards The analog synthesizer circuits are contained in five PCB’s, one for each major component (LFO, VCO1, VCO2, VCF, VCA+ENVS). The LFO also contains the noise circuit which drives the S&H LFO waveform and is a audio source into the VCF. Each PCB is 70 x 105mm and this is familiar analog electronics which is easy to get right. The CMOS logic that controls the analog circuits is contained on an ANALOG board which attached to the PANEL PCB, the analog PCB’s plug into the ANALOG board.

Processor Board The original 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. The original design pre-dates the availability of the Z80 microprocessor but luckily Tauntek has created a new processor PCB which uses a PIC microcontroller to replace a large proportion of the CMOS logic. This makes the build a lot easier and reduces the size of the PCB estate.

Whilst the Tauntek PCB is still too big for my project, I have redesigned the PCB to be narrow and it fits on the base of the KB-37. 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.

Synth PCB This PCB contains the discrete 6-bit DAC and the 18 Sample & Hold circuits for generating CV’s to control the analog circuits. It also contains the logic to drive the CMOS analog switches that replicate the hardware switches.

Panel PCB This is a wide PCB that holds all the switches and potentiometers as well as some of the scanning CMOS logic chips. Alpha 9mm pots and traditional DPDT and DP3T slide switches have been used.

Project Status & Outcome The design progressed during the Spring of 2021 and the details of the prototype build have been posted in my Blog. The aim is to complete the project by 31 May and offer a set of PCB’s for sale.

 

 

 

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